New Phytol. (1981), 87, 11-37 11

THE ROLE OF ELECTRICITY IN PLANT MOVEMENTS BY P. J. SIMONS Department of Chemistry, Imperial College, London SW7 2AZ, U.K.

{Accepted 24 August 1980)

SUMMARY A survey has been made of the different types of reversible movements in the plant kingdom (including those of carnivorous traps, leaves, floral parts, root tips, cytoplasmic streaming, cilia and fiagella) and it is suggested that many are mediated by changes in electrical potential. Many touch-sensitive movements in higher plants are regulated by propagating electrical signals (action potentials), which relay excitation from the site of stimulation to the motor organ(s), where collapse of sensitive motor cells is brought about. The resulting change in volume of the motor cells causes movement of the entire motor organ. In some cases additional localized changes in electrical potential (receptor potentials) can be induced by external stimuli, which convert specific characteristics of the stimulus into an electrical analogue before triggering an action potential. The role of electrical activity in other plant movements is more speculative. Action potentials do not arise in light- or endogenous rhythm-regulated movements, but changes in the membrane potential of motor cells concerned with these movements may also help to drive the ionic transport which leads to their turgor-mediated motion. The significance of electrical activity associated with intracellular movements is less certain, but may become the focus of increasing attention as the regulation of actin-directed cellular movements is further investigated. Electrical impulses comparable to those in higher plants may, however, regulate ciliar and Bagellar activity in the motile gametes and spores of lower plants. Thus, it is clear that electrophysiology is an important aspect of many plant movements, and bears a remarkable similarity to excitation in animal nervous systems. CONTENTS Introduction ...... 11 Glossary of nomenclature ...... 13 Measuring electric signals in plants ...... 14 Carnivorous plants .•••...... 15 Mimosa...... 17 Leaf movements regulated by light and endogenous rhythms ..... 20 Tanda effect ...... 22 Floral parts 22 Stomata. ••••••...... 23 Cytoplasmic streaming ...... 25 Tactic movements...... 27 Concluding remarks ...... 28 References ••••...... 32

INTRODUCTION The investigation of electric signals in plants has been neglected by many botanists particularly in the United Kingdom. This can be traced back to the opposition of the pioneering plant physiologists of the last century to any suggestion that plants could behave like animals (Simons, 1979). As a result, there has been a historical suspicion of nerve-like behaviour in plants, little helped by the often Abbreviations: ACh, acetylcholine; AChE, acetylcholinesterase; ATP, adenosine triphosphate; EDTA, ethylenediamine tetraacetic acid; EGTA, ethylene glycol bis (//-amin<)-ethylether)-Af,Ar'-tetraacetic acid.

0028-646X/81/010011 +27 $02.00/0 © 1981 The New Phytologist 12 P. J. SIMONS

Table 1. Latent time period hetween application of stimulus and start of motile response in reversihle plant movements. Note that figures only provide a rough guide, as the latent period will vary with temperature and age of material. Data from an animal is included for comparison

—'— f Latent i Species Stimulus Motile response period Key references ;

Albizzia julibrissin Red light to Leaflet closure 30 min Galston & Satter, 1976 I QarKiiesa A. julibrissin Darkness to Leaflet closure 5-10 min Galston & Satter, 1976 ; white light 1 Aldrovanda vesiculosa Electrical Trap closure 0 06 s Ashida, 1937 Berberis vulgaris Electrical Stamen bending 0 046 s Millet & Thibert, 1976 ; Biophytum sensitivum Mechanical l^eaflet closure 0 01 s Bose, 1913 Chara braunii Electrical Stoppage of c. 1 s Sibaoka & Oda, 1956 cytoplasmic streaming C. austratis Electrical Stoppage of 0-03 006s Tazawa & Kishimoto, 1968 cytoplasmic streaming Thermal 2s Insell & Zachariah, 1978 Dactylella braehopoga Trap closure Dionaea musciputa Mechanical Trap closure 0-4-12 s Sibaoka, 1966 Drosera intermedia Mechanical Tentacle 10 s- Williams & Pickard, 1972 bending several min Incarvillea grandiflora Mechanical Stigmatic lobe M-L3 s Sinyukhin & Britikov, closure 1965 a, b Mahonia aquifolium Electrical Stamen bending 0-052 s Millet, 1976 Mimosa pudica Wound or Leaflet closure 13 s Sibaoka, 1966 cooling M. pudica Red light to Leaflet closure (-. 5 min Eondeville, Borthwick & darkness Hendricks, 1966 Nitella stelligeria Electrical Stoppage of 1-2-2 s Franck & Auger, 1932 cytoplasmic streaming Pelargonium zonale Darkness Stomatal closure Not less Gregory and Pearse, 1937 than 1 min P. zonale Mechanical Stomatal closure (-. 1 mm Williams, 1949 P. zonale Wound Stomatal closure 1-4 min Williams, 1948 Phaseolus aureus Red light Root tip adhesior1 42 s Racusen & Etherton, 1975 Physarum polycephalum Electrical Cytoplasmic Up to 20 s Diannelidis & Umrath, 1953 streaming stoppage Sparmannia afrieana Mechanical Stamen bending 0-8 s Bunning, 1934 Stylidium crassifolium Mechanical Column bending 0-01 s Findlay & Findlay, 1975 S. graminifolium Mechanical Column bending 0-015-0-027 sFindlay & Findlay, 1975 Utricularia vulgaris Mechanical Trap door 0-006 s Lloyd, 1942 opening Zea mays 300 p.p.m. CO2 Stomatal closure 10 s Raschke, 1972 Z. mays Darkness to Stomatal opening 3-5 min Raschke, 1970 white light Animal Rana (frog) Electrical Sartorius musek 0-0028 Sandow, 1944 contraction 0-003 s Electricity in plant movements 13 gross exaggeration in popular literature. By describing the likely functions of electrical activity in reversible plant movements (that is, movements that can be performed more than once by the same motor organ), I hope to show that the plant kingdom has successfully exploited an electrical means of communication in many varied ways. The comparison between this activity in plants and nervous activity in animals is so striking that the relevant zoological terminology is also used in plant electrophysiology (see glossary). The role of electrical activity in plant movements has been dealt with in previous reviews by Bunning (1959), Umrath (1959), Sibaoka (1969), and Pickard (1973). These articles concentrate on seismonastic movements in higher plants, but many other types of movement are performed by a variety of species throughout the plant kingdom. The stimuli that induce these movements and the speed of movement vary considerably, and can even match some of the fastest animal movements (Table 1). The literature cited in this review is not meant to cover the whole field of electrophysiology of plant movements, and electrical data have been kept to a minimum. Reversible hygroscopic movements (such as the twisting of the awn of Avena fatua) and irreversible plant movements (such as fruit dehiscence in EcbalUum elaterium) have been omitted, as there is no evidence linking these movements with any electrical activity.

GLOSSARY OF NOMENCLATURE Action potential. Originally this term was applied to electrical events in nerves which cause a biological action, such as muscle contraction. There is little agreement, though, in the literature about the definition of an action potential, but the following criteria should give a general description of its properties: (i) transient and rapid change in transmembrane potential difference (Fig. 1); (ii) a typical voltage curve usually features a sharp spike, followed by a more gradual return to the original 'resting' potential (Fig. 1); (iii) can only be triggered after a critical level of excitation has been reached - a phenomenon known as the all-or-nothing response; (iv) if sub-threshold stimuli are delivered over limited periods, sufficient excitation can be accumulated to trigger an action potential;

20mV

One-second time intervals Fig. 1. Action potentials recorded from the surface of the trap of Diinuiea muscipula. The recording technique of Williams and Pickard (1972) was followed, using an extracellular electrode attached to a salt bridge and cotton wick, contacting the inside surface of one trap lobe. Mechanical stimuli were delivered by bending over one trigger hair twice with a glass rod. Each stimulus caused an action potential, but the electrical disturbance following the second action potential was caused by the trap lobes moving together and jerking the electrode (recording made by K. S. Rathore and P. J. Simons). 14 P. J. SIMONS (v) during and immediately after the passage of an action potential, it is not possible to elicit another action potential on the same membrane site without a rest (refractory) period; I (vi) can be triggered by electrical stimuli. 1

Depolarization. A decrease in electrical potential. As plant cell interiors are negative with respect to the outside medium, depolarization is generally a change to a more positive potential. |

Facilitation. An increase in membrane potential over limited periods of time with successive stimulation. |

Hyperpolarization. An increase in electrical potential. As plant cell interiors are negative with respect to the outside medium, hyperpolarization is generally a change to a more negative potential.

Receptor potentials. Transient changes in membrane potential which convert stimuli into electrical analogues. Receptor potentials always precede, and often trigger, action potentials, but they do not have the all-or-nothing, nor the propagating, properties of action potentials. The voltage curve of receptor potentials is highly variable and reflects specific properties of the stimulus. I Synaptic gap. These are discrete, but microscopical, junctions between nerve cells. Excitation is carried across synaptic gaps by chemical messengers - the neurotransmitters. |

Variation potentials. First described by Houwink (1935) in the wound-induced changes in membrane potential in Mimosa, these are apparently confined to plants. Variation potentials develop relatively slowly over a period of minutes, rather than seconds (as in action potentials). They do not conform to the all-or-nothing property of action potentials, but are conducted away from the site of stimulation. The shape of the voltage curve is extremely variable (hence the name), often consisting of many small spikes superimposed on a slowly developing hyper- or depolarizing potential. I

Neurotransmitters. A group of diverse chemicals concerned with transmitting excitation across synaptic gaps, muscle end-plates, or nerve-gland junctions. Neurotransmitters transfer excitation from one cell to another by binding to specific receptor sites on the plasmalemma of the receiving cell. The resulting change in membrane permeability regenerates the action potential.

MEASURING ELECTRIC SIGNALS IN PLANTS * Voltage changes in plants can be detected using an electrode placed either in a plant cell (intracellular electrode) or resting on the surface of a cell or group of cells (extracellular electrode). The electrodes can be of two types: polarizable (electro- chemically irreversible) or non-polarizable (electrochemically reversible). The potential of a polarizable electrode can change with time, or when current is passed through it, whereas that of a non-polarizable electrode remains virtually constant. As a result, if measurements of potential are made over relatively long periods (of Electricity in plant movements 15 the order of > 60 s), it is necessary to use a non-polarizable electrode to ensure that the change in potential is caused by the plant and not by the monitoring electrode itself. On the other hand, if signals of only brief duration (< 60 s) are being recorded (as is the case for many action potentials), it is possible to use either type of electrode. More detailed explanations of polarization in electrodes are given by Albery (1975). Another source of uncertainty arises from potential differences at boundaries, such as that between electrode and plant. These potentials are called junction potentials and exist even when no current is being passed through the circuit. Both electrode polarization and junction potentials can largely be overcome using a silver-silver chloride electrode or a calomel electrode (composed of mercury in contact with mercurous chloride), making contact with the plant via a salt bridge containing concentrated potassium chloride in agar jelly and held m a glass probe. In order to complete the electrical circuit a reference electrode is placed in solution near the plant; this electrode must also be chosen to minimize polarization and junction potentials. Silver-silver chloride or calomel electrodes are suitable for this purpose as well, but the stainless-steel forceps used by some researchers as reference electrodes are unsuitable as their electrical potentials and stability are completely unknown. The potential differences between recording electrode and reference electrode are extremely weak and need to be amplified before they can be recorded. A high-impedance amplifier (10" to 10" MQ) is suitable for this purpose as it draws virtually no current from the circuit whilst amplifying the signal picked up from the plant. Once amplified, the signal can be displayed on a chart pen-recorder or other recording instrument. Further descriptions of some of these recording techniques are given by many authors, including Hope and Walker (1975), Findlay and Hope (1976), Van Sambeek and Pickard (1976a) and Young (1973). More detailed explanations of electrode kinetics, junction potentials and other electrochemical phenomena are given by Maclnnes (1961).

CARNIVOROUS PLANTS Many carnivorous plants use rapid trap movements to ensnare their animal prey, and in some species it has been shown that the movements are mediated by electrical activity. The leaf blade oi Drosera is covered by numerous stalked glands (the tentacles), which secrete a sticky digestive fiuid from the gland surface at the tentacle head. An insect touching a tentacle on the leaf margin will become stuck to the digestive fluid and be pulled in towards the centre of the leaf as the tentacle gradually bends inwards. In the more sophisticated bilobed leaf trap of Dionaea muscipula (Venus' flytrap) an arthropod may bend over one or more of the trigger hairs located there, and if sufficient deflection occurs within an interval of 1 to 25 s, the trap lobes snap together and enclose the prey. Despite their different traps, the electrical activities of Dionaea and Drosera are remarkably similar. In both species mechanical stimuli initiate local graded receptor potentials, transducing certain physical parameters of the stimulus into electrical analogue form. In Dionaea the wave form of the receptor potential has been shown to be a function of the strength and speed of the stimulus (Jacobson, 1965), but so far this has not been quantitatively tested in Drosera. Given adequate i6 P. J. SIMONS | stimulation, a receptor potential generates a propagating action potential in Dionaea (Benolken and Jacobson, 1970) or a series of action potentials in Drosera (Williams and Pickard, 1972). The action potential in Dionaea (Fig. 1) is relayed from the stimulated trigger hair across the trap lobes (Burdon-Sanderson, 1873), whereas in Drosera the action potentials are contained within individual tentacles (Williams and Pickard, 1972). The frequency and number of action potentials control the rate and extent of movement in both Dionaea (Burdon-Sanderson and Page, 1876) and Drosera (Williams and Pickard, 1972). In both species the motor tissues become increasingly receptive to successive action potentials in a manner akin to facilitation in animals. i Stimuli presented over a length of time in Dionaea appear to be stored in a simple 'memory', which detects the accumulation of sufficient excitation in the trap before initiating movement. For example, two stimuli delivered at room tempera- S ture within 1 to 25 s of each other are needed to initiate closure (Sibaoka, 1966), but 14 stimuli are needed before movement begins if the stimuli are delivered once every 20 min (Brown, 1916), Since trap closure depends on the number of action potentials elicited, rather than the number of stimuli received (Jacobson, 1965), the memory is likely to involve an electrical component. In contrast, the Drosera trap lacks this sort of memory, as the tentacle visibly responds to all the action potentials it receives, no matter how slight the resulting movement (Williams and Pickard, 1972). ^ ; The trap of Aldrovanda (the Water Wheel Plant) closely resembles that of Dionaea, and performs similar movements in response to the same stimuli (Ashida, 1935). Mechanically induced action potentials have also been detected in Aldro- vanda (Williams, 1976; Sibaoka, 1980), preceding the sudden loss of turgor in the inner epidermis which allows the outer epidermis to expand, thereby bringing about movement of the trap lobes (Ashida, 1934). Since the mechanism of trap closure in Aldrovanda and Dionaea is likely to be similar, it is assumed that similar turgor losses induced by action potentials also account for trap movement in Dionaea (Lloyd, 1942). How the action potentials trigger the losses in turgor in the motor tissues of Dionaea, Aldrovanda and Drosera still remains a mystery. Following the rapid trap closure in Dionaea, the lobes tighten together, largely controlled by a chemotropic mechanism which mainly responds to nitrogenous compounds diffusing out from the prey (Robins, 1976; Lichtncr and Williams, 1977), Before it is killed though, the struggling victim probably delivers many more blows to the trigger hairs, thereby promoting trap tightening (AfTolter and Olivo, 1975), Action potentials resulting from these additional stimuli (but not from the chemical stimuli) have been recorded by AfTolter and Olivo (1975) using a silver wire glued to the outer trap surface. Lichtner and Williams also recorded mechanically induced action potentials during trap tightening using a non- polarizable Ag/AgCl electrode connected to the trap surface (presumably the outer surface, although this was not stated), but unfortunately they do not present their recordings. These action potentials were proposed by Lichtner and Williams also to initiate secretion of the acidic fluid out of the trap lobes; however no test was made to see whether this fluid contained digestive enzymes. Both acidic fluid and digestive enzyme secretion are largely controlled by a chemical mechanism (again, not mvolving action potentials), which responds to the presence and concentration of nitrogenous substances derived from they prey (Robins, 1976). Chemical stimulation also induces slow tentacle bending, tropic leaf margin rolling and increased gland secretion in Drosera (Darwin, 1875). It is not known Electricity in plant movements i7 if electric signals mediate gland secretion in Drosera, but applied electric shocks have been claimed to increase secretion (Gardiner, 1885). Common chemoreceptor mechanisms in Dionaea and Drosera have been proposed by Williams (1976), but the processes involved in exciting these receptors are obscure. Few electrical investigations have been made on the stimulation of other carnivorous plant traps. Williams (1976) failed to detect action potentials in the tentacles of DrosophvUuni (a genus closely related to Drosera), which lacks both slow and fast movements, and which does not secrete from its stalked digestive glands in response to either mechanical or chemical stimuli (Darwin, 1875). Mechanical stimulation of the sensory hairs of the underwater bladder trap of Utricularia triggers an astonishingly fast opening of the trap door, leading to capture of the prey. Transient changes in the electrical potentials of bladder wall cells during trap-door opening led Sydenham and Findlay (1973) to suggest that electrical excitation may mediate the movement. Although they were unable to detect an action potential following stimulation of the sensory hairs, their microelectrode measurements were made some distance from the trap door, where the excitation would be unlikely to pass. Unfortunately, insertion of an electrode in or near the sensory hairs triggers movement before recordings can be made. There is even doubt as to whether the triggering process is a purely mechanical one (as favoured by Lloyd, 1942), or involves an excitatory step. For example, bladders cannot be Rred by an electrical stimulus, which would not be expected if the process involved an electrical signal. Also, Merl (1922) failed to reduce the trap sensitivity at the low temperatures that Sydenham and Findlay later reported. Therefore, until a way is found reliably and temporarily to inhibit the opening response of the trap sufficient to insert an electrode into the bladder wall, it is unlikely that the presence of any electrical excitation will be detected. The sensitive three-celled ring trap of the carnivorous fungus Daetylella also responds to mechanical stimuli, rapidly inflating one or more of the trap cells so as to squeeze its prey tight. It is not known how the trap senses or responds so rapidly to its prey, but it may be worth considering the possibility of electrical excitation in future studies, since an electric shock has been reported to induce trap closure (Winkel, cited by MuUcr, 1958). Thus, Dionaea and Drosera are the only carnivorous plants in which both receptor and action potentials have been recorded. Receptor potentials serve to recognize suitable prey - only arthropods of suitable size can sufficiently stimulate the trap to induce the necessary receptor potential required to trigger an action potential. In Drosera the prey can be detained for some time by the sticky gland secretions, and the slower electrical signals reflect the slow rate of tentacle movement. In contrast, Dionaea relies almost entirely on speed alone to capture its victims, and its electrical signals are correspondingly faster than those in Drosera. By storing information on the action potentials in a primitive 'memory', Dionaea is also able to ignore single, chance stimuli, which might otherwise trigger trap closure and so waste considerable metabolic energy.

M I MOSA The compound leaf of Mimosa is sensitive to a variety of stimuli - including electrical, mechanical, chemical, thermal, wounding and light - which induce rapid movements of the leaflets, pinna-raches or petiole. These movements are performed by motor organs termed pulvini, which subtend tbe junctions of the i8 P.J.SIMONS petiole and stem (the primary, or main, pulvini), petiole and pinna-raches (the | secondary pulvini), and pinna rachis and leaflets (the tertiary pulvini, termed ^ pulvinules). Unless otherwise stated. Mimosa will refer to Mimosa pudica, although • many other mimosas are seismonastic to some extent and will presumably share similar electrical behaviour. There has been no report of a receptor potential initiated by a mechanical \ stimulus in Mimosa, but local changes in potential resembling receptor potentials | were detected by Sibaoka (1954) following electrical stimulation of the petiole, | although he did not describe these as receptor potentials. Once a critical threshold I in the intensity of the receptor potential had been reached, an action potential was } fired. Action potentials are also elicited by mechanical, chemical, thermal and | wounding stimuli, and are rapidly conducted to the pulvini, where they initiate | movement (Bose, 1907; Houwink, 1935). Action potentials arriving at the main j pulvini fire a separate pulvinar action potential, which is only conducted through | the lower half of the pulvinus, where sudden loss of turgor in the motor cells is I elicited (Oda and Abe, 1972). j In addition, a wound stimulus also initiates a variation potential, which iss thought to arise from the release of an unidentified hormone (the so-called Ricca's factor) into the transpiration stream (Houwink, 1935). The variation potential triggers an action potential in advance of its own passage, although only the variation potential is capable of passing through the entire pulvinus, regenerating an action potential on the other side of the pulvinus. In this way a wound stimulus can be carried throughout the entire shoot (Houwink, 1935; Sibaoka, 1953). Wound-induced variation and action potentials have also been detected in Mimosa spegazzinii (Umrath, 1928) and M. invisa (Umrath, 1931). The main pulvinus starts to bend 70 to 120 ms following the onset of thef pulvinar action potential (Oda and Abe, 1972), but the exact mechanism coupling the electrical excitation to motor cell movement is poorly understood. Three separate mechanisms have been proposed to account for the seismonastic response in Mimosa, and are briefly described here. Further details on the physiology of Mimosa leaf movements are given by Sibaoka (1969), Sanberg (1976) and Roblin (1979). I (1) Intercellular fiuxes of K+ have been demonstrated in the main pulvini using cytochemical (Toriyama, 1955), X-ray fiuorescence and radioactive label ("^K"'') (Allen, 1969) techniques. Migration of K+ occurs predominantly out of the motor cells in the lower half of the pulvinus, resulting in an efflux (presumably osmotic) of water (Allen, 1969). Campbell ei a/. (1980) have recently used X-ray microanalysis and ion microscopy to examine pulvinules frozen at minus 150°C in order to fix the motor tissues during movement and to avoid redistribution of difTusable ions after the movement has occurred. They found that motor cells contained large amounts of K"*" in their tannin vacuoles and that leaf movements were accompanied by lateral migration of K+ between opposing sides of the pulvinule. Toriyama (1962) found that K+ salts previously located inside vascular bundle parenchyma tissues were expelled into adjoining intercellular spaces and/or sieve tubes, following the passage of an action potential. Since these parenchyma tissues have also been shown to conduct the action potential in the petiole (Sibaoka, 1962), K"*" efflux is likely to be involved in the action potential process. If the pulvinar action potential also involves K+ migration, it is tempting to speculate that generation of the action potential may directly alter the osmotic potential of the motor cells so as to cause the observed changes in turgor. However, the following evidence Electricity in plant movements 19 suggests that the pulvinar action potential is not directly responsible for the ion fluxes associated with the turgor loss, for the following reasons. (a) The flux of ions required to generate a pulvinar action potential is unlikely to be sufficient to account for the magnitude of the turgor loss. This can be best illustrated in the giant internodal cells of Chara, in which the passage of an action potential results in comparable fluxes of K+ and Cl~ (Hope and Walker, 1975). Only a slight loss of turgor accompanies the action potential (a pressure loss of 1 to 3 kPa was detected by Barry, 1970), although the magnitude and duration of the action potential (and hence the associated ionic fluxes) is far greater than in Mimosa action potentials. (b) The K+ efflux continues long after the action potential has passed (Allen, 1969). (c) The refractory period for the movement is over 30 s longer than the refractory period for the pulvinar action potential (Oda and Abe, 1972). Thus, recovery processes for the action potential and for the movement do not completely match up. (d) The secondary pulvinus fails to respond to action potentials that pass through it (Simons, unpublished) even though there is no apparent cyto- logical explanation for this (Fleurat-Lessard and Bonnemain, 1978). Secondary pulvini do, however, perform nyctinastic ('sleep') movements, as do the pulvinules. (2) Numerous small vacuoles have been observed to contract in the excitable cells of the upper half of the pulvinule (Weintraub, 1951). The contractile vacuoles are believed to expel water rapidly from motor-cell protoplasts (Weintraub, 1951), perhaps in a manner akin to the contractile vacuoles in protozoans and many unicellular algae (in which such vacuoles play an important part in osmoregulation). In at least one protozoan. Amoeba proteus, episodes of membrane depolarization are associated with emptying of the contractile vacuoles (Josefsson, 1966) and electrical stimulation of the contractile vacuoles of Paramecium caudatmn acceler- ates their activity (Czarska, 1964), indicating that electrical events may control the activity of these vacuoles. (3) Ca^+-release from the membrane of a tannin vacuole into the central vacuole in the contracting motor cells of the main pulvinus has been claimed to contribute towards movement (Toriyama and JafTe, 1972). Microflbrils in the central vacuole are believed to contract during the collapse of the motor cell (Toriyama and Sato, 1969), and it was suggested that the liberation of Ca^+ into the central vacuole may bind to, and activate, these microflbrils (Toriyama and Jaffe, 1972). Toriyama and JafTe also suggested that Ca2+-migration may facilitate the K+-efllux from the cell. However, the material used by them had been treated with aqueous flxatives during cytochemical staining for calcium, so that artiflcial redistribution of Ca^+ may have occurred, and the signiflcance of their flndings is therefore not conclusive. Light-microscope and ion-microscope studies performed by Campbell, Sitka and Morrison (1979) on unstimulated main pulvini have located relatively large amounts of Ca2+ and K+ in the tannin vacuoles, plastids and cell walls of motor cells located in the outer zone of the cortex of the pulvinus. In contrast, cortical cells surrounding the central vascular strand (the inner zone) generally lack tannin vacuoles but contain a matrix material (microflbrils?) in their centra! vacuoles. Whether cells in this inner zone can be considered motor cells is not clear, but Campbell et al. (1979) found that their Ca^+ and K+ contents were segregated - Ca'"+ was largely conflned to the cell walls, whereas K"*" was mainly detected in the 2O P- J- SIMONS protoplasts. Ion microscopy has not been used yet to detect any changes in ion distribution following stimulation of the main pulvinus. The participation of Ca^'*' in the seismonastic reaction has been suggested by the inhibition of movement by a calcium chelating agent, lO^" M EDTA (Campbell and Thomson, 1977) (although ; Vanden Driessche, 1963, only obtained 'partial insensitivity' to mechanical; stimuli using 10"^ M EDTA), or by the calcium transport inhibitor, 10^''M lanthanum (Campbell and Thomson, 1977). However, in all these ion inhibitor studies it is difficult to known whether the observed effects are due to inhibition of the motor mechanism and/or the electrical excitability (and also whether the •: inhibiting agent is not affecting another, but completely separate, process). Apart a from ion microscopy, movement of Ca''^'*" during the seismonastic reaction might be established in future studies using electron microprobe analysis, aequorin microluminescence (as used by Ridgway and Durham, 1976, with slime moulds), I chlorotetracyclic microRuorescence (as used by Reiss and Herth, 1978, on pollen tubes), or "'•''Ca autoradiography. Much remains to be understood about the physiology of seismonasty in Mimosa and the role that electrical activity plays in it. Future studies might be expected to differentiate between cells of the outer and inner cortex of the main pulvinus since the microscopy studies of Campbell's group indicate that these two types of cell may differ in function, Seismonastic leaf movements also occur in several other members of the Mimosaceae and also of the Caesalpinaceae, Fabaceae and Oxalidaceae, although their sensitivity and range of leaf movement varies considerably. Action potentials associated with seismonasty have been recorded in Neptuniaplena (Umrath, 1928), Biophytum semitivum (Bose, 1907; Umrath, 1928; Guhathakurta and Dutt, 1963) and a species of Biophytum originally identified as B. somnulentum (Sibaoka, 1973), but which was later established as B, dendroides by J. F. Veldkamp (Sibaoka, pers, comm.). In these species pulvinule movements are elicited by action potentials, but the larger pulvini at the base of their pinna-raches remain stationary (although they can perform nyctinastic movements), 1 Sibaoka (1973) also recorded local changes in potential in Biophytum dendroides in response to electrical stimulation, and these can be identified as receptor potentials. The intensity of the receptor potentials reflected the intensity of the stimuli, and when stimulation exceeded a threshold an action potential was triggered. This pattern of excitation is similar to that in Mimosa, indicating that a common receptor mechanism may exist in other seismonastic leaf movements, Seismonasty (and its associated electric signals) has been claimed to protect young leaves from possible insect attack by scaring or shaking off intruders, as in Mimosa pudica (Pickard, 1973), However, it is difficult to imagine an insect being evicted from the leaves of say, Oxalis acetosella, which requires repeated shaking before its leaves begin even slowly to drop (Stiles, 1936). The function of seismonasty in some species is therefore curious. I

LEAF MOVHMHNTS REGULATED BY LIGHT AND ENDOGENOUS RHYTHMS Many leaf movements result from co-ordinated changes in motor cell turgor on opposite sides of a pulvinus. Changes in turgor in Albizzia julibrissin (Satter and Galston, 1971), Sanianea saman (Satter et al,, 1974) and Trifolium repens (Scott, GuUine and Robinson, 1977) are caused by the osmotic passage of water following large fluxes of K+ in or out of pulvinar motor cells. Similar K+-directed water Electricity in plant movements 21 movement has been found during the tropic sun-tracking and cupping movements oi Lupinus arizonicus (Wainwright, 1977). The displacement of charge caused by the movement of K+ is largely balanced by Cl'-migration in the same direction in Albizzia (Schrempf, Satter and Galston, 1976) and in Samanea (Satter et al., 1977). Although ion pumping is thought to account for the K+-flux during rhythmic and phytochrome-mediated nyctinastic closure in Albizzia (Satter and Galston, 1973), the forces regulating the ion pumps and the transport of K+ during other phases of leaflet movement, are obscure. Lanthanum inhibits nyctinastic leaflet closure of Mimosa leaves (Campbell and Thomson, 1977), sun-tracking and cupping leaf movements in Lupinus (Wainwright, 1977) and nyctinastic and endogenous rhythm (but not phytochrome-promoted) leaflet movements of Albizzia (Campbell, 1979). Thus, Ca'^+ transport may play an important role in many leaf movements, but movement of Ca'''+ was not detected during movement of Albizzia pulvini using electron microprobe analysis (Satter, Marinoft and Galston, 1970). The older literature indicates that changes in potential measured on the outside of a pulvinus precede the movement of the pulvinus (for example, Yamaguti, 1932). It has since been demonstrated that intracellular potentials of motor cells on opposite sides of the pulvinus of Samanea (Racusen and Satter, 1975) and Trifolium (Scott et al., 1977) oscillate rhythmically during leaf movements regulated by endogenous rhythms. The change in electrical potential of Samanea flexor motor cells (those that contract during leaflet closure) matches the leaflet movement, increasing in polarity during leaflet opening and depolarizing during closure (Racusen and Satter, 1975). Plexor motor cells (those that expand during leaflet closure) also respond rapidly to switches between white light and darkness, and between red and far red light (Racusen and Satter, 1975). The electrical potential of the extensor motor cells (those that expand during leaflet opening) are about 8 h out of phase with the rhythm in flexor cell potential, increasing in potential just before opening is detectable. Changes in electrical potential may help to drive K+ ions into or out of the motor cells, but the extent of electrical control over K+ movement is not certain. Studies on the photochrome-mediated closure of Samanea leaflets indicate that K+ flux is much slower than the change in potential (Racusen and Satter, 1975), suggesting that other events, possibly electrical, precede and cause the K+ flux. It would be interesting to see whether leaflet movements can be altered in any way using electrical methods, such as voltage clamping, in order to test for the significance of the electrical potential of the motor cells in pulvinar movements. One possible mechanism for driving rhythmic changes in electrical potential in pulvini has been proposed by Racusen and Galston (1977), who found that depolarization of motor cells by applied sucrose was strongly dependent on the pH of the bathing medium. The magnitude of the sucrose-induced depolarization varied in a circadian manner, and it was suggested that sucrose may be co- transported across motor cell plasmalemmae with protons. The resulting change in membrane potential may contribute towards the rhythmic changes in pulvinar potential, but this needs further verification. The co-ordination of ionic fluxes throughout the pulvinus is also unclear. For instance, stimulation of Samanea pulvini with light fails to elicit action potentials (Racusen and Satter, 1975), but in Mitnosa stimulation of a localized area of a pulvinule with a narrow beam of light induces a bending response throughout the entire pulvinule (Watanabe and Sibaoka, 1973). An external stimulus can therefore 22 P. J. SIMONS 1 be conducted through the motor tissues of the pulvinus, but the nature of this conduction is unknown. Similarly, the mechanism co-ordinating complementary changes in ion fluxes on opposite sides of a pulvinus is also unknown, although recent evidence using X-ray microanalysis of frozen pulvini of Samanea has shown that the apoplast of the motor tissue is an important pathway for the lateral migration of K"*" and Cl" across the pulvinus (Satter, Campbell and Garber, 1980).

TANADA EFFECT I Tanada (1968) first described phytochrome-mediated adhesion of mung bean root tips to charged glass surfaces. He found that the red-light-induced adherence of root tips required a specific composition of the solution bathing the root tips, and a phosphate pretreatment of a glass beaker. The adhesion of the root tips is associated with a sudden change in surface charge of the epidermal cells of the root cap (Racusen and Etherton, 1975), although measurements of the electrical potential on the root surface have shown only small (about 1 mV) changes in potential (Jaffc, 1968), In addition, phytochrome-mediated adhesion is also associated with an increase in acetylcholine content (Jaffe, 1970), H+ release into the bathing solution, increased Oj consumption and increased ATP consumption (Yunghans and JafTe, 1972), These findings suggest a causal sequence of events which may have some features in common with nerve excitation, although there is no conclusive evidence to support this notion. It would be interesting to see whether the light-induced adhesion of Spirogyra filamentous cells to glass, as reported by Nagata (1977), is also mediated by electrical events similar to those in mung bean root tips. In this connection Fujii. Shimmen and Tazawa (1978) have reported light-induced changes in membrane potential in Spirogyra filamentous cells, ys

V LOUAL PARTS ^ Certain members of the following families perform rapid and reversible floralj movements in response to mechanical stimulation, in order to promote pollination:} stamen filament movements in the Berberidaceae, Cactaceae, Cistaceae, Compo- sitae, Cucurbitaceae, Malvaceae, Portulaceae, and Tiliaceae; stigma movements in the Bignoniaceae, Lentibulaceae, Liliaceae, Martyniaceaeand Scrophulariaceae; style movements in the Compositae and Mimulaceae; fused stamen/style move- ments in the Stylidiaceae; corolla tube contraction in the Gentianaceae; labcllum movements in the Orchidaceae, I Few electrical investigations have been made on these motor responses, but in" those cases that have been investigated, electrical changes have been found to precede movement. The contractile stamen filaments of Sparmannia africana and Berberis vulgaris generate a localized action potential at their bases when they are mechanically or electrically stimulated (Bunning, 1934), The bending movement closely follows the passage of an action potential, and as movement has not been observed without recording a preceding action potential, it seems reasonable to assume that the action potential is a requirement for movement. Repeated action potentials do not elicit further movement until the motor tissue has completed their own refractory period (Umrath, 1937), There is also a significant lag phase between the passage of an action potential and movement in both Sparmannia (Bunning, 1934) and Berberis (Bunning, 1959), Thus the mechanism coupling the action Electricity in plant movements 23 potential to movement is not likely to be a simple one and is not yet understood. Electrical sbocks also induce stamen contraction in Centaurea (Burdon-Sanderson, 1 911; Stern, 1924) and Mahonia (Millet and Tbibert, 1976), and action potentials associated witb stamen bending bave recently been detected in Mahonia (Millet, pers. comm.). Sinyukbin and Britikov (1967a, b) have also measured localized action potentials following mecbanical stimulation of tbe stigmatic lobes of Incarvillea, prior to the lobes shutting together. Tbe mechanism controlling the changes in rnotor cell turgor of a motile floral organ bas recently been investigated by Findlay and Pallagby (1978) in the extraordinarily rapid bending of tbe fused stylar-stamen column of Stylidium graminifolium. Stimulation of the region around the base of the column causes a rapid loss of K+ and Cl^ from the motor cells, sufficient to account for the corresponding water loss. There has been no published account of electrical activity associated with this movemetit, but electrical shocks also initiate movement (Kabasch, 1861; Findlay and Findlay, 1975), although this alone cannot be taken as proof of mediation by endogenous electrical signals. On the basis of investigations on Berberis, Sparmannia, Mahonia and Incarvillea it is likely that many, if not all, seismonastic floral movements are mediated by electrical impulses, but so far this prospect has not been fully explored.

STOMATA Active uptake of K"*" into guard cells is now accepted as providing the necessary osmoticum required for guard cell expansion (and hence opening of the stomatal pore), while loss of K+ accounts for the loss of guard cell turgor (pore closure). It has been suggested that the movement of K+ is balanced by a concomitant transport of anions (particularly Cl^) in tbe same direction and of cations in the opposite direction, or synthesis of malate anions in the guard cells (see Hsiao, 1976; Raschke, 1976). However, the mechanism driving the transmembrane K+ flux is far from clear. Relatively few electrophysiological studies have been carried out on stomata, probably because tbe tbickness of the cell wall presents a formidable obstacle to successful microelectrode impalement. Penny and Bowling (1974) found no difference in electrical potential between guard cells, subsidiary cells and sur- rounding epidermal cells during equilibrium states, and Pallagby (1968) could not detect changes in guard cell potential with changes in light intensity or in the presence of bicarbonate ions. However, substantial and rapid changes in guard cell membrane potential following switches between light and darkness have been reported for Tradescantia (Gunar, Zlotnikova and Panichkin, 1975) and Allium (Zeiger et al., 1977; Moody and Zeiger, 1978). Zeiger's group considered that the rapidity of the electrical response (having no detectable latency period) was probably caused by an electrogenic proton pump at the plasmalemma, although the mechanism triggering this pump is not known. The role of the membrane potential in guard cell movement is still highly speculative. Zeiger et al. (1977) and Dittrich, Mayer and Meusel (1979) consider that the increase in membrane potential can drive the movement of potassium ions into tbe guard cells. However, Rascbke and Humble (1973) found tbat increasing tbe level of K+ in tbe solution batbing epidermal strips of Vicia increased the amount of H+ released. Fusicoccin-induced acceleration of K+ uptake is also correlated with an increase in H+ expulsion (Raschke, 1976); these results suggest 24 p. J. SIMONS * that K+ exchanges for H+ at the guard cell plasmalemma instead of (or in addition to) the proposed proton pumping. Nevertheless, rapid electrical responses at the guard cell plasmalemma may be involved with transducing the environmental signals which trigger the subsequent ion fluxes. Other electrical investigations of stomata have relied on application of electrical shocks to the leaf epidermis, Darwin and Action (1894) were the first to report the induction of stomatal closure by electrical shocks in Allium, and similar results . have been obtained by other workers (Koketsu, 1923; Umrath, 1959; Pallaghy, 1968), but it is not known whether these shocks mimic natural processes involved in guard cell movement, or whether they introduce artificial excitation. The nature of the communication between stomatal complexes has been poorly studied, A local reduction in light intensity on one part of a leaf can induce a decrease in stomatal aperture in another part, even when separated by an area under , high light intensity (Heath and Russell, 1954), The intercellular spaces of the leaves I were continuously swept with CO.^-free air so as to eliminate the possibility of stimulus transmission by reductions in COj levels, and Heath and Russell • proposed that either a chemical or electrical messenger could account for this * phenomenon. Unfortunately, the speed of stimulus conduction was not reported, f but propagating transient photo-induced changes in potential have been detected - in many photosynthetic tissues and are able to travel through darkened or non-green I cells (for example. Waller, 1925; Brinkmann and Luttge, 1974), It would be - interesting to know whether these photo-induced electrical fluctuations could ' induce changes in guard cell potential, possibly leading to stomatal pore i movement, ! Woundmg leaves results in the conduction of a stimulus which induces partial ' stomatal closure (Scarth, 1932; Williams, 1948), It is strange, though, that the slow conduction of the stimulus (about 0-1 mm s"') measured by Williams in Pelargonium zonale did not match the transport of Ricca's factor (about 3 to 4 mm s"') that Van Sambeek and Pickard (1976b) found was released when leaves of several species of higher plants were wounded, and which led to the propagation of both variation and action potentials. This discrepancy may be partly explained by the lag period (about 1 to 5 min) between receipt of the wound-induced excitation and the start of stomatal movement, that Williams recorded using a pressure sensing porometer. Van Sambeek and Pickard (1976c) also found that arrival of a variation potential at a leaf distant from the one wounded was soon followed by increased respiration, decreased photosynthetic COj uptake and decreased transpiration. Whether the decrease in transpiration was directly caused by the variation potential or by the mcrease m ambient CO,^ concentration, was not ascertained. Since applications of Ricca's factor induces stomatal closure in a variety of species (Umrath, 1966) and depolarization of individual leaf cells (Cheeseman and Pickard, 1977), electrical events may play a part in the stomatal response to wounding. There have also been reports of seismonastic reactions in stomata (for example, Knight, 1916), and although this response may be partly due to bursts of COof released as a result of mechanical stimulation (Williams, 1949), studies using different wind velocities have indicated that stomata are at least partially sensitive to mechanical stimuli (Martin and Clements, 1935), Possible electrical activity associated with the stomatal response to mechanical stimulation has not been investigated so far. Electricity in plant movements 25

CYTOPLASMIC STREAMING The relationship between cytoplasmic streaming and associated electrical activity is still very confused. Studies of cytoplasmic streaming in plants have concentrated on slime moulds and the internodal cells of the characean algae; the mechanics and biochemistry of the motive forces that drive streaming in these organisms have been recently reviewed (R. D. Allen and N. S. Allen, 1978; N. S. Allen and R.D.Allen, 1978; Vanden Driessche, 1979). In slime mould plasmodia cyto- plasmic streaming back and forth along protoplasmic strands is responsible for the slow movement of the plasmodia, and occurs as a result of strands of endoplasm being squeezed by alternating contractions of Ca'-^-sensitive actomyosin fibres at the two ends of the strands (Nachmias and Asch, 1974; Fleischer and Wohlfarth- Botterman, 1975). Cytoplasmic streaming of plasmodia is highly sensitive to applied calcium ions and ceases at concentrations < 10"" M (Hatano, 1970). The membrane potentials of the two ends of the endoplasmie strand of Physarum polycephalum have been recorded using external electrodes and found to oscillate •with approximately the same frequency as the streaming rhythm (Iwamura, 1949; Kamiya and Abe, 1950; Ridgway and Durham, 1976). In addition, Ridgway and Durham (1976) detected oscillations in endogenous Ca'^'^ concentrations using an aequorin-luminescence technique, which matched the frequency of the electrical rhythm. As maximum Ca^"*" concentrations precede maximum recordings of hyperpolarizations in the amoeba Chaos chaos, Nuccitelli, Poo and Jaffe (1977) suggested that current enters during the rise in Ca^+ due to a local Ca^"*" leak at the plasmalemma, and that a similar sequence of events may occur in Physarum. Cobbold (1980) though, has recently found from calibrated aequorin measurements on tbe amoeba Chaos carolinense that cytoplasmic movement occurs without changes in the free Ca'^^ concentration of the cytoplasm, and suggested that previous proposals of calcium control of motility in other types of cells based on aequorin studies are now in doubt. However, a causal link between electrical events and Ca''^"'' transport is far from certain. Kamiya and Abe (1950) found that the electrical rhythm of Physarum plasmodia lagged behind the streaming rhythm. The electrical oscillations also remained unaltered whether the cytoplasm was allowed to flow freely, brought to a standstill, or even reversed by means of pressure applied across the ends of the strand. This evidence suggested that streaming and the electrical potential were independent of each other, but Kishimoto (1958) found that the electrical rhythm could be affected by applied pressures and argued that Kamiya and Abe's results were largely invalidated. Nevertheless, Kamiya and Abe also found that a potential difference of between 0 01 to 4 V applied for a short period across a plasmodium did not produce any noticeable effects on the pattern of streaming. In addition, Iwamura (1949) found that the electrical rhythm was disrupted by applying 1 M NaOH or HCl, but streaming was unaffected. These two observations seem to indicate that the electrical potential does not directly control protoplasmic streaming. Whether streaming causes the rhythm in potential is not known for slime moulds, but comparable oscillations in electrical activity continue in Nitella when streaming is inhibited by application of c^'tochalasin B (Ogata and Kishimoto, 1976). If this is also true of slime moulds, then streaming and membrane potential are likely to be governed by a common, but separate mechanism. Further evidence against electrical mediation of streaming in slime moulds comes from studies on the chemotaxis of plasmodia. Once a critical threshold of 26 p. J. SIMONS I a particular chemical has been exceeded, a receptor potential is triggered and is shortly followed by a change in the rate and direction of streaming, resulting in a chemotactic response (either towards or away from the chemical stimulus) (Ueda et al., 1975), It is puzzling, though, that no matter whether the chemical stimulus induces repulsion or attraction of the plasmodium, it always triggers a depolarizing receptor potential. In addition, increasing the concentration of the chemical stimulus beyond the receptor potential threshold increases the magnitude of the potential, but not the magnitude of the streaming response (Ueda et al,, 1975), The chemically induced change in membrane potential may therefore serve no function in chemotaxis, and may only be the insignificant byproduct of another process, For example, the co-transport of a sugar molecule with a proton may depolarize the plasmalemma (as has been shown in another fungus, Neurospora - see Slayman and Slayman, 1974), In contrast, evidence in favour of a causal sequence of events linking changes in membrane potential to streaming has been found from studies of streaming in the giant characean internodal cells. Electrical or mechanical stimuli can trigger an action potential which is then conducted along the plasmalemma, and sometimes passing beyond into neighbouring cells. The start of an action potential always precedes the stoppage of streaming, which usually occurs at the peak of the action potential (Tazawa and Kishimoto, 1968), Pickard (1969), though, was able to induce stoppage of streaming in Nitella without triggering an action potential, by clamping the voltage of the vacuolar potential and steadily depolarizing the potential over relatively long (1000 s) periods. Depolarization of the vacuole beyond a critical voltage induced an abrupt halt in streaming, and Pickard reasoned that changes in the plasmalemma conductance to ions accompanying the passage of an action potential was responsible for the stoppage in undamped cells, Barry (1968) had previously reached a similar conclusion, as bathing Nitella cells in Ca^+, Sr^+, Mg^"*" and Ba^"*" chlorides resulted in increased action potentials in the above order, but only action potentials triggered in Ca^^ or Sr'^+ media were capable of stopping streaming. So it appears that the action potential in some way opens up a passage in the plasmalemma to allow entry of Ca'^^, where it somehow reacts with an actomyosin contraction system (see N. S, Allen and R, D. Allen, 1978- Vanden Driessche, 1978), | How membrane permeability to specific ions is suddenly increased at a critical threshold of electrical excitation is not known, but is reminiscent of smooth muscle contraction in which actomyosin contraction is also triggered by a critical membrane voltage in an all-or-nothing fashion. However, contractions in both muscle cells and in slime moulds are activated by Ca2+ concentrations > 10"' M, whereas similar (Za}+ concentrations in characean cells inhibit streaming (and also autonomous chloroplast movements) (Hayama, Shimmen and Tazawa, 1979), Cytoplasmic streaming also continues unabated in cells treated with the Ca^^- chelating agent EGTA, even following excitation (Tazawa, Kikuyama and Shim- men, 1976), So it appears that a critical Ca^+ concentration can turn off the motive force m characean cytoplasmic streaming and that electrical events are unlikely to play a part in internally regulated streaming, but are involved in the perception ot at least some external stimuli. Fibrils similar to those responsible for streaming in the characean cells and slime mould plasmodia have also been observed in higher plants (O'Brien and McCulley, 1970), and which may also contain actin (see Hepler and Palevitz 1974) Little IS known about the control of streaming in higher plants, but electric shocks have Electricity in plant movements 27 been found to stop streaming in the leaf cells of Elodea and young staminal hairs of Tradescantia (see Pfeffer, 1906; Kamiya, 1959). It is not known whether endogenous electrical activity also affects streaming, but the controlling mechanism islikely to be similar to that in characean cells. For example, Fordeand Steer (1976) demonstrated that streaming in Elodea canadensis was inhibited by the same agents that inhibit characean streaming, namely cytochalasin B (an inhibitor of actin activity), or 10^^ M Ca'^"*", whilst 10"'' M EDTA or Mg'^+ (concentration not given) stimulated streaming (as in Chara — see Williamson, 1975).

TACTIC MOVEMENTS The complete mechanisms of ciliary and Hagellar beating is not understood but Naitoh and Eckert (1974) have established that both orientation and frequency of ciliary beating in the protozoan Paraniecium are controlled by electrical impulses carried along the plasmaleinma. Adverse external stimuli (such as mechanical knocks) cause increased membrane permeability in the region stimulated. As a result, an ionic current is generated that creates a depolarizing receptor potential, which is then conducted electrotonically along the membrane (without the need for an all-or-nothing action potential). Ca'^^-specific ion channels in the membrane are sensitive to changes in the membrane potential and depolarization results in a rapid and large influx of Ca''^"''. The subsequent rise in intracellular concentration activates an ATP-dependent motor process bringing about reversal of ciliary beating, which results in the tactic movement of the organism away from the stimulus. The rise in internal Ca^+ concentration finally induces a transient increase in K"*" conductance, resulting in a return of the membrane potential to its original 'resting' level, so terminating the Ca'^^-mediated ciliary reversal. A second mechanism is involved in controlling the frequency of ciliary beating in Paramecium. Hyperpolarizing currents, or depolarizing currents greater than several mV, increase the beating frequency, whereas milder depolarizing currents inactivate or slow cilia. The beating frequency is closely related to intracellular Ca^+ (and possibly Mg^+) concentrations, so that the frequency may also be coupled to the membrane potential through Ca''^+ (and possibly Mg^"*") fluxes (Eckert, Naitoh and Machemer, 1976). Similar electrical events may also control flagellar movements and recent evidence from a number of studies on unicellular plant cells supports this view, Ca^"*" has been shown to play an important part in the phototaxis of Chlamydomonas, by acting as the agent coupling flagellar reversal to photostimulation in a manner akin to the role of calcium in coupling ciliary reversal to membrane stimulation in Paramecium (Schmidt and Eckert, 1976). Applied electric fields inhibit the phototactic response of Chlamydomonas (Marbach and Mayer, 1971), and flagellar movements in Euglena in an electric field mimics the photophotic response of the flagellum (Bancroft, 1913). Injection of a hyperpolarizing current into Euglena results in a reduced or inactivated flagellar activity, whereas injection of a depolarizing current increases flagellar beating (Nichols and Rikmenspoel, 1977). Similarly, an applied hyperpolarizing current inhibits the fiagellar activity of bull spermatozoa (O'Day, Rikmenspoel and Lindemann, 1976). In contrast, an injection of hyperpolarizing current into Paramecium results in increased ciliary beating in the forward swimming position (Eckert and Machemer, 1975). More direct evidence for electrical control of flagellar activity comes from extracellular electrode recordings made on Haematococcus pluvialis by Litvin, 28 P- J- SIMONS f Sineschekov and Sineschekov (1978). These workers found that many of the factors that affect phototaxis (such as duration and intensity of a light stimulus) induced changes in membrane potential. Furthermore, the action spectrum of the photo-induced changes in potential resemble the action spectrum of the phototactic response. These findings indicate that electrical events are probably involved in the control of Ragellar activity. It would be interesting to see if the membrane potentials of unicellular plant cells respond to chemical and mechanical stimuli in the same electrical fashion as in Paramecium (Naitoh and Eckert, 1969; Eckert, 1972), so allowing a direct comparison of the mechanisms controlling Ragellar and ciliary activity in these organisms. Tactic movements are also performed by organisms lacking cilia and flagella. Slime mould taxis has already been dealt with. The filamentous blue-green alga Phormidium uncinatum performs phototactic movements - a switch from light to darkness generally causes the filaments to reverse their previous path of movement (Hader, 1978b). Both intra- and extracellular electrode recordings have detected photo-induced changes in electrical potential, with lag phases roughly corres- ponding to the lag phase of the phototactic reaction (Hader, 1978a). In addition, the phototactic reaction is inhibited by applied electric fields (Hader, 1977). These findings indicate that electrical events are involved in the phototactic movement of the filaments, but unfortunately it is not clear what part they play. They may, for instance, involve perception of the stimulus, or conduction of the excitation to the motor region, or be a part of the motor reaction itself.

C O N C L U D I N G R E M A R K S * Many of the events leading to seismonastic movements are likely to be shared by a variety of higher plants - such as Dionaea, Aldrovanda, Mimosa, Biophytum and Stylidium - and probably occur in many other species as well (Fig. 2). The electrical characteristics of this sequence are surprisingly similar to those of animal ^ nervous systems (Fig. 3), albeit on a simpler and slower scale (Table 2). In both animals and plants a receptor potential perceives a stimulus, firing an action . potential after sufficient stimulation which then carries the excitation signal to the | organs/tissues concerned with the response. It appears from work on the Dionaea '•- 'memory', and the behaviour of Mimosa pulvini, that the mechanism coupling ' the action potential to changes in motor cell turgor in plants is not simply due to the ionic movements accompanying the action potential. Rather, it would appear that some other event is involved in triggering the motor cell response, perhaps the release of an excitatory chemical such as a neurotransmitter, or an 'excitatory ion' such as Ca'^+. Neurotransmitter-like substances are present in the pulvini of Mimosa (Trabucchi, 1948) and several other plant species (Applewhite, 1973), but the literature on this subject is too large to cover here. What is not certain is whether these chemicals play any part in plant movements, and combined biochemical and electrophysiological investigations will be needed to test this possibility. There is, however, a danger in drawing too many parallels with excitation in animals, as both ACh and AChE have been detected in relatively high concentrations in non-motile plant tissues, particularly actively growing tissues (see Fluck and Jaffe, 1976), in which their significance is also obscure. The role of Ricca's factor in plant movements is also of considerable interest, as this agent or group of agents has not only been shown to generate the variation potentials which lead to pulvinar movements in Mimosa, but also to induce Electricity in plant movements

Physical stimulus

Receptor potential

Action lotL'iitial

Coupling ot" action potential to increased motor cell permeability to K*,C\-

Passage of water into or out of motor cell

> Change in motor cell turgor

t Movement ol' motor organ

Fig, 2, A generalized sequence of e\ ent,s leading to seismonastic tnnovemetits in higher plants. The scheme represents common features from a variety of species, and does not include unusual events peculiar to a particular species, Stomatal closure in a variety of species (Umrath, 1966) and stamen bending in Berberis (Umrath, 1943). Considerable progress has now been made in charac- terizing Ricca's factor obtained from Mimosa, and which has been found to include gentisic acid among other agents (Schildknecht, 1978), so that it should now be possible to investigate its physiological properties more thoroughly. K+ and Cl" fluxes are apparently common to all reversible turgor-mediated movements that are sensitive to mechanical stimulation. Circumstantial evidence has indicated that Ca^+ may also participate in these movements, perhaps by altering membrane properties so as to trigger the large fluxes of K"*" and Cl", The driving forces for these ionic movements are poorly understood, although it is known that considerable ATP hydrolysis accompanies many turgor-mediated movements, such as leaflet opening in Albizzia (Satter and Galston, 1973), Dionaea trap closure (Jaffe, 1973) and Mimosa pulvinus bending (Lyubimova et al,, 1964). Electrogenic ion pump activity has been implicated in these movements (as in rhythmic and nyctinastic leaf movements — see Galston and Satter, 1976), but changes in membrane potential are likely to drive at least some of the ionic transport, perhaps by regulating the activity of the ion pumps, as has been indicated by studies on Nitella (Spanswick, 1972) and Halicyctus (Graves and Gutknecht, 1977), Control of ionic movement across the plasmalemma by electrical potential is believed to play an important part in the transmission of action 3° P. J. SIMONS

Physical stimulus

Receptor potential

Action potential

Chemical transmission across synapses hy neurotransmiller

Action potential re-propagatcj

Fig. 3, Sequence of salient events occurring during sensory excitation in higher animals. potentials in excitable cells in animals - a phenomenon known as 'voltage-gating' (see Armstrong, 1975; Ulbricht, 1977). The conflicting evidence concerning electrical control of Ca^"'"-mediated cyto- plasmic streaming activity in slime moulds and characean algae indicates that too close an analogy with the sequence of events leading to actomyosin contraction in animal muscles cannot be made (see, for example, Wilkie, 1968). Yet understanding the mechanisms controlling actin or actomyosin contraction in plant cells has important implications, since actin has been linked witb chromosome movement (and hence cell division) in Haemanthus kaiherinae (Forer and Jackson, 1976), as well as the movement of chloroplasts (Wagner, Haupt and Laux, 1972; Schonbohm, 1973) and possibly even vesicles (see Quatrano, 1978). The control of ciliary beating in Paramecium by electrical activity lends strong support to similar control of unicellular plant ciliary and flagellar activity. If this is so, then electrically regulated movements will be widespread throughout the plant kingdom, since male gametes of all but the gymnosperms and angiosperms are propelled by either cilia or flagellae. In addition, electrical impulses also trigger the bioluminescence flash and tentacle contraction in the dinoflagellate Noctiluca militaris in response to mechanical stimuli (Eckert, 1965a, b; Eckert and Sibaoka, 1967; Sibaoka and Eckert, 1967). In contrast to the vascular cryptogams, rapid and reversible movements are relatively rare in higher plants, and as a result the electrophysiology of theif movements has tended to be of only minority interest. Yet, despite the speculative nature of much of this review, it is clear that electrical activity plays an important part in many plant movements and has helped to prompt investigations into electrical activity in other aspects of plant physiology. Action potentials in higher Electricity in plant movements 31 Table 2 Conduction velocities of electrical excitations mediating reversible plant movements Note that considerable variation in these velocities will occur due to temperature, humidity, intensity of stimulus and age of specimens, and that data given here are meant to indicate the order of velocities involved. Two examples from excitation in animals have been included for comparison

Excitation velocity Species Organ Stimulus (mm s"') Key reference j4tdrovaiida vesiculosa Trap lobe Electrical 40-120 Sibaoka, 1980 Biophytum dendroides Pinna-rachis Electrical 3-5 Guhathakurta & Dutt, 1963 Chara braiitiii Internode Electrical 10-18 Sibaoka, 1958 Dionaea muscipula Trap lobe Electrical 60-170 Sibaoka, 1966 Drosera ititerniedw Tentacle stalk Electrical c. 10 Williams & Pickard, 1972 Incarvillea grandiflora Stigrnatic lobe Mechanical 18 Sinyukhin & Britikov, 1965a, b IMimosa pudica Petiole Electrical 6-44 Sibaoka, 1950b M. pudica Stem Electrical 40-50 Umrath, 1937 M. pudica Stem Wound (cut 20-30 Sibaoka, 1950a or burn) M. pudica Stem Severe wound 100-400 Sibaoka, 1954 (cut or bum) M. invisa Pinna-rachis Wound (cut) 7-5 Umrath, 1959 Neptunia plena Petiole Electrical 9-12 Umrath, 1928 Nitella flexilis Internode Electrical 40-48 Sibaoka, 1958 A • 1 Animals Column Electrical 121 146 Parker, 1918 Anemone Felix (cat) Dorsal spino- Electrical S.S00O Grundrest& Campbell, 1942 cerebellar 165 000 tract

plants have been admirably reviewed by Pickard (1973), but the possible functions of electric signals in systems lacking motile properties is largely a mystery. On the other hand, electrical activity is believed to play an important part in cell development (see Jaffe and Nuccitelli, 1977). Clearly, many perplexing problems in plant electrophysiology remain to be solved.

NOTE ADDED IN PROOF Literature covering a wide variety of reversible and irreversible plant movements has recently been reviewed in Encyclopedia of Plant Physiology, New Series, vol. 7, Plant Movements (ed. by W. Haupt and M. E. Feinleib), Springer-Verlag, Berlin, Heidelberg and New York (1979).

ACKNOWLEDGEMENTS I am grateful to Dr M. J. Earnshaw for critical reading of the manuscript, Dr M. Spiro for technical advice and help in translation, Mr N. Billingham for advice on animal biochemistry, and Mr and Mrs A. M. Setchell for technical assistance. 32 p. J. SIMONS |

REFERENCES AFFOi.THn, J. M. & Oi.ivo, R, F. (1975). Action potentials in Venus's flytraps; long-term observations following capture of the prey. American Midland Naturalist, 93, 443-445, Ai.BERY, W, J, (1975), Electrode Kinetics. Oxford Chemistry Series. Oxford University Press, London, Ai.i.EN, N, S, & ALLEN, R. D. (1978). Cytoplasmic streaming in green plants. Annual Review of Biophysics and Bioengineering, 7, 497-526. ALLEN, R. D. (1969). Mechanism of the seismonastic reaction in Mimosa pudica. Plani Physiology, 44, 1101-1107, ALLEN, R, D, & ALLEN, N. S. (1978), Cytoplasmic streaming in amoeboid movement. Annual Review of Biophysics and Bioengineering, 7, 469^95. APPLEWHITE, P. B. (1973). Serotonin and norepinephrine in plant tissues, Phytochemistry, 12, 191-192, ARMSTRONG, C. M.(1975). I onie pores, gates, and gating currents, Quarfcr/yWcDjVtoo/Bi'op/ij'.w'cs, 7,179-210. ASHIDA, J. (1934), Studies on the leaf movement of Aldrovanda vesiculosa L, II. Process and mechanism of the movement. Memoirs oj the College of Science, Kyoto Imperial University, Series B, 9, 141-244. ASHIDA, 1. (1935). Studies on the leaf movement of Aldrovanda vesiculosa L. 11, Effects of mechanical, electrical, thermal, osmotic and chemical influences. Memoirs of the College of Science, Kyoto Imperial University, Series B, 11, 55-113, ASHIDA, 1. (1937), Studies on the leaf movement of Aldrovanda vesiculosa L. III. Reaction time in relation to temperature. Botanical Magazine, Tokyo, 51, 505-513, BANCROFT, F. W. (1913). Heliotropism, differential sensibility, and galvanotropism in Euglena. Jowrna/o/ Experimental Zoology, 15, 383-428, BARRY, P. H, (1970). Volume flows and pressure changes during an action potential in cells of Chara australis. 1, Experimental results, J'ourHa/ of Membrane Biology, 3, 313-334. BARRY, W. H. (1968). Coupling of excitation and cessation of cyclosis in Nitella: role of divalent cations. Journal of Cellular Physiology, 72, 153-159. BENOLKF.N, R. M. & JACOHSON, S. L. (1970), Response properties of a sensory hair excised from Venus' fly-trap. Journal of General Physiology, 56, 64—82. BosE, J, C, (1907), Comparative Eleclrophysiology. Longman, Clreen and Co., London, ,J . BOSE, J, C. (1913), Researches on Irritability of Plants. Longman, Green and Co., London, "^ BRINKMANN, E. & LOTTCU, U. (1974). Light-triggered membrane potential oscillations and electric coupling in variegated photosynthetic mutants of Oenothera. Planta (Berlin), 119, 47-57, BROWN, W. H. (1916), The mechanism of movement and duration of effect of stimulation in leaves of Dionaea. American Journal of Botany, 3, 68-90, ; BuNNiNO, E. (1934). Elektrische Potentialanderungen an seismonastisch gereitzen Staubfaden. Planta (Berlin), 22, 251-268. BuNNiNO, E. (1959), Die seismonastisch Reaktionen. In; Handbuch der Pfianzenphysiologie, vol. 1, Part 1 ' (Ed. by W, Ruhland), pp, 184-242. Springer, Berlin. \ BuRtJON-SAN't:>riRSON, J, (1873), Note on the electrical phenomena which accompany stimulation of leaf of : Dionaea muscipula. Proceedings of the Philosophical Transactions of the Royal Society of London, 21, • 495-496, ' t BURDON-SANDERSON, J, (1911). The excitability of plants. In; Sir John Burdon-Sanderson, a Memoir . . .with i a selection from his papers (Ed. by G. Burdon-Sanderson), pp, 187-198, Clarendon Press, Oxford, BuRDON-SANDIiR,soN, 1. & PAGE, F, J. M, (1876). On the mechanical effects and on the electrical disturbance consequent on excitation of leaf of Dionaea muscipula. Proceedings of the Philosophical Transactions of the Royal Society of London, 25, 411-434. ia.l,, N. A. (1979). Tbe effects of lanthanum on the leaf movements of Albizzia. Plant Physiology, Supplement, 63, 125. H'LI., N. A.,GAiinER,R, C,,SITKA, K, M,, SATTF.R, R, L.& MORRISON, G, H.(198()), Ion redistribution in the tertiary pulvinus of Mimosa during rapid leaf movement. Plant Physiology, Supplement, 65, 163, CAMPDELL, N. A, & THOMSON, W, W. (1977). The effects of lanthanum and ethylenediaminetetraacetate on the leaf movements of Mimosa. Plant Physiology, 60, 635 639. CAMPtiELL, N. A., SiTKA, K, M, & MoRRKSON, G, H. (1979), Calcium and potassium in tbe motor organ of the sensitive plant; localization by ion microscopy. Science (Washington), 204, 185-187, CHEESEMAN, 1. M. & PICKARD, B, G, (1977), Depolarization of cell membranes in leaves of Lycopersicon by extract containing Ricca's factor. Candian Journal of Botany, 55, 511 519. CoBHOLD, P, H, (1980). Cytoplasmic free calcium and amoeboid movement, Nature (London), 285, 441-446, CzARSKA, L, (1964). Role of tbe K"^ and Ca'''^ ions in the exeitability of a protozoan cell. Chemical and electrical stimulation of contractile vacuoles. Acta Protozootogica, 2, 287 296. DARWIN, C. (1875), Insectivorous Plants. J. Murray, London. DARWIN, F, & ACTON, E, H, (1894), Practical Physiology of Plants. Cambridge University Press, London. DiANNFi.iDis, T, & UMRATH, K. (1953), Uber das elektrisehe Potential und uber den Erregungsvorgang hei dem Myxomyceten Physarum polycephalum. Protoplasma (Wien), 42, 312-323 Electricity in plant movements 33

DiTTRicH, P., MAYER, M. & MEUSEL, M, (1979), Proton-stimulated opening of stomata in relation to chloride uptake by guard cells, Playita (Berlin), 144, 305-309, EcKERT, R, (1965a). Bioelectric control of bioluminescence in the dinoHagellate Noctiluca. 1. Specific nature of triggering events. Science (Washington), 147, 1140-1142. EcKERT, R. (1965b). Bioelectric control of biolumiticscence in the dinoflagellate Noctiluca. II, Asynchronous flash initiation by a propagated triggering potential. Science (Washington), 147, 1142 1145. EcKERT, R, (1972), Bioelectric control of ciliary activity. Science (Washington), 176, 473-481. EcKERT, R. & MACHEMEU, H, (1975). Regulation of ciliary beating frequency by the surface membrane. In; Symposium of the Society for General Physiology (Ed. by R. Stephens & S. Inoue), pp, 151-163. Raven Press, New York. EcKERT, R., NAITOH, Y, & MACHEMER, H, (1976), Calcium in the bioelectric and motor functions of Paramecium. Symposium of the Society for Experimental Biology, 30, 233-255, EcKERT, R. & SIBAOKA, T. (1967). Bioelectric regulation of tentacle movement in a dinoflagellate, Jo»r»o/ of Experimental Biology, 47, 433-446. FINDLAY, G. P. & FINDLAY, N. (1975). Anatomy and movement of the column in Stylidium. Australian Journal of Plant Physiology, 2, 597-621. FINDLAY, G. P. & HOPE, A. B. (1976). Electrical properties of plant cells: methods and findings. In: Encyclopedia of Plant Physiology, New Series, vol. 2, part A (Ed. by U. Luttge & M, G. Pitman), pp, 53-92, Springer, Berlin, FINDLAY, G. P. & PALLACHY, C. K. (1978). Potassium chloride in the motor tissue of Stylidium. Australian Journal of Plant Physiology, 5, 219-229. FLEISCHER, M, & WOHLFARTH-BOTTERMAN, K. E. (1975). Correlation between tension force generation, fibrillogenesis and ultrastrueture of cytoplasmic actomyosin during isometric and isotonic contractions of protoplasmic strands. Cytobiologie, 10, 339-365. FLEURAT-LESSARD, P. & BONNEMAIN, J,-L. (1978), Structural and ultrastructural characteristics of the vascular apparatus of the sensitive plant (Mimosa pudica L,), Protoptasma (Wien), 94, 127-143, FLUCK, R. A, & JAFFE, M. J. (1976), The acetylcholine system in plants. In: Commentaries in Plant Science (Ed. by H. Smith), pp. 119-136. Pergamon Press, Oxford, FoNDEViLLE, J. C, BoRTiiwicK, H. A. & HENDRICKS, S. B, (1966), Leaflet movement of Mimosa pudica L, indicative of phytochroine action. Plant (Berlin), 69, 357 367. FORDE, J. & STEER, M, W. (1976). Cytoplasmic streaming in Elodea. Canadian Journal of Botany, 54, 2688-2694. FoRER, A, & JACKSON, W, T. (1976). Actin filaments in the endosperm mitotic spindles in a higher plant, Haemanthus katherinae Baker. Cytobiologie, 12, 199-214. FRANCK, F. & AUGER, D. (1932). Analyse cinc'matographique des mouvements du protoplasme en rapport avec la variation electrique dans l'excitation chez Nitella. Comptc Rendu de VAcademie des Sciences, Paris, 195, 1321-1322. FUJII, S., SHIMMEN, T. & TAZAWA, M, (1978). Light-induced changes in membrane potential in Spirogyra. Plant atid Cell Physiology (Tokyo), 19, 573-590. GALSTON, A. W, & SATTER, R, L. (1976). Light, clocks and ion flux: an analysis of leaf movement. In: Light and Plant Development (Ed. by H, Smith), pp. 159-184. Butterworths, London, Boston. GARDINER, W. (1885). On the phenomena accompanying stimulation of the gland cells in the tentacles of Drosera dichotoma. Proceedings of the Royal Society (London), 19, 229 334. GRAVES, J. S, & GUTKNECIIT, J. (1977), Current-voltage relationships and voltage sensitivity of the Cl" pump in Halicystis parvula. Journal of Membrane Biology, 36, 83-96. GREGORY, F, G, & PEARSE, H, L, (1937), The effect on the behaviour of stomata of alternating periods of light and darkness of short duration. Annals of Botany (London), N.S,, 1, 3-10. GRUNDFEST, H. & CAMPHELL, B, (1942), Origin, conduction and termination of impulses in the dorsal spino-cerebellar tract of cats. Journal of Neurophysiology, 5, 275-294. GUHATHAKURTA, A, & Dtrrr, B, K, (1963), A study of the multiple electrical response of Biophytum sensitivum. Transactions of the Bose Research Institute, 26, 85-96, GuNAR, 1. 1., ZLOTNIKOVA, I. F. & PANICHKIN, L, A, (1975). Electrophysiological investigation of the- stomate complex in spiderwort, Soviet Plant Physiology, 22, 704—707. HADER, D.-P, (1977). Influence of electric fields on photophobic reactions in blue-green algae. Archives of Microbiology, 114, 83-86. HADER, D.-P, (1978a), Evidence of electrical potential changes in photophohically reacting blue-green algae Archives of Microbiology, 118, 115-119. HADER, D.-P, (1978b), Extracellular and intracellular determination of light-induced potential changes during photophobic reactions in blue-green algae. Archives of Microbiology, 119, li~T). HATANO, S. (1970), Specific effect of Ca" on movement of plastnodial fragment obtained by cafleine treatment. Experimental Celt Research, 61, 199-203. HAYAMA, T., SHIMMEN, T, & TAZAWA, M. (1979), Participation of Ca" in cessation of cytoplastiiic streaming induced by membrane excitation in Characeae internodal cells, Protoplasma (Wien), 99, 305-321,

2 ANP87 34 P- J- SIMONS |

HEATH, O. V. S. & RUSSELL, J. (1954). Studies in stomatal behaviour. VI. An investigation of the light responses of wheat stomata with the attempted elimination of control by the mesophyll. 1. Effects of light independent of carbon dioxide and their transmission from one part of tbe leaf to another. Journal of Experimental Botany, S, 1-15. HEPLER, P. K. & PAi.EVirz, B. A. (1974). Microtubules and microfilaments. Annual Review of Plant Physiology, 25, 309-362. HOPE, A. B. & WALKER, N. A. (1975). The Physiology of Giant Algal Cells. Cambridge University Press, London, New York. HOUWINK, A. L. (1935). The conduction of excitation in Mimosa pudiea. Recueil des Travaux hotanique neerlendais, 32, 51-91. HSIAO, T. C. (1976). Stomatal ion transport. In; Transport in Plants. Vol. II. Part B, Tissues and Organs (Ed. by U. Luttge & M. G. Pitman), pp. 195-221. Springer-Verlag, Berlin, Heidelberg, New York. INSELL, J. P. & ZACHARIAH, K. (1978). The mechanism of the ring trap of the predacious hypbomycete Dactylella brochopoga Dreschler. Protoplasma (Wien), 95, 175-191. IwAMURA, T. (1949). Correlations between protoplasmic streaming and bioelectric potential of a slime mould, Physarum polyeephatum. Botanical Magazine (Tokyo), 62, 126-131. JACOBSON, S. L. (1965). Receptor response in Venus's i\ytrap. jfournal of General Physiology, 49, 117-129. JAFIE, L. J. & NticciTELLi, R. (1977). Electrical controls of development. Annual Review of Biophysics and Bioengineering, 6, 445-476. JAFEE, M. J. (1968). Phytochrome-mediated bioelectric potentials in mung bean seedlings. Science (Wash- ington), 162, 1016-1017. JAFFE, M. J. (1970). Evidence for tbe regulation of phytochrome mediated processes in bean roots by the neurobumor, acetylcboline. Plant Physiology, 46, 768-777. JAFFE, M. J. (1973). The role of ATP in mechanically stimulated rapid closure of the Venus's flytrap. Plant Physiology, 51, 17-18. JosEF.ssoN, J. O. (1966). Some bioelectric properties o( Amoeba proteus. Acta Physiologica Scandinavica, 66, 395-405. JosEPHSON, R. K. (1962). Spontaneous electrical activity in a bydroid polyp. Comparative Biochemistry and Physiology, 5, 45-58. KABSCH, W. (1861). Anatomiscbe und physiologische Untersuchungen uber einige Bewegungserscheinungen im Pflanzenreiche. Botanische Zeitung, 19, 369-375. KAMIYA, N. (1959). Protoplasmic streaming. Protoptasmatologia, 8, 3a. KAMIYA, N. & ABE, S. (1950). Bioelectric phenomena in the myxomycete plasmodium and their relation to protoplasmic How. Journal of Colloid Science, 5, 149-163. KisiiiMOTO, U. (1958). Rhythmicity in the protoplasmic streaming of a slime mold, Phvsarum polvcephaluni. 11. Theoretical treatment of the electrical potential rhythm .Journa/ of General Physiology, 41,1223-1244. KNiGHr, R. C. (1916). On the use of the porometer in stomatal investigation. Annals of Botany (London), O.S., 30, 57-76. KoKETsu, R. (1923). tlber die Wirkungen der Elektrischen Reizung an den Pflanzlichen Zellgebilden. Journal of the Department of Agriculture Kyushu Imperial University, 1, 1-133. LicHTNER, F. T. & WILLIAMS, S. E. (1977). Prey capture and factors controlling trap narrowing in Dionaea (Droseraceae). American Journal of Botany, 64, 881-886. LirviN, V. F., SiNESCHEKOv, O. A. & SiNESCHEKov, V. A. (1978). Pbotoreceptor electric potential in the phototaxis of the alga Haematococcus pluvialis. Nature (London), 271, 476-478. LLOYD, F. E. (1942). The Carnivorous Plants. Chronica Botanica Co., Waltham, Massachusetts. LYUBIMOVA, M. N., DEMYANOVSKAYA, N. S., FEDOROVICH, I. B. & ITOMI.ENSKITE, I. V. (1964). The part played by ATP in the motor function of the Mimosa pudica leaf (in Russian). Biokhimya, 29, 114-119. MACHUMER, II. & ECKERT, R. (1973). Electrophysiological control of reversal ciliary beating in Paramecium. Journal of General Physiology, 61, 572-587. MACINNES, D. A. (1961). The Principles of Electrochemistry, 2nd edn. Dover Publications, New York. MARBACH, I. & MAYER, A. M. (1971). Effect of electric field on the phototactic response of Chlamydomonas reinhardii. Israel Journal nf Botany, 20, 96-100. MARTIN, E. V. & CLEMENTS, F. E. (1935). Studies on the effects of artificial wind on growth and transpiration in Helianthus anuus. Plant Physiology, 10, 613-636. MERL, E. M. (1922). Biologische Studien uber die UtriculariahVAsi:. Flora, 115, 59-74. MILLET, B. (1976). Etude cinetique de la courbure des etamines du Mahonia aquifolium (Pursh.) Nutt. stimulces electriquement. Annales Scientifiques de I'Universite Besanfon, 17, 75-80. MILLET, B. & THIIIERT, P. (1976). Enregistrement de !a reponse mecanique des etamines de Berberis stimulees electriquement. Comptes rendus des Seances de ta Societe de Biologie, 170, 87-94. MOODY, W. & ZEIGER, E. (1978). Electrophysiological properties of onion guard cells Planta (Berlin) 139, 159-165. MuLLER, H. G. (1958). The constricting ring mechanisms of two predacious hyphomycetes. Transactions of the British Mycological Society, 41, 341-364. Electricity in plant movements 35 NACHMIAS, V, & AscH, A, (1974). Actin mediated calcium dependency of aetomyosin in a myxomycete. Biochemical and Biophysical Research Communications, 60, 656-664. NAGATA, Y, (1977). Light-induced adhesion of Spirogyra cells to glass. Plant Physiology, 59, 680-683. NAITOH,' Y, & ECKI:RT, R. (1969). Ionic mecbanisms controlling bebavioural response of Paramecium to mechanical stimulation. Science (Washington), 164, 963-965, NAITOH, Y. & ECKERT, R. (1974), Tbe control of ciliary activity in Protozoa, In; Cilia and Flagella (Ed. by M, A, Sleigh), pp. 305-352. Academic Press, London and New York. NICHOLS, K, M. & RIKMKNSPOEL, R, (1977). Mg'-+-dependent electrical control of flagellar activity in Euglena. Journal of Celt Science, 23, 211-226, NucciTEl.Ll, R,, Poo, M,-M, & JAI'FE, L. F. (1977), Relations between ameboid movement and membrane- controlled electrical currents. Journal of General Physiology, 69, 743-763. O'BRIHN, T. P, & McCuLLEV, M, E, (1970), Cytoplasmic fibres associated with streaming and saltatory- particle movement in Heracleuni mantegazziamim. Planta (Berlin), 94, 91-94. O'DAV, P. M,, RIKMENSPOEL, R. & LINDEMANN, C. B. (1976). Effects of direct current injection on tbe activity of bull spermatozoa. Biophysical Journat, 16, 119a, ODA, K, & AHE, T, (1972). Action potential and rapid movement in the main pulvinus of Mimosa pudica. Botanical Magazine (Tokyo), 85, \3S'14(>. OGATA, K, & KISHIMOTO, U, (1976). Rhythmic change of membrane potential and cyclosis of Nitella internode. Plant and Cell Physiology (Tokyo), 17, 201-207, PALLAGHV, C, K, (1968), Electrophysiological studies in guard cells of tobacco, Planta (Berlin), 80, 147-153, PARKER, G. H. (1918), Tbe rate of transmission in the nerve net of tbe Coelenterates. Journal of General Physiology, 1, 231-236, PENNY, M, G. & BOWLING, D. J. E. (1974). A study of potassium gradients in the epidermis of intact leaves of Commelina communis L. in relation to stomatal opening. Planta (Berlin), 119, 17-25. PFEFFER, W, F, P. (1906). The Physiology of Plants, vol. 3, English translation by A, J, Ewart, Clarendon Press, Oxford, PICKARD, B. G, (1973), Action potentials in higher plants. Botanical Review, 39, 172-201. PICKARD, W, F, (1969), The correlation between electrical behavior and cytoplasmic streaming in Chara hraunii. Canadian Journal of Botany, 47, 1233-1240. QuATRANO, R. S, (1978), Development of cell polarity. Annual Review of Plant PhyKuohgy, 29, 487-510, RACUSEN, R, H. & ETHERTON, B. (1975). Role of membrane-bound fixed-charge changes in phytochrome- mediated mung bean root tip adherence phenomenon. Plant Physiology, 55, 491-495, RACUSEN, R. H, & GALSTON, A, W, (1977), Electrical evidence for rhythmic changes in tbe cotransport of sucrose and hydrogen ions in Samanea pulvini. Planta (Berlin), 135, 57-62, RACUSEN, R, H. & SATTI-R, R. L, (1975). Rhythmic and pbytochrome-regulated changes in transmembrane potential in Samanea pulvini. Nature (London), 255, 408-410, RASCHKE, K, (1970). Stomatal responses to pressure changes and interruptions in the water supply of detached leaves of Zea mays L, Plant Physiology, 45, 415-423, RASCHKE, K, (1972). Saturation kinetics of the velocity of stomatal closing in response to CO,^. Plant Physiology, 49, 229-234. RASCHKE, K, (1976). Transfer of ions and products of photosynthesis to guard cells. In: Transport and Transfer Processes in Plants (Ed, by 1. F. Wardlaw & 1. B. Passioura), pp, 203-215. Academic Press, New York, London. RASCHKE, K, & HUMBLE, G. D, (1973). No uptake of anions required by opening stomata of Vicia faba: guard cells release hydrogen ions. Planta (Berlin), 115, 47 57, REISS, D, H. & HERTH, W. (1978). Visualization of the Ca''+ gradient in growing pollen tubes of Lilium longiflorum witb chlorotetracycline fluorescence. Protoplasma (Wien), 97, 373-377, RiDGWAV, E. B. & DURHAM, A. C. H. (1976). Oscillations of calcium ion concentrations in Physarum polycephalum. Journal of Cell Biology, 69, 223-226. ROBINS, R. J. (1976), The nature of the stimuli causing digestive juice secretion in Dionaea muscipula Ellis (Venus's-flytrap), Planta (Berlin), 128, 263-265, RoBLiN, G, (1979). Mimosa pudica: a model for the study of tbe excitability in plants. Biological Reviews of the Cambridge Philosophical Society, 54, 135-153. SANBERG, P, R. (1976), 'Neural capacity' in Mimosa pudica: a review. Behavioral Biology, 17, 435-452, SANDOW, A, (1944), Studies on the latent period of muscular contraction. Method, General properties of latency relaxation. Journal of Cellular and Comparative Physiology, 24, 221-256. SATTER, R, L., CAMPBELL, N. A. & GARBER, R. C, (1980). Apoplastic transport of ions in tbe pulvinus of Samanea during rhythmic leaf movements. Plant Physiology, Supplement 65, 163.- SATTER, R, L, & GALSTON, A. W. (1971). Potassium flux: a common feature of Albizzia leaflet movement controlled by pbytochrome or endogenous rhythm. Science (Washington), 174, 518 — 520, SATTER, R, L, & GALSTON, A. W. (1973), Leaf movements: Rosetta stone of plant behavior? BioScience, 23, 407-416. SATTER, R, L., GEDALLE, G, T,, APPLEWHITE, P, B, & GALSTON, A, W. (1974). Potassium flux and leaf movement in Samanea saman. I, Rhythmic tnovement. Journal of General Physiology, 64, 413-430, 36 P. J. SIMONS SATTER, R. L,, MARINOFF, P, & GALSTON, A. W, (1970), Phytochrome controlled nyctinasty in Albixzia > julibrissin. II, Potassium flux as a basis for leaflet movement. American Journal of Botany, 57, 916-926. SATTER, R, L,, SCHREMPF, M,, CHAUDRI, J. & GALSTON, A, W, (1977). Phytochrome and circadian clocks in Samanea. Rhythmic redistribution of potassium and chloride within the pulvinus during long dark periods. Plant Physiology, 59, 231-235, ScARTH, G, W. (1932). Mechanism of the action of light and other factors on stomatal movement. Plant Physiology, 7, 481-504, SCHMIDT, J, A, & ECKERT, R, (1976), Calcium couples flagellar reversal to photostimulation in Chlamydomonas reinhardtii. Nature (London), 262, 713-715, ' ScHONBOHM, E, (1973), Kontractile Eibrillen als aktive Elemente bei der Mechanik der Chloroplasten- veragerung, Berichte der deutschen botanischen Gesellschafi, 86, 407-422, ' SCHREMPF, M,, SATTER, R. L, & GALSTON, A, W, (1976), Potassium-linked chloride fluxes during rhythmic leaf movement of Albizzia julibrissin. Plant Physiology, 58, 190-192. ( ScHii.DKNiiCHT, H. (1978). Uber die Chemie der Sinnpflanze Mimosa pudica L. Sitzungsberichte der Heidetberger Akademie der Wissenschaften, 6, 371^02. •' SCOTT, B, I. H,, GULLINE, H. F, & ROBINSON, G. R. (1977). Circadian electrochemical changes in the -., pulvinules of Trifolium repens L. Australian Journal of Plant Physiology, 4, 193-206. '| SIBAOKA, T. (1950a). Action potential and conduction of excitation in the leaf of Mimosa pudica. Science Reports of the Tohoku University, 4th Series (Biology), 18, 362-369. ^' SIBAOKA, T. (1950b), Nature of conduction of excitation in tbe petiole of Mimosa pudica. Science Reports of the Tohoku University, 4th Series (Biology), 18, 370-376. i SiDAOKA, T, (1953), Some aspects of the slow conduction of stimuli in the leaf of Mimosa pudica. Science • Reports of the Tohoku University, 4th Series (Biology), 20, 72-88, ; SIBAOKA, T. (1954). Conduction mechanism of excitation in the petiole of Mimosa pudica. Science Reports of the Tohoku University, 4th Series (Biology), 20, 139-157, :' SlUAOKA, T, (1958), Conduction of action potential in the plant cell. Transactions of the Bose Research Institute, 22, 43-56, t SIBAOKA, T, (1962). Excitable cells in Mimosa. Science (Washington), 137, 226, SIBAOKA, T, (1966). Action potentials in plant organs. Symposium of the Society for Experimental Biology, 20, 49-74, SlUAOKA, T, (1969), Physiology of rapid movements in higher plants. Annual Review of Plant Physiology, . 20, 165-184, SIBAOKA, T, (1973), Transmission of action potentials in Biophytum. Botanical Magazine (Tokyo), 86, 51-62. SIBAOKA, T. (1980), Action potentials and rapid plant movements. The Tenth International Conference on Plant Growth Substances. (To be published,) SIBAOKA, T, & ECKERT, R, (1967). An electrophysiological study of the tentacle-regulating potentials in Noctiluca. Journal of Experimental Biology, 47, 447-459, SIBAOKA, T, & ODA, K, (1956), Shock stoppage of the protoplasmic streaming in relation to the action potential in Chara. The Science Reports of the Tohoku University, 4th Series (Biology), 22, 157-166. SIMONS, P. J, (1979), Electrical behaviour in carnivorous plants. The Carnivorous Plant Society Journal, 2, 7-10, SlNVUKHiN, A, M, & BRITIKOV, E. A, (1967a), Generation of potentials in tbe pistils of Incarvillea and lily in connection witb the movement of the stigma and pollination, Soviet Plant Physiology, 14, 393-403. SINYUKHIN, A, M, & BRITIKOV, E. A. (1967b), Action potentials in the reproductive system of plants. Nature (London), 215, 1278-1280, SLAVMAN, C, L. & SLAYMAN, C, W. (1974), Depolarization of the plasmamembrane of Neurospora during active transport of glucose: evidence for a proton-dependent cotransport system. Proceedings of the National Academy of Science, USA, 71, 193.S-1939, SPANSWICK, R, M, (1972), Evidence for an electrogenic ion pump in Nitella translucens. 1, The effects of pH, K+, Na"*", ligbt and temperature on the membrane potential and resistance, Biochimica et Biophysica Acta, 288, 73-89, SYDENHAM, P, H, & FINDLAY, G, P. (1973), Tbe rapid movement of the bladder of t/fricu/ono sp. ^ujtra/i'aM 5 Journal of Biological Sciences, 26, 1115-1126. STERN, K, (1924), Elektrophysiologie der Pflanzen. Monographien aus den Gesamitgebeit der Pbysiologie der Pflanzen und der Tiere, vol. iv. Springer, Berlin, STILES, W, (1936), Introduction to the Principles of Ptant Physiology. Metbuen, London. TANADA, T. (1968). Substances essential for a red, far-red light reversible attachment of mung bean root tips to glass. Plant Physiology, 43, 2070-2071. TASAKI, I, & KAMIYA, N. (1950), Electrical response of slime mold to mechanical and electrical stimuli, Protoplasma (Wien), 39, 333 343, TAZAWA, M,, KIKUYAMA, M, & SHIMMEN, T, (1976). Electric characteristics and cytoplasmic streaming of Characeae cells lacking tonoplast, Celt Structure and Function, 1, 165-176, TAZAWA, M, & KISHIMOTO, U. (1968), Cessation of cytoplasmic streaming of Chara internodes during action potential. Plant and Cell Physiology, Tokyo, 9, 361-368, ToHiYAMA, H. (1955), Observational and experimental studies of sensitive plants, VI, Migration of potassium in the primary pulvinus. Cytologia, 20, 367-377, Electricity in plant movements 37

To IYAMA H (1962). Observational and experimental studies of sensitive plants. XV. The migration of potassium in the petiole of Mimosa pudica. Cytotogia. 27, 431-449. T AMA H & JAFFIS, M. J. (1972). Migration of calcium and its role in the regulation of seismonasty in the motor cell of Mimosa pudica. Ptant Physiology, 49, 72-81. T YAMA H & SATO, S. (1969). Electron microscope observation of the motor cell of A/miosa put/jVa. III. Th" fine structure of the central vacuole in the motor cell. Proceedings of the Japanese Academy, 45, 175-179. T ABUCCHi C. (1948). I mediatori chimici e la motilita delle piante. Archivio di scienze Biotogiche (Botogna), 32, 278-290. II A T TERAYAMA, K., KURIHARA, K. & ROHATAKF, Y. (1975). Threshold phenomena in chemoreception and taxis in slime mold Pliysarum potycepbalum. Journal oj Generat Physiotogy, 65, 223-234. II BRicHT W (1977). Ionic channels and gating currents in excitable membranes. Amniat Review oj Biophysics and Bioengineering, 6, 7-31. UMRATH K (1928). Uber die Erregungsleitung bei sensitiven Pflanzen, mit Bemerkungen zur Theorie der Erregungsleitung und der elektrischen Erregbarkeit in allgemeinen. Ptanta (Bertin), 5, 274—324. U RATH K (1931)- Der Obergang der Blatter in Schlafstellung. Ptanta (Bertin), 13, 169-192. UMRATH' K. (1937)- Der Erregungsvorgang bei hoheren Pflanzen. Ergehnisse der Biotogie, 14, 1-142. U RATH' K (1943). iJber die Erregungssubstanz von Bert>eris. Protoptasma (Wien), 37, 346-349. UMRATH' K (1959). Die Erregungsvorgang. In: Handhuch der Pflanzenphysiotogie, vol. 17, part 1 (Ed. W Ruhland), pp. 24-110. Springer, Berlin. UMRATH K. (1966)- Durch Erregungssubstanz ausgeloste SpaltoffnungsschlieBbewegung. Zeitschrift fur Pflanzenphysiotogie, 55, 217-223. SAMBEEK J W. & PICKARD, B. G. (1976a). Mediation of rapid electrical, metabolic, transpirational and hotosvnthetic changes by factors released from wounds. I. Variation potentials and putative action potentials in intact plants. Canadian Journal oj Botany, 54, 2642-2650. V N SAMBEEK J W. & PICKARD, B. G. (1976b). Mediation of rapid electrical, metabolic, transpirational and photosynt;hetic changes by factors released from wounds. II. Mediation of the variational potential by Ricca's factor. Canadian Journal oj Botany, 54, 2651-2661. VAN SAMBEEK J. W. & PICKARD, B. G. (1976C). Mediation of rapid electrical, metabolic, transpirational and photosynthetic changes by factors released from wounds. 111. Measurements of COj and HjO Hux. Canadian Journal oj Botany, 54, 2662-2671. VANDEN DRIESSCHE, T. (1963). Implication of contractile proteins in the leaf movement in Mimosa pudica L Annales de Phvsiologie Vegctale de t'Universite de Bruxeltes, 8, 101-112. VANDEN DRIESSCHE, T. (1978). The molecular mechanism of Mimosa leaf seismonastic movement. A re-evaluation. Archives de Biotogie (Bruxettes), 89, 435-449. VANDEN DRIESSCHE, T. (1979). Deux mouvements protoplasmiques; le mouvement de navette des myxomycetes et la cyclose des cellules vcgetales. Annee Biologique, 8, 417-445. WAGNER G HAUPT, W. & LAUX, A. (1972). Reversible inhibition of chloroplast movement by cytochalasin B in'the green alga Mougeotia. Science (Washington), 176, 808-809. WAINWRIGHT CM. (1977). Sun-tracking and related leaf movements in a desert alpine (Lupinus arizonicus). American Journal oj Botany, 64, 1032-1041. WALIER I C (1925). Plant electricity. I. Photo-electric currents associated with the activity of chlorophyll in plants. Annals oj Botany (London), O.S., 39, 515-538. WATANABE S. & SIBAOKA, T. (1973). Site of photoreception to opening response in Mimosa leaflets. Ptant and Cetl Physiotogy (Tokyo), 14, 1221 1224. WEINTRAUB, M. (1951). Leaf movements in Mimosa pudica L. New Phytohgist, 50, 357-382. WiiKiE D. R. (1968). Muscle. The Institute of Biology's Studies in Biology, no. 11. .^rnold, London. WiLl IAMS, S. E. (1976). Comparative sensory physiology of the Droseraceae - the evolution of a plant sensory system. Proceedings oj the American Philosophical Society, 120, 187-204. WiLi IAMS, S. E. & PICKARD, B. G. (1972). Receptor potentials and action potentials in Drosera tentacles. Planta (Berlin), 103, 193-221. WiLi IAMS W. T. (1948). Studies on stomatal behaviour. I. Stomatal movement induced by heat-shock stimuli, and the transmissioti of such stimuli across leaves of Pelargonium zonale. Annals oj Botany (London), N.S., 12, 35-51. WILLIAMS, W. T. (1949). Studies on stomatal behaviour. III. The sensitivity of stomata to mechanical shock. Annats of Botany (London), N.S., 13, 309 328. WILLIAMSON, R. E. (1975). Cytoplasmic streaming in Chara\ a cell mode! activated by ATP and inhibited by cytochalasin B. Journal of Cell Science, 17, 655-668. YAMAGUTI, Y. (1932). Uber elektrische Potentialveriinderungen an periodisch sich bewegenden Primar- blattern von Canavalia ensiformis D.C. Botantical Magazine (Tokyo), 46, 216-223. YOUNG, S. (1973). Electronics in the Life Sciences. Macmillan Press, London. , YuNGHANS, H. & JAFFE, M. J. (1972). Rapid tespiratory changes due to red light or acetylcholine during the early events of phytochrome-mediated photomorphogenesis. Plant Physiology, 49, 1-7. ZEIGER, E., MOODY, W., HEI'I.ER, P. & VARELA, F. (1977). Light sensitive membrane potentials in onion guard cells. Nature (London), IIQ, 270-271.