IAWA Bulletin n.s., Vol. 6 (4), 1985 293

HYDRAULIC CONDUCTANCE AND XYLEM STRUCTURE IN TRACHEID-BEARING PLANTS·

by

Arthur C. Gibson, Howard W. Calkin and Park S. Nobel Department of Biology and Laboratory of Biomedical and Environmental Sciences, University of California, Los Angeles, California 90024, U. S. A.

Summary To understand water flow in tracheary ele­ chymatous tissue. Elongate, hollow water con­ ments, hydraulic conductances per unit length duits with appropriate hydrostatic pressure were measured and then compared with theo­ gradients permit relatively large volumes of retical values calculated from xylem anatomical water to flow per unit time. measurements using the Hagen -Poiseuille rela­ Imperforate tracheids have traditionally been tion for nine species of pteridophytes, including treated as suboptimal water conduits compared Psi/a tum and eight species of ferns. In ferns the with perforate vessel elements, because for water potential gradients were essentially con­ every several millimetres of axial flow, water stant from the root tips to the distal portion of must move through cell walls to pass from one the leaf rachises, although somewhat larger gra­ tracheid to another. In a vessel, the uninter­ dients were found from the petiolule onward. rupted conductance pathway is much longer, Although tracheid number and diameter appa­ i. e., many centimetres to metres (Zimmermann, rently controlled water flow in xylem, estimates 1971,1982,1983; Zimmermann & Jeje, 1981; of hydraulic conductance per unit length pre­ Zimmermann & Potter, 1982). Some support dicted from tracheid numbers and diameters for this interpretation comes from measure­ were generally twice those actually measured ments of lower flows in tracheid-bearing plants from plants under steady-state conditions. A than in vessel-bearing plants (Zimmermann, model was developed to account for this dis­ 1971, 1983). Nonetheless, the methods used to crepancy for Pteris vittata, indicating that pit obtain values of sap ascent, either as a velocity membrane resistances may contribute 70% of or a volume flow, have not yielded the data the total resistance to water flow in this fern. needed to analyse the performance of individu­ This may account for the generally observed de­ al tracheids and vessels. viation of tracheid performance from that pre­ To help understand flow in capillaries, one dicted for ideal capillaries of uniform diameter. should know the hydraulic conductance per Key words: Cyrtomium, fern, hydraulic con- unit length, Kh (m4 MPa- 1 8-1 ; Gibson et ai., ductance, pit membrane, Psi/otum, Pteri­ 1984, 1985; Calkin et al., 1985; presented in dium, tracheid, vessel, water relations, xylem Zimmermann, 1971, 1983 as hydraulic conduc­ anatomy. tivity). Hydraulic conductance indicates the ability to permit water flow, which is primarily In troduction a function of conduit diameter and number. The origin and subsequent pre-eminence of Under steady-state conditions, the volume flow­ sporophytic land plants have been attributed to ing per unit time q (m 3 S-I) is a suite of evolutionary innovations enabling ~\}I~\}I photosynthetic shoots to remain hydrated while q = Kh ~x = r Eqn. 1 emergent from water (Bailey, 1953; Barghoorn, where ~ \}I is the water potential difference (MPa) 1964). Among these was the development of over the length ~x (m) and r (MPa s m-3 ) is xylem to facilitate rapid transport of water and the resistance to water flow. Because q and minerals to green, transpiring shoots. Presence ~ \}I / ~x can be quantified along a plant, one of long tracheids is considered crucial for land can calculate Kh for various segments. Follow­ invasion by large sporophytes, because the ing this, a theoretical Kh can be obtained from water supply for erect green shoots cannot be lumen measurements using the Hagen-Poiseuille adequately met by diffusion through paren- relation (Nobel, 1983; Zimmermann, 1983):

* This research was supported by National Science Foundation grants DEB 81- 09281 and PCM- 8406351 and Department of Energy contract DE-AC03-76-SFOOOI2.

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1T~di 4 fore, to provide high values of q and d'll / dx. Kh = 12811 Eqn.2 Also, this environment produced steady-state conditions during the late morning, when the where 11 (MPa s) is the viscosity of the solution water relations parameters were measured. A and the summation is over all water-conducting value of q was obtained for whole plants using tracheary elements (diameter di (m) for the ith a steady -state porometer to determine the element) at a given level. Values of Kh obtain­ amount of water lost from all portions of a ed experimentally can then be compared with shoot above a certain level, thus indicating the those expected from tracheary element numbers amount of water passing that level. Selected and diameters. Moreover, a theoretical flow portions of the plant, e. g., certain pinnae on a model, analogous to one devised for conifer rachis, were covered with aluminum foil; these tracheids (Bolton & Petty, 1975; Bolton, 1976), pinnae would then presumably reach equilibri­ can be constructed for fern tracheids and used um with the plant axis water potential at the to analyse the importance of conductance be­ point of insertion. The water potential of the tween tracheids. Although the Hagen-Poiseuille covered plant parts could then be measured relation is usually based on hydrostatic pres­ with a Scholander-type pressure bomb or a sure differences, here the latter were equated series of leaf psychrometers and d'll / dx calcu­ to d'll's because osmotic potentials and the lated. After measuring q and d'll / dx along a gravitational contributions were essentially plant axis, values ofKh were calculated (Eqn.I). constant. Also, any interference to water flow Cross sections were also made of xylem to mea­ by cavitation or other blockage needs to be sure the lumen diameters of all tracheary ele­ considered, because only conducting elements ments and thereby to determine a theoretical shoulq be used for Hagen-Poiseuille calcula­ Kh for each level (Eqn. 2). For calculating the tions. predicted value of Kh in stipes of Pteris vittata, Studies on water flow in tracheid-bearing the narrowest diameter of each tracheid lumen plants (reviewed in Zimmermann, 1983; Tyree was used (Gibson et aI., 1984; Calkin et aI., et aI., 1983; Ewers & Zimmermann, 1984a & b) 1985). have generally used woody stems, in which For plants with long axes without leaves workers often could not study q under careful­ (roots, internodes, petiole), the method just de­ ly controlled conditions and could not know scribed cannot be used to determine Kh along precisely which tracheids were actually con­ the axis. Hydraulic conductance of whole axes ducting. To circumvent this difficulty, Gibson and segments of axes were therefore measured et al. (1984, 1985) used holly fern, Cyrtomium immediately after excision by forcing a 20 mM falcatum, a tracheid-bearing fern that lacks sec­ KCl solution (Zimmermann, 1978) through each ondary growth, so that they could assume that segment using a vacuum of 10 to 80 kPa (Dry­ essentially all of the tracheids were still func­ den & Van Alfen, 1983). Wherever paired com­ tioning in water conduction. The initial investi­ parisons were made, e.g., along the rachis, val­ gation was followed by similar studies on other ues of Kh measured from whole plants and ferns (Calkin et aI., 1985) and Psi/otum on from excised segments were very similar. How­ which whole-plant profiles of Kh are relatively ever, such measurements on excised segments easy to obtain. could not be used with materials having large air-filled lacunae in the parenchyma because Materials and Methods under negative pressure the lacunae conducted Plant material. - Detailed studies were con­ much water. ducted on the following fern species: Adiantum capillus-veneris L., Asplenium nidus L., Cyrto­ Modelling water flow in tracheids. - A model mium falcatum L. f., Dennstaedtia davallioides was constructed from morphological data of (R. Br.) T. Moore, Onoclea sensibilis L., Poly­ tracheids in the mid stipe of Pteris vittata (Calkin podium aureum L., Psi/a tum nudum (L.) Beauv., et aI., 1985). Relationships among maximum Pteridium aquilinum (L.) Kuhn var. pubescens tracheid diameter, tracheid length, decreasing Underw., andPteris vittata L. (sources indicated diameter near tracheid ends, and number and in Gibson et aI., 1984 a; Calkin et aI., 1985). sizes of lateral-wall pits were determined from Psi/a tum nudum (Psilophyta) was vegetatively xylem macerations using Jeffrey's solution (J 0- propagated from specimens at the UCLA Bo­ hansen, 1940); the macerated tissue was stained tanical Garden. with safranin. Three-dimensional shapes of pits Physiological studies. - Plants were grown as well as the areas and thicknesses of pit mem­ and experiments conducted under moderately branes were measured on scanning electron high photosynthetically active radiation and wet photomicrographs of stipe tracheids (Fig. I) soils to enhance transpiration rates and, there- obtained after fixing the tissue in 2.0% giutar-

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Fig. I. Scanning electron photomicrographs of metaxylem from Pteris vittata. - A: RLS showing angular tracheids with scalariform, intertracheal, lateral-wall pitting; x 610. - B: TS of tracheids with intermediate diameters, showing the angular outlines of the lumen and the extensive lateral­ wall pitting; x 460.

Pc rpm r pc r1 1 1

tI/. r pc rpm r pc r '2 2 2 2 rtY2

!::.'1f r pc rpm r pc 3 3 3

r pc rpm r Pc n n n

Fig. 2. Electrical circuit model for water flow from the middle of one tracheid to the middle of the next tracheid. The resistance of a tracheid lumen, rtl, is divided into equal portions, one on each side of a pit network. The n pits represent parallel pathways, each consisting of a series arrangement of a pit membrane resistance, rpm, and two pit canal resistances, rPc, one within each tracheid.

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aldehyde (v/v) in 50 mM sodium cacodylate where rtl is the resistance of a tracheid lumen, buffer (pH 6.9); the fixed material was dehy­ and r~c and r~m are the resistances of the i th drated in ethanol, critical-point dried, coated 1 1 with 200 A of gold-palladium by a Hummer I pit canal and pit membrane, respectively (Fig. sputter coater, and viewed with an E.T.E.C. 2). In these calculations, pit canal was the term Autoscan scanning electron microscope at 10 used for the pit canal (s. s.) plus the pit cham­ kV. The following observations were used in ber, and the increasing dimensions near the pit constructing the model: 1) tracheid lengths membrane were ignored. ranged from 3 to 15 mm; 2) length was pro­ The resistance of the pit canal was calculated portional to diameter [length (mm) = 0.169 from an analogue of the Hagen-Poiseuille relation diameter (p.m) + 0.535, r2 = 0.78] ; 3) total (Eqn. 2) adapted from Langlois (1964), assum­ pit area between two adjacent tracheids was ing an elliptical cross section for the pathway. about 30 times the tracheid lumen cross-sec­ tional area; and 4) pit membranes were ap­ r= .1x = 12811.1x a2 +b2 Eqn.4 proximately 0.1 J.LITI thick and they were per­ Kh 1T a 2 b2 2 ab forated by minute pores (cf. Petty, 1974; But­ where a and b are the major and minor axes, terfield & Meylan, 1982; Van Alfen et al., 1983). respectively. The resistance of the i th pit mem­ The scalariform pit-pairs represent parallel brane was calculated as a set of pores 0.05 to pathways between tracheids and collectively 0.85 JJ.m in diameter and of negligible length are in series with the resistance of the tracheid (because the pit membrane is so thin) by an lumens (Fig. 2). The series and parallel resistan­ analogue of the Hagen-Poiseuille relation (Vogel, ces may be combined and total resistance per 1981): tracheid (r') calculated using equations derived pm 2411 r. = for electrical circuits: 1 Eqn. 5 ~ (d ~m Pi)3 r'=rt1 + 1 j J \' -;- pm Eqn.3 +r. where d?m Pi is the diameter of the j th pit L2r.1 1 J membrane pore.

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o o 10 20 30 Distance along soil-plant-atmosphere continuum (cm)

Fig. 3. Water potential along the soil-plant-atmosphere continuum for Cyrtomium falcatum (A) and Adiantum capillus-veneris (B). Xylem water potentials are plotted at various points along a plant axis from the soil to the tip of the rachis (0); water potentials measured on whole transpiring pinnae (.III.) are plotted at mid pinnal position. The soil distance represents the distance from the root tips to psychrometric soil probes. Means ± SE are shown for n = 4 to 10.

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Results the greatest drop from root tip to pinnae was 1.18 MPa. Water potential gradients along the Water relations of Cyrtomium falcatum plant axis were quite constant (see Fig. 3B fqr Cyrtomium falcatum had a whole-leaf tran­ Adiantum capillus-veneris); in six species, they spiration rate of 309 ± 14 x 10-12 (mean ± SD) averaged 1.03 ± 0.04 MPa m-I . For the remain­ m 3 S-I and a change in water potential from ing two species, the smallest stipe water poten­ -0.19 MPa at the root tips to -0.58 MPa at the tial gradient occurred in Pteris vittata (0.29 distal end of the rachis. Water potential gra­ MPa m-I) and the largest occurred in Adian­ dients along most of the plant axis were quite tum capillus-veneris (2.11 MPa m-I). constant (Fig. 3A). However, about 40% of the This survey showed that the water potential overall water potential drop occurred between drops along different plant parts can be related the rachis and the site of evaporation in the la­ to plant morphology and habit. For example, minae of the pinnae (a distance averaging 3.1 in six species the root drops were 0.09 to 0.15 cm). This A'It includes all parts of the pathway MPa, or IS ± 5% of the whole-plant drops in from the point where the water enters the pe­ water potential; but in the two epiphytes, As­ tiolule to the evaporating surface of the leaf, plenium nidus and Polypodium aureum, the as no method could be found that could give a root drop was 0.5 MPa, which made up 52% separate A'It for the xylem only. and 43% of the whole-plant drops, respective­ Kh increases to a peak in the distal third of ly. In Asplenium nidus, which has entire leaves the stipe and then decreases in a gradual man­ and an extremely short stipe, the small water ner to the distal tip of the rachis (Fig. 4A). The potential drop between the midrib and the la­ summation of Kh's over the 4 to 5 adventitious mina (0.1 0 MPa) corresponded to the pinna roots that supply each leaf is approximately drops in the other species; and its water poten­ equal to the Kh at the base of the stipe. Petio­ tial gradient along the stipe and rachis was sim­ lule Kh'S were only about I % as large as stipe ilar to Onoclea sensibilis, even though the ster­ values, whereas flow through a typical petiolule ile leaf of 0. sensibilis had a prominent stipe was 7.1 % of the flow through a stipe. and a high stipe-to-rachis length ratio (1.4) Both the measured and the predicted values whereas Asplenium nidus had an extremely low of Kh varied in a similar pattern along the leaf ratio (0.04). In Onoclea sensibilis, the fertile axis. The ratio of predicted to measured values leaf, which had the lowest whole-leaf transpira­ of Kh ranged from 1.7 to 2.6. This ratio was tion (30 x 10-12 m 3 S-I) of any fern measured, obtained for a wide range of xylem structure, had an appropriately small water potential gra­ from stipe sections having 1200 tracheids with dient (0.19 ± 0.04 MPa m-I). some of them more than 40 Ilm in lumen diam­ The whole-plant profiles of Kh for all species eter to distal sections having fewer than 200 of ferns were quite similar (Fig. 4A- E), although tracheids with no lumen wider than IS Ilffi. each species differed in the maximum value of Hence, water flow varied in a consistent man­ Kh. Characteristically, the highest value of Kh ner with tracheid number and diameter at each occurred in the stipe and most often near its level in the plan t. midpoint (between 0.2 and 0.4 of the normal­ The amount of xylem per total leaf area dis­ ised distance along the leaf). In Asplenium nidus, tal to that point was greater in the distal half which lacks a prominent stipe, the highest val­ of the rachis than in the proximal half. Conse­ ue still occurred about 0.3 of the distance from quently, the vascular supply to the distal pinnae the leaf base. Pteris vittata (Fig.4D) had the appeared to be greater than to the proximal highest in vivo Kh (4.1 x 10-9 m4 MPa- 1 S-I), ones. Nonetheless, when water-stressed, the which was significantly greater than the maxi­ terminal pinnae of this pinnately compound mal in vivo Kh in other ferns, including Pteri­ leaf wilted and dried out before the lower ones. dium aquilinum, which has vessels (Fig.4C). The amount of xylem at a petiolule base was The highest measured flows occurred in stipes directly proportional to the leaf area of the of Pteridium that had been backflushed. pinna. Values of Kh that were predicted from xyla­ ry element diameters were always higher than Water relations of other ferns those calculated from measurements of q and Patterns of xylem water potential and hy­ A\}I / Ax at any point along the plant axis (Fig. draulic conductance in seven additional species 4A-E). To standardise interspecific analyses, of ferns were very similar to those obtained for predicted and measured values of Kh were Cyrtomium falcatum (Calkin et aI., 1985). The compared at mid stipes, where maximal Kh oc­ seven species had water potentials averaging curred. The maximal Kh predicted from the -0.22 ± 0.05 MPa at the root tips and -0.74 ± Hagen-Poiseuille relation was 1.8 to 4.2 times 0.25 MPa at the distal end of the rachis, and higher than the corresponding measured Kh for

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"0 - J: .£: :!! "0 CJI I" I" ~ IAWA Bulletin n.s., Vol. 6 (4), 1985 299 the seven, stipe-bearing species (five in Fig. 4) just below the first branch dichotomy. Each of and was 3.8 times higher for Asplenium nidus. the two branches of the dichotomy had ap­ The majority of the values were around 2. The proximately the same Kh. Before each subse­ highest values were obtained for Pteridium quent dichotomy, the number of tracheids and aquilinum, the vessel-bearing species; however, Kh increased; after the dichotomy, the two when stipes were backflushed by forcing basi­ branches had less than half of the Kh of the petal flow of 20 mM KCl solution through the source, i. e., there was an anatomically observ­ xylem, the ratio of predicted to measured Kh able vascular constriction. Regardless of these decreased to 2.1. Backflushing also was effec­ periodic changes in Kh along the plant axis in tive in increasing flow in stipe segmen ts in cer­ the aerial portion of the plant, Kh decreased tain other tracheid-bearing species in which ab­ very gradually from the first branch dichotomy normally low flows were initially obtained. In to the shoot tips. all species with pinnate or deeply lobed leaves, The ratio of predicted to measured Kh in the the ratio of predicted to measured Kh in the lower portion of the aerial axis was often around rachis was greater than that of the mid stipe. 2.6 to 3.2 (Fig. 4F). However, it could not be Despite the great differences in maximal Kh determined which tracheids were functioning in the stipes and rachises of different fern spe­ in water transport. The metaxylem elements of cies and within a single species, water flow P. nudum frequently have very thick walls and through plant axes was closely correlated with appear to be more like fibres than tracheary the Hagen-Poiseuille estimates. This strongly elements. Consequently, the predicted values suggested that a common relationship existed of Kh obtained from Hagen-Poiseuille calcula­ between the hydraulic demand of transpiration tions may be unreliable. and the numbers and diameters of xylary ele­ ments. To test this, the values of predicted Kh Modelling resistances in tracheids of Pteris vit­ were plotted versus transpiring leaf area (Fig. 5), tata indicating a close correlation (r2 = 0.88) be­ The large number of measured and predicted tween the transpiration of leaves, which is of­ values of Kh for Pteris vittata facilitated esti­ ten proportional to leaf area, and their xylem mation of the importance of resistances to flow supply. The only species that did not fit the contributed by the pit canals and the pit mem­ general line was Pteris vittata (Fig. 5), which branes. Assuming that each canal had a uni­ exhibited a higher Kh relative to flow and rela­ form cross section, the resistances of pit canals tive to leaf area, although the reasons for the were small, averaging 2% of r' (Eqn. 3), and the differences were not apparent. apparent contribution decreases more if one in­ cludes the widening of the pit chamber or con­ Water relations of Psilotum nudum siders deviations of the cross section from the To test the generality of the fern tracheid assumed ellipse. To account for the observed results, a study was made of Psilotum nudum, difference between predicted and actual Kh for which has leafless stems. These stems exhibit stipes of P. vittata (Fig. 40), a pit membrane very low maximal transpiration rates (24 ± 3 x resistance per unit area of 5.0 x 10 6 Pa s m- 3 10-12 m 3 S-I) and concomitantly low flows in was required, in which case the pit membranes the xylem. Under conditions of maximal tran­ would comprise 70 % of the total resistance to spiration, the water potential drop from subter­ water flow . ranean axes to aerial shoot tip was still very small (0.20 ± 0.04 MPa), and the correspond­ Discussion ing water potential gradient was 0.6 MPa m- I . Water flow in tracheary elements has long Measured values of Kh for P. nudum (Fig. been described by some analogue of the Hagen­ 4F) were closest to the values obtained for Poiseuille relation (Zimmermann, 1971, 1978a Onoclea sensibilis (Fig. 4E). In P. nudum, Kh & b, 1983). However, some data do not closely started at a low value and reached a maximum fit the Hagen-Poiseuille predictions (Giordano

Fig. 4. Hydraulic conductance per unit length at intervals along the leaf of five species of ferns (A-E), and along the aerial stem for Psilotum nudum (F). Values were predicted from the Hagen­ Poiseuille relation (Eqn. 2; 0) or calculated from water flows and water potential gradients (Eqn. 1; D) for excised segments. An arrow indicates the position of the stipe-rachis junction along a leaf, ex­ cept in leafless P. nudum where the position of the first stem dichotomy is plotted. For P. nudum Kh's of all branches at a given level were summed for each point. Means ± SE are shown (unless eclipsed by the symbol) for n = 3 to 6.

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N ! c: , -CD 11 E 10- CD... ~ I/) c CD E ~ 10-2 ~ vvP >- <> VV )( o O~ <> 0 -c .tP~·<>A<> V • A -I/) ~ 10-3 ,,,,/~ V CD c... • A c /~~ -cu A ...J A 10-4 10-12 10-11 10-10 10- 9 10-8 Predicted hydraulic conductance per unit length (m4 MPa-1 5-1)

Fig. 5. Xylem hydraulic conductance per unit length predicted by the Hagen-Poiseuille relation (Eqn.2) for different transpiring areas. Points along the rachises provided a range of transpiring areas and Kh's for each species. Data are for Adiantum capillus-veneris (0), Asplenium nidus (0), Cyrtomium falcatum (.), Dennstaedtia davallioides (6), Onoclea sensibilis (0), Polypodium aureum (-"), and Pteridium aquilinum (e); the indicated regression line (r' = 0.88) does not include the data for Pteris vittata (\7).

et aI., 1978). In fact, the wide variance between xylem area divided by fresh weight of transpir­ experimental results and expected values has ing leaves) and 'leaf-specific conductivity' (hy­ created uncertainty about the reliability of the draulic conductance divided by fresh weight Hagen-Poiseuille relation to describe the fme of transpiring leaves). Huber thus recognised details of water flow, leading to the suggestion the need to relate xylem structure to transpi­ that the water flow characteristics must be in­ ration, and the leaf-specific conductivity is a terpreted on a species-by-species basis. In the morphological approach to obtain a rough com­ current studies on pteridophyte tracheids, sim­ parison of transpirational loss of the leaves to ilar ratios of predicted to measured values of the amount of xylem supplying the leaves. This Kh have been obtained for numerous species, is a particularly convenient method of analys­ even though the external morphology and stelar ing water flow in large woody plants (Ewers & organisation was very diverse and the maximal Zimmermann, 1984a, b). The good correlations volumetric flow rates ranged broadly, from plants of conductance per unit length with transpiring with large leaf areas and large water demands area (r2 = 0.88; Fig. 5) and with transpirational to axes with no conspicuous transpiring lamina flow (r2 = 0.93 ; Calkin et aI., 1985) more di­ (Psilotum and the fertile spike of Onoclea ; Gib­ rectly show that as portions of the total flow son et aI., 1984, 1985; Calkin et aI., 1985). are diverted to portions of the transpiring area, Huber (1928) and later Zimmermann (1978, the conductance of the axial pathway remains 1982, 1983) have described the methods for proportional to the flow required by transpira­ determining the 'Huber value' (cross-sectional tion in the remaining distal areas.

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Using a model, the discrepancy between ac­ tected in any region. Thus, if constrictions are tual and predicted Kh in stipe tracheids could present, they have only miilor effects on the be accounted for by a pit membrane resistance water potential gradient and hence on water per unit area of 0.2 x 10-6 m Pa- I S-I. Pub­ flow in pteridophytes. lished values for pit-membrane pore-diameters range from 0.02 to 0.85 J.Ull (Petty, 1974; But­ References terfield & Meylan, 1982; Van Alfen et aI., 1983). Bailey, I. W. 1953. Evolution of the tracheary The above pit membrane resistance could be tissue ofland plants. Amer. J. Bot.40: 1-8. achieved by 1) a frequency of 45 pores J.Ull- 2 Barghoorn, E. S. 1964. Evolution of cambium in for 0.05 !lm diameter pores, 2) 5.6 pores J.Ull- 2 geological time. In: The formation of wood of 0.1 J.Ull diameter, 3) 0.7 pores /-lm- 2 of 0.2 in forest trees (ed. M. H. Zimmermann): 3- J.Lffi diameter, and 4) 0.09 pores /-lm- 2 of 0.4 17. Acad. Press, New York. J.Lffi diameter. At a pore diameter of 0.4 /-lm, pit Bolton, A. J. 1976. Biological implications of a membranes of even a large (40 J.Lffi diameter) model describing liquid flow through coni­ tracheid would have only 1.6 pores to give the fer wood. In: Wood structure in biological above resistance. Preliminary scanning electron and technological research (eds. P. Baas, A. J. photomicrographs suggest that pit membranes Bolton & D. M. Catling): 222-237. Leiden of Pteris vittata contain pores of about 0.05 /-lm. Bot. Ser. 3. Leiden Univ. Press. The actual value of pit membrane resistance - & J.A. Petty. 1975. Structural components can be determined with future ultrastructural influencing the permeability of ponded and analyses of pit membranes as well as more di­ unponded Sitka spruce. J. Microscopy 104: reet physiological experiments on water flow. 33-46. The relatively high values of Kh in ferns con­ Butterfield, B. G. & B. A. Meylan. 1982. Cell tradict the widespread impression that tracheids wall hydrolysis in the tracheary elements of are poor water conduits. Also, hydraulic con­ the secondary xylem. In: New perspectives ductances of tracheids and vessels of ferns de­ in wood anatomy (ed. P. Baas): 71-84. M. viate from those of ideal capillaries of similar Nijhoff/Dr. W. Junk, The Hague. diameter to about the same extent as tracheids Calkin, H.W., A.C. Gibson & P.S. Nobel. 1985. in conifers (Ewart, 1905 ; Miinch, 1943) and Xylem water potentials and hydraulic con­ vessels in dicotyledons (Tyree & Zimmermann, ductances in eight species of ferns. Canad. 1. 1971;Petty, 1978). CarJquist (1975) attributed Bot. 63: 632-637. the ecological success of Pteridium in dry habi­ Carlquist, S. 1975. Ecological strategies of xylem tats to the presence of vessels, but physiologi­ evolution. Univ. Calif. Press, Berkeley. cal experiments do not show that these particu­ Dodd, R. S. 1984. Radial and tangential diame­ lar vessels are inherently better for water flow ter variation of wood cells within trees of than fern tracheids of the same diameter; and Acer pseudo platanus. IAWA Bull. n.s. 5: higher values of Kh have actually been measured 253-257. in Pteris vittata. Nobel et al. (1985) showed Dryden, P. & N.K. Van Alfen. 1983. Use of the that Pteridium and other ferns of open, dry pressure bomb for hydraulic conductance habitats, unlike those of shaded, moist habitats, studies. J. Exper. Bot. 34: 523- 528. have appreciable stomatal closure as the leaf­ Ewart, A.J. 1905. The ascent of water in trees. air water vapour concentration difference in­ Trans. Philos. Soc. London B 198: 41-45. creases, and hence a way to avoid excessive Ewers, F.W. & M.H. Zimmermann. 1984a. The water loss. hydraulic architecture of balsam fu (Abies Numerous authors (Larson & Isebrands, 1978; balsamea). Physiol. Plant. 60: 453-458. Zimmermann, 1982, 1983 ; Zimmermann & - & - 1984b. The hydraulic architecture of Sperry, 1983 ; Tyree et aI., 1983; Ewers & Zim­ eastern hemlock (Tsuga canadensis). Canad. mermann, 1984a, b;Dodd, 1984; Niklas & Banks, J. Bot. 62: 940-946. 1985) have attempted to understand the phys­ Gibson, A. C., H. W. Calkin & P. S. Nobel. 1984. iological significance of vascular constrictions Xylem anatomy, water flow, and hydraulic located at junctions between main and lateral conductance in the fern Cyrtomium fa1ca­ axes, e.g., at the bases of leaves or branches. tum. Amer. J. Bot. 71: 564-574. The above authors suggest that air embolisms - , H. W. Calkin, D. O. Raphael & P. S. Nobel. can form at the constrictions and cause dys­ 1985. Water relations and xylem anatomy function to the lateral structure while preserv­ of ferns. Proc. Roy. Soc. Edinb., Spec. Vol. ing the main stream of xylem from cavitation. Biology of Pteridophytes. In press. In the present studies on pteridophytes under Giordano, R., A. Salleo, S. Salleo & F. Wander­ conditions of maximal water flow, however, no lingh. 1978. Flow in xylem vessels and Poi­ large local drops in water potential could be de- seuille's law. Canad. J. Bot. 56: 333-338.

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