3°° Experiments with the Stipes of Fucus and Laminaria

3°°

EXPERIMENTS WITH THE STIPES OF FUCUS AND LAMINARIA BY E. MARION DELF (MRS PERCY SMITH), D.SC. (LOND.), F.L.S.

(Received znd February, 1932.) (With Two Text-figures.) INTRODUCTION. MARINE algae may be briefly characterised as either intertidal or totally submerged forms. The intertidal algae have a dual existence: they are land plants at low tide and aquatics when covered by the sea. The proportion of time spent in the aerial or aquatic medium must vary with the position on the shore and the factors influencing tidal periodicity. The more exposed plants must have a tolerance for various light intensities and for a wide range of conditions affecting water loss; and they must also, if fixed to the substrate, be able to withstand more or less violent movements of the water. These movements may be partly due to currents and partly to waves, frequently reinforced by wind. Currents are said to have little influence on littoral plants, but wave action is certainly an important factor, influencing the mechanical stresses and strains to which the plants are subjected. These stresses differ from those to which land plants are exposed in being predominantly a series of intermittent pulls or jerks, applied to plants with little initial rigidity and great flexibility. In the experiments about to be described, an attempt is made to determine the " ultimate strength " (p. 302) and the elastic properties of the stipes of several of the larger brown algae, differing in habit, habitat and construction. These may be briefly indicated as follows: (a) two plants of closely similar structure but different positions on the shore, ex. Fucus serratus L., Fucus vesiculosus L.; (b) two plants of somewhat similar structure and corresponding positions on the shore, ex. Fucus serratus L. and Ascophyllum nodosum Le Jol.; (c) comparison of plants of dissimilar habit and construction, ex. Laminaria digitata1 Lamour., Fucus, and HaUdrys siliquosa Lyngb. The strength of a material is usually measured by testing the pull, the compression or the torsion which it can sustain. For the present purpose, the first was the method selected. When a steel wire is moderately loaded, a slight stretching results and on removing the load the wire will return to its original length. By increasing the load 1 Laminaria digitata appears to be a somewhat ill-defined species, and it is probable that two of the stipes used were in reality hybrids between the typical L. digitata and L. Clomtord. I am indebted to Dr V. M. Grubb for the identification of the doubtful cases. Experiments with the Stipes of Fucus and Laminaria 301 progressively and removing it between each addition, a point is found at which the length is permanently increased. The limit at which this occurs is the limit of perfect elasticity (the R limit of engineers). If, instead, a wire is continuously loaded, for each added weight there is a corresponding slight increment of length which is propor- tional to the increment of weight, until a point is reached where this proportion is exceeded. This is the limit of linear elasticity (the so-called P limit), and is higher than the R limit. Further increase in the load gives a point at which a slight added load gives a sudden large extension—the yield point—and a further increase of load will soon induce rupture. There are thus two main phases in extension under pulling strain, first that which is within the elastic limits (P and R), and secondly that in which the extension may be relatively much greater, leading to the final rupture. The latter phase is well marked in ductile metals, but is only slight in brittle materials. A plant tissue is far from isotropic and is therefore not strictly comparable with a wire, but we may expect to find a certain parallelism in its reaction to mechanically imposed strains. When wires are stretched to breaking point, it is found that in hard non-ductile metals such as steel the fracture is a clean break transverse to the length. In ductile metals on the contrary, the substance yields by shearing on an inclined plane, often with a ring-like crater on one side and a truncated cone projecting on the other side of the fracture. It is interesting to note that in these experiments both types of fracture have been found; the clean break with a straight pull on Laminaria where the structure is typically parenchymatous, but the truncated cone and irregular surface of fracture in both the species of Fucus examined, where the central core of filaments was the last to yield. It was at first thought that this would be correlated with the difference in structure, but in AscophyUum a clean break was nearly always found, and here the structure resembles that of Fucus. The difference may be in the fundamental nature of the cell walls.

EXPERIMENTAL PROCEDURE. Specimens were sent fresh from Plymouth with its sheltered harbour and from Aberystwyth where the coast is more exposed. Precautions were taken to keep the material moist during handling, and in the more protracted determinations (e.g. of elasticity) the stretching strip was moistened between the readings. It was found that by keeping the material on ice covered with wet cotton-wool kept cool by ice, the plants could be kept fresh for several days. For the most part the plants were well- grown adults, and all the plants of a consignment were used excepting any obviously diseased or injured. As far as possible the whole stipe was used—ije. the region between the hapteron and the lowest node or dichotomy. In Fucus vesiculosus the stipe was sometimes too short for fixing, and then the lowest practicable region was used, and the same was always true for AscophyUum andHalidrys: in these cases, the lowest available part of the frond was pulled, but there is little differentiation between frond and stipe in this region. The stipes of the larger specimens of Laminaria were too strong for any weights at my command and had to be split 302 E. MARION DELF longitudinally into four or more parts. Each part was then broken separately and the total load of all equated to the area of cross-section of the whole stipe. With the stipes of Fucus, this method was unsatisfactory, for the split stipes gave values quite incommensurate with their expected strength, probably owing to the inevitable cutting of many of the medullary filaments. The stipes of the strongest specimens of F. vesiculosus were thus too tough for estimation with the means at my disposal. In order to estimate the strength of a stipe, a scale pan was attached by a strong wire passing over a steel pulley to a grip which held one end of the stipe, the other being firmly held in a vice screwed to the edge of a bench. Weights were gradually added to the scale pan, with intervals of 30-60 sec. between each addition, until at last a rupture was produced. The area of cross-section was estimated from previous measurements, with a micrometer screw, of the diameters in two principal planes taken at each end of the stipe within the region needed for fixing. The load per unit of area of cross-section which just produces fracture with continuous loading may be called the "breaking stress"; it corresponds with the "ultimate strength," a term applied to certain materials used by engineers, and it just exceeds the "tenacity" or greatest longitudinal stress which can be borne per unit of area icithout rupture. In reckoning the load, the weight of scale pan, wire and grip was included. On mechanical grounds one would expect that the breaking stress would vary with the method of application. In my experiments, the initial load was small, usually 2 or 4 lb., in relation to the final load expected. Thereafter weights were added cautiously, without jerking, in increments of 2 lb. or 1 lb. With slender stipes, such as that of Halidrys, ounce weights were used and finally loose shot. Occasionally the load would be jerked off by a sudden slip, and on re-setting a premature break nearly always occurred. This is in accordance with the well-known mechanical principle that a strain set up by a sudden application of force is much greater than (and may be double) that of the same force applied gradually. A point of practical difficulty was to get a firm enough grip of an end without actually crushing the tissues. At first the more slender stipes of Fucus and Ascophyl- lum were set in cubes of plaster of Paris to avoid compression, but the plaster itself broke with loads of 10-15 lb. and the vice was then used direct, the rough inner surface of its jaws being carefully protected with a layer of sheet rubber and the end closely wrapped in cotton-wool before insertion. It became a matter of experience to judge the pressure required to give a firm hold without crushing. In the latter case a very slight load was sufficient to cause sudden rupture, and the experiment was then ignored. With practice it was also possible to set the stipes so as to avoid torsion; unless this was done, the resulting fracture was oblique and was apt to be premature if the torsion was at all considerable. The variability in area of section was a difficulty in calculating the mechanical stresses involved. Mostly there was a more or less elliptical cross-section tapering slightly in the upward direction (Fucus, Laminaria) or downwards (Ascophyllum). The diameters in two planes at right angles were measured in advance, near the upper and lower ends, where it would clear the supports, and when the fracture occurred midway between the two ends, the mean area was obviously a suitable basis; Experiments with, the Stipes of Fucus and Laminaria 303 but when the break occurred definitely at either end, the area of section at that end was used. The basis is thus the nearest approximation to the original cross-section at the point of fracture; not to the sectional area after the strip has been stretched (as was done by Willecs)). In Fucus and Laminaria, however, the diminution in cross- section due to stretching is so slight that it would make no appreciable difference to the results. Separate determinations of variability of area show that so long as the whole area is not too far from the standard area (1 sq. cm.) the error involved lies within the order of accuracy of the experiments. In the case of Halidrys, however, this cannot be claimed, and the error may reach 30-50 per cent, on the load per unit of area. In determinations of the elastic limits and total extensibility, a similar arrangement was used, but for this purpose the application of each load was followed by a measurement of the distance between two marks; the strip was then released from the load and after an interval of 30-60 sec. the relaxed length was again read. The same time was allowed for the stretching after loading as for the relaxing after un- loading. Ultimately by this intermittent method the breaking point is also reached, but this is considerably below the value as found by the method of tension under continuous loading, as is also the case for inert materials when loaded and unloaded repeatedly at short intervals beyond their elastic limits. Very few observations appear to have been made on the strength of the fronds and the stipes of the marine algae. The work of Wille (5) in the Norwegian language includes some observations of the kind1, but his experiments were mainly planned for finding the elastic limits and he used almost entirely the intermittent method of loading. Actually his figures are not very comparable with mine, for he used thin strips of tissue with a sectional area of 2-3 sq. mm., and no details are given as to how the strips were selected, for instance, from among the different tissues of the stipe of Laminaria, or Fucus, where there is considerable differentiation in the inner and outer regions. The well-known experiments of Schwendenertj) are not concerned with the marine algae.

CALCULATION OF BREAKING STRESS. Each type examined exhibited a considerable variation in the breaking stress. It is probable that a part of this variability may be due to assignable causes, but in the first instance an attempt has been made at comparison on general lines. For this purpose the individual values were graded into strengths of 10-19, 20-29, 3°~39> etc. lb. per sq. cm., the percentage number of cases in each group being then calculated (cp. Table I). By expressing these values as percentages of all the cases and plotting graphs, using these percentages as ordinates and the corresponding strengths as abscissae, a mean value wa3 found such that 50 per cent, of all cases examined are stronger and 50 per cent, of all cases are weaker than this mean value. The correction for the mean was found in the usual manner and the standard deviation {a) calculated

1 I have to thank Miss Astrid Karlsen of Bergen for help with the relevant matter when she was in England. 304 E. MARION DELF as the square root of the variance (Fisher (o). The errors of the mean (cr/yn) and of the standard deviation (cr/Vzn) were then calculated and are shown in their appropriate columns (Table I). Table I. Breaking stress o/Fucus serratus.

No. of Breaking cases Frequency Total Frequency (F) Variance strength (- fire- as % of from x deviation (D) -FxD*-25 lb /sq. cm. quency) all cases beginning

10-19 4 4 1 x 63 = -63 3969 20-29 0 4 3O-39 0 40-49 1 4 8* 1 x 33 - 33 1089 SO-S9 0 60-69 5 20 28 5 x 13 -65 845 70-79 16 ix 3 — 12 36 80-89 0 24 6x7 + 42 204 90-99 1 4 72 1 x 17 + 17 289 100-109 3 12 84 3 x 27 + 81 2187 no—119 1 88 1 x 37 + 37 1369 120-129 2 t 96 2 x 47 + 94 2209 130-139 0 0 140-149 1 4 100 1 x 67 + 67 4489 Total number Algebraic Total of cases = 25 Sum - 16776 Variance I+338 -671 = 165

Correction for mean > •& = 6-6. 25 Standard deviation (cr) • V671 -25-8. Standard error on deviation -8 V2 x n T

1 Excepting in the case of Halidrys. Experiments with the Stipes of Fucus and Laminaria 305 remaining. In these plants, thickness may be some indication of age, for the fronds usually last more than one season and there is progressive increase in diameter. The dichotomies of the thallus in Fucus frequently occur so low down that the stipe may be only 2-3 cm. in length and in such cases it was found impossible to fix in the apparatus; thus the lowest internode above it was used for the experiments. This occurred in F. serratus (Nos. I, XXI and XXII) and in 9 plants of F. vesiculosus, always in old bushy plants. Several attempts were made to compare the breaking stress at the two places, but mostly the plants were unsuitable, the lengths of each segment being so short that in using either, the other was crushed too much in the vice or clamp for subsequent testing. In the two examples of F. vesiculosus, where the comparison was made with apparent success, the breaking stress of the stipe was much less than that of the next internode above it (No. XXIII, stipe 59, internode 115; No. XXVI, stipe 43 and internode 120, lb. per sq. cm.). This was at least in part due to the tearing away of the cortex in the region of the disk so that the medullary filaments were torn in part from the firmlyhel d disk like the wick from a short end of candle; and it seems that unless the vice can be fixed well above the expanded part of the disk there is no indication of the full mechanical strength1. Most of the plants of Laminaria were in the adult stage and fertile, with the stipes about 40 sq. mm. or more in cross-section. If we exclude three cases of young plants, there was no apparent correlation between area of section and ultimate strength. For instance, the largest examined, of sectional area 207 sq. mm., had the same strength (102 lb. per sq. cm.) as another of only 75 sq. mm. in sectional area. The three young plants which seemed exceptional had sectional areas of 30 sq. mm. or less, and their breaking stress was between 17 and 25 lb. per sq. cm. Two of these plants were collected from Seaford in April, where the conditions are so un- favourable that although young plants were numerous, few were found of full size. They were kept for 3 days moistened with sea water before use and were among the first used in this series of experiments. They were so different from subsequent plants received from Aberystwyth that they were disregarded, until much later another young plant of similar dimensions was sent which with careful handling gave the same low figure of 25 lb. to the sq. cm. This suggests that the young plants of L. digitata are for a time at least much less mechanically efficient than at a later stage, but more evidence is clearly needed. The two plants of HaUdrys gave ten experiments with the slender much-branched thallus. Ignoring all but those experiments in which the breaking stress was determined to within an ounce, the two plants of Halidrys gave eight values varying from 127 to 179 lb. wt. for the breaking stress, taken at the lowest region of the slender branches, near the disk. At first sight this might appear to have some significance, although from the statistical point of view it can give little or no information, but on careful 1 Himanthalia lorea is peculiarly suitable for such a purpose owing to the long straight internodes, and here a marked diminution has been demonstrated not only between successive internodes of an old plant over 2 metres long, but between the upper and lower regions of the same internode; and this is what must be expected to occur on general grounds elsewhere, though presumably to a less extent in the much less elastic tissues of Fucus. 306 E. MARION DELF measurement the diameters along the short lengths used varied by as much as i-8 per cent, in one and 2-7 per cent, in another, the latter being the least and the former the most regular of the pieces used. Since the total area of section was of the order of 5 sq. mm. or less, the most favourable cases (129, 167 lb. wt. with sectional areas 5-78 and 4*78 sq. mm. respectively) when reduced to the common basis (129, 167 lb. wt. per sq. cm.) have an error of at least 30 and possibly 50 per cent. Unless therefore material can be found with stipes of more uniform diameter, this plant can hardly be compared on the present basis.

Table II. Breaking stress calculated as lb. per sq. cm.

No. of Mean breaking stress Standard Plant plants Correction deviation Error on examined Un- for mean a co rrected Corrected

-F. vesiculosus (a) 30 90 100-3 5-5 3S-2S 46 39 103 109 94 107 W 25 us 116 S-4 27-2 3-8 3-6 F. serratus 25 83 90 2S-8 A. nodosum 8 77 83 IO-I 28-3 41 L. digitata (d) 20 98 92-4 57 25-2 4-0 to 17 104 119 4-4 18 31 H. siliquosa 2 i4S F. vesiculosus. (a) Using stipes only. (A) As (a) but including also 9 plants where lowest internodea only could be used, (c) As (6) but excluding all cases of slipping. F. terrains. Including five cases where only internodes could be used and two cases of slipping. A. nodosum. Twenty-four experiments were performed with these 8 plants. L. digitata. (d) All cases, (e) As (d) but excluding three cases of doubtful significance. H. siliquosa. Ten experiments were performed with these plants.

If we consider the figures in the table as approximately true for the samples of plants concerned, the following conclusions may be drawn: (1) The stipes of F. vesiculosus appear to be appreciably stronger than those of F. serratus. This is probably underestimated, for the strongest plants were un- breakable with the resources available and so were omitted. It seems reasonable to suppose that the lowest internodes were not very different in strength from the stipes immediately below them, and if so, the inclusion of these should not seriously affect the comparison. (2) F. vesiculosus occurs higher on the shore than F. serratus, but the difference in strength cannot be correlated with the difference in habitat, in the light of other evidence. (3) The strength of stipe of F. serratus is appreciably greater than that of the basal region of the larger thongs of A. nodosum. Both are surf plants occurring in similar positions on the shore, but Ascophyllum favours more sheltered positions than F. serratus. (4) The adult stipes of Laminaria are about as strong as those of F. vesiculosus. Experiments with the Stipes of Fucus and Laminaria 307 The latter occurs at the upper tidal zone, the former at the lowest of the intertidal stretches. The former is a prostrate plant mostly emergent, the latter has an erect stipe even during its short periods of exposure. (5) Halidrys has a tough thallus of considerable strength, but on the present evidence it cannot be compared numerically with the other types. It occurs chiefly in pools towards the low tide levels and is subjected to little exposure, often remaining covered (at least in the south) at low spring tides.

ELASTICITY AND EXTENSIBILITY. When a wire is loaded beyond its elastic limits, the total extension per unit length produced by the load gives a measure of its extensibility. For each total extensibility there is also a small amount of permanent extension found by measuring the length after release from the load. Some measurements of the elastic limits and the extensibility of thin strips of tissue were made by Wille(s) and are quoted for comparison. Table III. Elasticity and extensibility according to Wille ((5), pp. 8, 9).

Region Extension, cm. per ioo cm. Plant tested Temporary Permanent Total

Laminaria saccharina Middle 3 9 26 L. digitata 127 28-5 SarcopkylUt edulit Lamina S7 7-1 33 Porpkyra lacirdata 11 3-i 25 Aspidistra lurida Petiole 71

In another place, Wille states that in Laminaria a load of 50 gm. per sq. mm. (about 2-2 lb. per sq. cm.) exceeds the limits of elasticity, and involves an extension of 3 per cent. Also that a strip of L. digitata can be stretched until it increases by 48 per cent, of its length with a permanent lengthening of 25 per cent. It is not clear whether this refers to the stipe or to the lamina, but presumably it is to the latter. In my earlier experiments on the ultimate strength of Fucus and AscophyUum a marked difference was noted in the amount of stretching before fracture occurred and a few measurements were made. The latter behaved somewhat like the Laminaria described by Wille, having a high total extensibility (about 20 per cent.); the former had a much lower extensibility, about 5 per cent, or in one young plant as much as 9 per cent. The record of a typical experiment is appended (p. 308). The method of intermittent loading has already been described (p. 303). In F. serratus, No. XXVII, two white marks 4 cm. apart were made on the stipe such that they could be measured with a small scale when in position for stretching. The successive loads are shown in column 2; the corresponding increments in length measured 1 min. after loading and calculated as a percentage of the original length are in column 3. Weights, scale-pan, wire and grip were then removed, and the distance between the marks again read after a minute of relaxation. This was repeated 3o8 E. MARION DELF until the stipe broke; which happened suddenly, the fracture occurring just beyond one of the white marks, so that the final length could be read. The limit of tensile strength in this plant is very near the limit of elasticity; in Ascophyllum, on the other hand, there is possible a total extension of 2-3 times that at the elastic limit before fracture occurs (Table V). Subsequently these observations were extended to other plants of which the breaking stress was being determined.

Table IV. Extensibility of Fucus serratus. No. XXVII. Bushy plant with short stipe; used lowest internode; sectional area at break 27-27 sq. mm.

Length Load Total Relaxed Permanent cm. lb. lb /sq.cm. cm./ioo cm. length cm./ioo cm.

4 0 — — 4 5 49 0 4-°5 3-5 12-81 1-25 40 0 410 5-5 24-13 2'5 4-0 0 4-15 5 27-45 3'75 4-0 0 4-17 T8-5 31-11 4-25 40 0 4-18 95 34-77 450 40 0 4-20 105 38-43 50 4-0 0 4 20 "•5 42-02 5° 4-05 1-25

Table V. Limits of elasticityand extensibility.

Extension at Load Maximum exten- elastic limit sion (M.E.) cm. % Length (BX.) cm. % lb./cm.' Plant No. cm. Tem- Per- At At Tem- Per- porary manent EX. M.E. porary manent

F. serratus XXV i-94 8-2 i-5 148 158 93 i-5 XXVI 3-25 5-25 0 97-5 97-5 5-25 0 XXVII 4-0 5° 1-25 38-4 42-9 50 125 Ascopkyllum EXa 3-1 806 161 391 75'9 216 — 1X6 2-8 10-5 i-8 27-2 40-0 27 — Laminaria XVII 10 4-8 0-15 14-85 55-5 13-12 426 XIX 6 664 i-66 2665 51-2 162 5 Halidrys III 4-8 5-2 1 04 56 144-9 129 4-16 IV 4-0 6-25 2-5 78-1 122-8 18-75 8-75 Aucuba I i-45 — — 107-3 107-3 138 10-3 II 0-65 i-o 0 221 154 231 I5-3 Aspidistra 40 i-o 0 221 551-2 i'37 —

In order to compare the extensibility of the different types, a number of graphs have been made in typical cases, plotting the extension per unit length against the load per unit area. The course of these graphs reveals in a striking way the contrast between the behaviour in the different plants (Fig. 1). In Wille's account, Ambronn's figures are quoted for Aspidistra htrida (cp. Table I) as being provided with collenchyma around the bundles which are the main source of mechanical strength. I made a few observations on the common Aspidistra '-3 ASC:IXa b.4 ABSCISSE LBS PER SQ CM. ASC: IX b ,I ORDINATES CMS PER 100CM. C:1

9 9 2 % F Q0 ASP: , Q 3 ._ -- .------FL I I I I I 0 20 40 60 80 100 120 140 160 180 200 220 240 260 rE!

Fig. I. Graphs showing come of extension under intermittent loading. Each curve repreaenta an actual experiment, excepting that of Aucuba (Auc.) where it 13 the mean of two. The point of fracture is indicated thue <. 5 P,' 310 E.MARION DELF cultivated in England as pot plants, but an adult leaf in the prime had bundles heavily encased in lignified fibres. These strands were always the last to break. In one petiole of sectional area i3-3i4 sq. mm., a load of 36-5 lb. gave an extension of only 0-5 mm. on an initial length of 4 cm. and was then within the limits of perfect elasticity. The breaking stress was found by cutting the petiole vertically into strips and handling each strip separately. The total load thus carried was 73-5 lb., i.e. about 552 lb. per sq. cm.1, a very different result from that of Ambronn for his strips of Aspidistra lurida, and typical of lignified tissue; thus the tensile strength of the oak is said to have a value varying from about half a ton to one and a half tons per sq. cm. (Llandolt and Bornstein(a)). A difficulty in these experiments is the lack of uniformity of the tissues over any considerable length. In the stipes of Fucus, the behaviour seemed uniform over the length, which was necessarily limited; but in Ascophyllum, the region below the lowest node (often 5-10 cm.) exhibits a visible difference in elasticity, one end of the strip stretching much more than the other if strips longer than 3-5 cm. are used. The same applies to Laminaria (cp. No. XVII, Table V), where most of the elon- gation in the piece occurred in the morphologically upper 5 or 6 cm.; the percentage is thus too low as calculated. On the other hand, with shorter lengths, any error in reading the marks becomes correspondingly magnified in calculating percentages. However, in spite of these difficulties, it is clear from the figures in Table V that there are two types of behaviour represented, the one seen in Fucus which has a slight total extensibility but generally a high tensile strength, the other seen in Ascophyllum, and to a less extent in Laminaria and Halidrys, in which the maximum extension is considerable, the load required to produce the extension being variable, and least in Ascophyllum. Within the limits of elasticity the extensibility is about the same (i.e. about 5 per cent.) in Fucus (stipe and lowest internode), Laminaria (stipe), and Halidrys (base of thallus). In Ascophyllum, the extreme base of the thallus has an extension within the elastic limits of 8-10 per cent., ije. nearly double that of the others. As the extension beyond the elastic limits is such a marked feature of the tissues, a further comparison has been made, namely between the extensibilities at hah0 the total maximum extension, the values being read from the graphs of extension in Fig. 1. These values are given in Table VI. If we compare these algae with some inert substance such as steel, we find a great contrast in strength, but a curious resemblance in their elastic properties. According to the literature, the tensile strength of steel is very variable, mild steel having an ultimate strength of about 3 tons to the sq. cm., steel wire 11-16 tons per sq. cm. and piano wire as much as 23 tons to the sq. cm. The range of extensibility is much less, apparently varying from 15 to 30 per cent, on the original length, when measured at the limit of perfect elasticity. This is much nearer to that of the algae at the same elastic limits than would be expected from the excessive difference in their tensile strength. Moreover, the algae examined (excepting Fucus) have a considerable 1 This is rather more than the value given for cast lead, viz. 484 lb. per sq. cm., according to Sir William Thomson (4). Experiments with the Stipes of Fucus and Laminaria 311 capacity for extension beyond the elastic limits. In inert material, it is the elastic limits which largely determine how much load can be borne without permanent strain, but in marine algae, capacity for extension without injury beyond these limits may also have a biological importance.

Table VI. Linear extensibility (at half maximum extension) in relation to load producing it.

Extensions Ratio Ratio of Plant Load (L1) (E) = cm./ioo Mean lb./an. cm. E/L means

•F. serrattu XXV 105 4-65 0-044! 0-038 XXVI 78 2-61 0-033) 13 XXVII 2I-S 2-S o-n 37 A. nodosum DCfl 68 16-25 0-239} 0-300 100 DC* 36 131 0-361/ L. digitata 31 8-66 0-28 93 Aucuba I 76 7-25 0-095) 0-094 3i II no 10-25 0-093) Atpidittra 207 0-625 0-003 1 H. siliquosa 76 69 0-091 3° • Nos. XXV, XXVI, stipes used; No. XXVII, lowest internode at base of thallus.

INFLUENCE OF TIME. When a load is stretching a wire within its limits of elasticity, the resulting stress can be borne for an indefinite period, but when these elastic limits are sufficiently exceeded, the time factor becomes of importance and a slow gradual extension may result, leading to ultimate fracture. Ambronn states that he took a "strip of collenchyma" (Wille(5), p. 10) and hung on a weight which he estimated exceeded the limits of elasticity for 2-3 days and the extension at the end was not greater than in the first hour. Wille, performing similar experiments with Laminaria digitata and other seaweeds, says: "I hardly believed my own eyes when I found that a weight large enough to procure permanent lengthening did not cause further extension if left on for a long time." So far as I have been able to look into this question I can only conclude that the length of time which an overstretched wire or tissue can hold a weight depends greatly on the extent by which the elastic limit is exceeded, but that this extent is an ill- defined quantity to be determined within wide limits only by practical experience. The graphs in Fig. 1 show something of the kind for the thallus of Ascophyllum nodosum where in the first case (IXa) the load held safely for an hour and in No. 1X6 where a relatively smaller load (but also beyond the elastic limits) held for 45 hours and then suddenly yielded, giving a clean transverse fracture. In Fig. 2 the slow extension which occurred—mostly in the first few hours—is plotted against time. From the biological point of view (as was suggested by Wille), this capacity to 312 E. MARION DELF endure excessive strains in nature allows time for the recovery, e.g. after a storm, or for the strengthening of the stretched walls by the deposition or intussusception of new particles. The same would doubtless be true in the intertidal periods between exposure to direct wave action, and it may be that we have here the biological equi- valent of the well-known effects of " tempering" in metals by repeated blows. On the whole, the elastic properties oiFucus are characterised by tensile strength rather than by extensibility, in this resembling the lignined tissues of the higher plants. Ascophyllum and Laminaria exceed in extensibility the petiole of Aucuba which has collenchymatous mechanical tissue. Halidrys resembles Laminaria in elastic properties rather than Fucus which it resembles in structure. It thus1 seems that the strength and elastic properties of these marine algae reside in the nature of their cell walls rather than in the nature or arrangement of their tissues—possibly in the proportions of mucilaginous material in their composition.

25.

ABSCISS/E_HRS ORDI NATES _ CM S/lOOCM 15 10 20 30 40 50 Fig. 2. Graph showing slow extension in Atcopkyllum nodosum, No. TXb loaded beyond the elastic limits (40 lb. per sq. cm.) for 45 hours. Compare dotted vertical line in Fig. 1.

CONSIDERATIONS OF FORM AND STRUCTURE. Externally Fucus, Ascophyllum and Halidrys are similar in possessing a well- marked discoid hapteron and a much divided, more or less strap-shaped frond, so pliant as to remain prostrate when exposed by the tide. In all three the disk is composed of branching filaments interwoven together, some in the vertical, others in the transverse direction. In Fucus and Halidrys, the disk is compact, but in Ascophyllum the disk is irregular in shape and splits easily in the vertical direction, between the bases of the individual thalli. The structure of the thallus is characterised by a well-developed filamentous medullary tissue; it has been likened to a hempen rope, but it differs from a rope in having numerous filaments interweaving between the vertical strands, the whole being intricate but loosely woven together, giving the great flexibility of the typical surf plant. When one considers how the surf plant is thrown about, twisted and tossed by the waves, it is evident that there must be a strong shearing force developed between the cortex and medulla, and it is here that we may expect the transverse filaments to play an important part. This shearing force tends to increase with greater development of the lamina. It may become very considerable in Fucus, for in 1 The suggestion that the elastic properties arise from the nature of the cell walls was made by Prof. T. G. Hill when the substance of this paper was read to the Society of Experimental Biologists, December, 1931. Experiments with the Stipes of Fucus and Laminaria 313 many of the well-grown specimens used the number of dichotomies varied from 300 to 500, without counting tufts of small outgrowths due to secondary proliferations. Ascophyllum and Halidrys differ from Fucus in combining flexibility with a much greater extensibility. The adult plant of Laminaria differs from most other marine algae in possessing a stipe sufficiently rigid to stand erect when emergent at low water. The stipe is supported at the base by a series of outgrowths or crampons, and these bear slender finger-like extremities which find their way in all directions, fitting accurately into crevices by the unequal growth of the surfaces at points of contact with some substrate. Both crampons and their finger-like extremities become firmly cemented to the substrate by the outgrowth of short hairs with mucilaginous cell walls from all the superficial cells at a surface of contact. These crampons form at successively higher levels after the manner of the successive prop roots which support the stem of Zea mais. Both crampons and stipes are almost entirely parenchymatous in nature, and the filamentous central region of the so-called "trumpet hyphae" forms an insig- nificant part of the adult whole, mechanically speaking. The rigidity appears to be maintained by the thick-walled cells of the cortex, and is peripheral rather than central in disposition. The stipes thus exhibit a mechanical structure suited to resist bending strains, and the development of the characteristic features of the crampons point in the same direction.

SUMMARY. 1. The breaking stress or ultimate strength of a number of algae has been determined by the method of fracture by tension. The magnitude of the breaking stress was not correlated with exposure due to the locality or to the position on the shore. 2. The elastic limits and the extensibility have been determined for Fucus, Laminaria, Ascophyllum and Halidrys in a few cases; and, for comparison, the petiole of Aucuba with collenchymatous and of Aspidistra with lignified tissue. 3. Fucus has relatively the least power of extension and the greatest resistance to stretching of the algae examined. 4. Ascophyllum and Laminaria have much greater power of extension and less power of resistance to it. 5. Attention has been drawn to the long endurance of the base of the lamina of Ascophyllum to a load which caused stretching well beyond the elastic limits. 6. Some quality in the cell wall is invoked to account for (a) the form of break on rupture by tension, (b) the breaking stress, and (c) the characteristics of the elasticity and extensibility. REFERENCES. (1) FISHER, R. A. (1930). Statistical Methods for Research Workers. (2) LLANDOLT and B6RNSTEIN (1923). Physikalische Chemische Tabellen. (3) SCHWENDENER, S. (1874). Das Mechamsche Princip. (4) THOMSON, SIR WILLIAM (1890). Mathematical and Physical Tables, 3. (5) WIIXB, N. (1884). Bidrag til Algernes Physiologiske Anatomi. Kongl. Svensk Vet. Akad., 21, 12,490. I EB- IX lii 20