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, fre- quently 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 sili- quosa 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 sur- face 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 sur- face 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.

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