ABSTRACT
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.
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. Cloustoni. I am indebted to Dr V. M. Grubb for the identification of the doubtful cases.
I have to thank Miss Astrid Karlsen of Bergen for help with the relevant matter when she was in England.
Excepting in the case of Halidryt.
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.
This is rather more than the value given for cast lead, viz. 484 lb. per sq. cm., according to Sir William Thomson (4).
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.
