PLANTS of the sea have one great problem to solve : having found a suitable place, to stay there; a plant needs to be attached to a rock and to allow the water to flow over it. A drifting plant just moving with the current is, for all practical purposes, in stagnant water and soon uses up the food supplies in its immediate neighbourhood. It is easy enough to hold on in quiet water, but an exposed coast is an impossible locality for plants without a strong and efficient holdfast system. We find that the finest and largest plants in the sea are at a level where light and atmospheric gases are plentiful. The associated disadvantage is wave action, which will tear weak plants from their moorings and break up those which are not flexible enough to stand the constant washing to and fro. An alternative equipment is that of growing close to the sea-bottom, either as a skin on rocks or stones or in moss-like masses on which the surf beats harmlessly. The family of the kelps, which includes the longest plants we know, is represented on the shores of the British Isles by a number of species. They are found within the influence of wave-action at and below low-water mark and are, characteristically, large, slippery, flexible plants with a mass of strong-like branches by which they are attached to rocks and stones. It is no uncommon thing to find a dense growth of a single species with no other large plants able to compete successfully at that tide-level.
These and other sea-plants are subject to a further effect of their method of life and that is weighing their moorings. We might perhaps say ‘take up ‘instead of using the nautical term, but we should gain nothing in accuracy and lose a graphic metaphor. What commonly happens is that a sea-plant grows until it has acquired sufficient buoyancy to float the rock or stone to which it attached itself in infancy. It is then subject to some of the disadvantages of a drifter. If the currents carry it off-shore it may lose enough buoyancy, by decay or other damage, to sink in deeper water than that in which it has grown. This is not necessarily fatal : there may be enough light in the new habitat for the plant to grow up to the surface again. This is the sort of thing which happens to the giant kelps in the Antarctic Ocean. Plants of something like a thousand feet long are reported in those conditions.
It may be remarked here that we ought to be very wary of considering a change as advantageous. At first sight we may be tempted to think that there is an advantage in being floated to a place where it is possible to grow very big; but the increase in size is the price of survival in the deeper water. We do, very readily, think of survival as an advantage in itself, but the notion of advantage gained from our observation of human affairs is apt to be misleading when transferred to other species of living organisms. In any case, the habit of weighing moorings is likely to be occasionally fatal when the currents are on-shore instead of off.