CHAPTER XXXII.

IN THE SEA

All the plants which grow in the sea are hastily grouped together by most people under the name “seaweeds.” We know that there are many kinds of seaweeds, and yet even to one who has not studied them, they do not seem to differ so much from each other as to deserve special classes. And this general view is quite a correct one, for with very few exceptions, all the plants which actually live in the sea are algæ, and so belong to the simplest family of plants (~see~ Chapter XXVII.). Yet they are not without interest and individuality. In the sea these simple plants have everything to themselves; and it is there that we get them developed in a very special way.

You must have noticed that you never find seaweeds actually rooted in the sand (except in protected marshes, where the sea samphire and some flowering plants may grow), because sand is always shifting and being churned up by the waves, so that they cannot get a firm hold. This is almost the same on the pebbly shores where the stones are rolled over by the waves, and so would batter any unfortunate plant growing on them. If you go along a rocky coast at low water, however, you will find countless true seaweeds, growing so thickly that the rocks are covered by their slimy masses, while in the rock pools are beautiful tufts of more delicate seaweeds of all colours (~see~ Plate VII.).

Examine a single plant of bladderwrack or fucus, and pull it up if you can. You will find that it is very slimy and slips out of your fingers, and then, that when you have got a firm hold on it, it sticks so fast to the rocks that it is difficult to get it off without breaking it. Does this mean that it has roots which go right into the rock as the roots of land-plants go into the soil? Find a plant growing on a small stone, if possible, and look closely at it; the “root” does not go into the stone at all, but is much divided and clasps round it, bending into every little crevice and sticking tight. Note, too, that there are no root hairs as there are in land-plants, which is natural enough when the whole plant is growing in water, and can therefore absorb it through all its surface. All that is required from the “root” is that it shall hold firmly on to the rocks and keep the plant from being dashed on to the shore by the waves. The “root” is not a true root, but is really only a part of the simple body, which is specially adapted for attachment.

The many large bladders on the plant are filled with air, as you will see if you split them open, and they help to buoy it up in the water. Notice, too, how flat the whole plant is; it is really a single sheet of tissue or “thallus,” which is much divided, but does not branch in many directions as a land-plant does. All these characters are those of the simple family of algæ, to which all the seaweeds belong. Though in some cases they may form what look like very complicated structures, yet they are always built upon these simple lines.

Often you may find little plants growing on the bigger ones; sometimes a well-established weed may be almost covered by small seaweeds of many kinds, brown, green, or red. These attach themselves to the big plant in much the same way as they would to a rock, but only use it as a place of anchorage, and do not tap its food supply, as the parasitic mistletoe does to the land-plants. In the same way you may find numbers of seaweeds planted on shells or growing on the backs of crabs.

As the tide goes out it gradually exposes the rocks and pools with their innumerable inhabitants. Now in the case of those which are first uncovered, a long time must pass before the water returns, while those quite near the low water level are only uncovered for a little while. Follow the falling tide some day, and look for the effect which this difference (in the time for which they are exposed) has on the plants growing at different depths.

As you go out towards the low water mark you will find first and commonest the bladderwracks, which get more luxuriant where they are a little removed from the region of the pounding waves at the actual shore. Then further out you will find that the bladderwrack gives up its place to another plant very like it, but with more jagged margins. Beyond this you will come to the big strap-shaped laminarias, which never grow where they are very long exposed without water (~see~ fig. 149).

These different regions of seaweeds (some of which are only laid bare by the tides which go very far out) really depend on the fact that the different levels of the shore are left exposed for varying lengths of time according to their depth. If the shore is flat or gently sloping, then the tide has a very great distance to recede before the same ~depth~ is reached as would be attained much nearer in where the shore slopes steeply (~see~ fig. 150). This explains how it is that in one place you may have to walk out a quarter of a mile till you come to the region of laminarias, while in another you need walk no distance, but merely clamber down the rather steep rocks to get to it. But as the actual time taken by the falling tide is the same in both cases, the plants at any level are left exposed for almost the same time whatever the kind of shore may be.

One thing that may perhaps puzzle you about the seaweeds is their colour; some few of them are green, but most are blackish, brown, or even red. How then do they build their food? It is found that true chlorophyll is present as well as the other colours, and that though they hide the green tone from our eyes, they do not hinder its activity in the plant. You can see that the brown bladderwrack is really a green plant if you soak some of its tissues in hot water; the brown colour will be washed out and will leave the plant bright green. In almost all cases these simple algæ living in the sea are self-supporting plants, which have adapted themselves to the special conditions in the depths of the sea where no flowering plants can live, and there they reign supreme.