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.
[Illustration: Fig. 149. The Laminarias, which are only exposed at quite low water.]
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.