All these forms must remind you of the plants which were characteristic of dry regions; how is it that these plants, often actually growing in the water, should yet be specialised in the same way? It is because all the water they get is salt, and it is very difficult for them to live in it. They can only use a relatively small quantity, otherwise they would be forced to take in too much salt, so they must prevent their leaves from transpiring much and using the water up. In this way they are really in the same kind of position and so require to have the same kind of leaves as a plant growing where very little water of any kind is to be had. They are in the same difficulty as the Ancient Mariner, with “Water, water everywhere, nor any drop to drink.” Pull up a marsh samphire, and you will see that it has a very much branched, spreading root, which gives the plant a firm grip on the sand or mud, but it has not long roots like the sand-dune plants, for all the water which it can use is to be had quite easily and is near at hand. You may notice, too, on these mud flats the mingling of plants from land and sea. When the marsh samphire and sea-daisy invade the flats which are covered every day by the tide, they are entering the region of the sea-plants, and you may find them growing side by side with the true seaweeds, and even in some cases we may notice the bladderwrack seaweed further in toward the shore than the samphire, which has ventured far out to sea. As you will find in everything in nature, it is always difficult to draw a fixed line and say that on one side lies one type of thing, and on the other side something different; so, in dealing with different “plant associations,” we find that they have their special regions, but that they tend to cross over any limiting lines set between them. In deep water and on high, dry land, we find quite different kinds of plants which never mix with each other, but on the border land between such regions the boundary is not strictly kept, and we sometimes find plants growing where we might expect the conditions to be unsuited to them. [Illustration: PLATE VII. BLADDERWRACK GROWING ON THE ROCKS EXPOSED AT LOW TIDE.] 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.