Even those plants which, like the water marestail, can stand up by themselves some way out of the water, yet have softer stems than most land-plants, and far fewer well-developed “water-pipe” cells, because they are so surrounded by water that they can get it easily. Both these plants and the water-lilies, as well as many others, store air rather than water in their stems, and often the spaces in the meshes of the stem-tissue are filled with air, which acts both as an air reservoir and a buoy to float the leaves. We find all through the plant-world that the structure of a plant depends very much on the kind of conditions under which it is living, and in the case of those growing in the water, it is quite clear how the soft, air-filled stems are one result of their mode of life, and are well adapted to it. [Illustration: Fig. 142. Duckweed, with simple leaves, and long roots hanging in the water.] In the ponds you will often find that the duckweed grows in large masses on the surface. Each plant seems to consist of but one leaf and a slender root about an inch long, hanging freely in the water. Sometimes two or more of the leaves are attached and form a little cluster, but it is exceedingly rare to find the duckweed in flower. Simple as it is, almost suggesting the algæ rather than the flowering plants by its general appearance, yet the duckweed is really a flowering plant. It is, in fact, one of the very tiniest of flowering plants which are known. Floating with the duckweed are frequently many fine, thread-like algæ, sometimes quite free, and sometimes attached to stems or rocks. They are very delicate, unprotected plants, their whole body consisting of simple rows of cells. Notice how their feathery tufts cling together in a close mass when they are taken out of the water; they require its support and protection to enable them to live. [Illustration: Fig. 143. Creeping rhizome of the Bulrush, which pushes out towards the middle of the pond.] There are many plants growing round the borders of the pond, half in and half out of the water, such as the reeds and sedges, irises and the tall marsh buttercups. Watch how these plants gradually grow further and further in towards the middle of the pond. They advance with their creeping underground stems (~see~ fig. 143), and collect mud, dead leaves, and stalks around them, gradually building up a little firm soil round their roots. Slowly these accumulations from different plants meet, and the whole gets more compact, till the plants from the shore which require soil are able to grow with them. [Illustration: Fig. 144. A water channel grown over by floating plants and the advancing reeds and rushes.]