Fig. 139. Duckweed, with simple leaves and long roots hanging in the water.
In the ditches you will probably find a number of green, thread-like algæ; these may also remain on the mud for some time when they are dried up, and in their case some of the cells at such times get a specially thick coat, and remain living for long. Then, if the water returns, it is again the home of these algæ, which rapidly grow out from their protected cells.
So that you see, even if you had no plants but those in the hedges and ditches to study in their homes, yet you could manage to find many examples of living plants which are trying to fit themselves to their ever-changing surroundings. Those that cannot succeed must die away in that spot, and confine themselves to some other place where the struggle is not too hard for them. All the plants which we find anywhere living together are, therefore, those which are suited to the conditions in that place, and all such plants growing together in this way form what is called a “plant association.”
CHAPTER XXIX.
MOORLAND
Fig. 140. A moorland stream. Notice the low growth of all the plants.