To the general observer, plants seem to be distributed in a promiscuous and haphazard way, without law or order. This is because he does not see and consider.
The world is now full of plants. Every plant puts forth its supreme effort to multiply its kind. The result is an intense struggle for an opportunity to live.
Seeds are scattered in profusion, but only the few can grow. The many do not find the proper conditions. They fall on stony ground. In [Fig. 250] this loss is shown. The trunk of an elm tree stands in the background. The covering of the ground, except about the very base of the tree, is a mat of elm seedlings. There are thousands of them in the space shown in the picture, so many that they make a sod-like covering which shows little detail in the photograph. Not one of these thousands will ever make a tree.
Fig. 250. A carpet of young elms, all of which must perish.
Fig. 251. A plant society waiting for the spring.
Since there is intense competition for every foot of the earth's surface that is capable of raising plants, it follows that every spot will probably have many kinds of plant inhabitants. Plants must live together. They associate; they become adapted or accustomed to each other. Some can live in shade; they thrive in the forest, where sun-loving plants perish. Others prefer the sun, and thereby live together. There are plant societies.