CHAPTEE XXXIV 

 SOCIAL HABITS OF PLANTS; COMPETITION AND INVASION 



422. Social habits. Those plants which live associated with 

 many individuals of the same species are called social plants. 

 Those kinds which are not social usually occur as members of 

 plant communities, or assemblages of two or more species. The 

 vegetation of the earth mainly consists of such assemblages, and 

 the total number of solitary plants is comparatively small. 



Adult seed plants are usually incapable of locomotion, and 

 only a small proportion of all the kinds of seeds (though a some- 

 what larger proportion of fruits) is equipped with means for 

 carrying them on long journeys. It is therefore natural that 

 the offspring of any plant or plant community should generally 

 be found near the parent plants. It is not easy to trace the 

 working of this gradual spread of the successive broods in 

 the neighborhood of the parents where there is already dense 

 vegetation. But in any region where there are considerable areas 

 destitute of any given vegetation form, as in cleared land, the 

 young seedlings of an oak, a hickory, or a black walnut may 

 often be detected in many places near the parent tree. 



423. Competition. Every one knows, in a general way, that 

 in a state of nature plants often greatly crowd each other. This 

 is evident enough from mere inspection of most meadows, 

 thickets, or tracts of woodland or waste land ; but in order to 

 realize how few of all the bidders for each square foot of ground 

 actually find a chance to occupy it, a little calculation is needed. 

 A single annual seed plant usually matures hundreds and often 

 thousands of seeds. One common weed of the Middle West, the 

 Russian thistle 1 (Fig. 340), often produces as many as 25,000 



1 Sdlsola Kali var. Tragus. 

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