SEED DISTRIBUTION 



167 



usually remain from year to year without great changes ex- 

 cept those which are brought about by human interference. 

 This fact is evidence enough that seeds in unimaginable num- 

 bers must be scattered in such a way as to make good the 

 losses in the plant population of the world due to all destruc- 

 tive causes. The means by which this distribution of the 

 seeds is secured will be taken up in sections 162 and 163. 



FIG. 147. Dandelion fruits 



a, akeiie ; b, beak of pappus ; br, bracts ; p, pappus (representing the limb of the 

 calyx) ; r, common receptacle for all the fruits. Twice natural size 



160. The struggle for existence. Only a small proportion 

 of all the seeds annually produced can have a chance to grow. 

 The resulting contest among plants for a foothold and for the 

 means of subsistence forms one portion of what the great Eng- 

 lish naturalist, Charles Darwin, called the struggle for existence. 

 It is shown by careful calculation that about 5,300,000 acres 

 of land could be sown with the wheat grown at the end of 

 fifteen years from a single parent kernel, if every grain were 

 to grow and live. But the wheat plant does not produce a 

 very large number of seeds. The so-called Russian thistle 



