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. 
AW : 
wr Se 
b 
Y= 
"a 
Fic. 147. Dandelion fruits 
a, achene; 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 
