116 



PLANT BIOLOGY 



to be continued, there would be 243,000,000,000,000,000 morning 

 glory seeds. 1 



It is evident, however, that no pea vine or morning glory plant, 

 if left to itself, would be able to produce anything like the number 

 of seeds we have named, for otherwise at the end of a short term of 

 years there would not be room on the whole surface of the globe for 

 any other kinds of plants than these. As a matter of fact, the num- 

 ber of individuals of a given kind of organism does not vary much 

 from year to year. In the first place, many seeds are eaten by birds 

 and other animals. Again, many other seeds are not carried to a 

 place where they find all the conditions that are essential for germi- 

 nation (118). Still other seeds, even if planted in good soil 

 and in favorable surroundings, fail to germinate. Because of the 

 great losses of seeds in one or the other of these three ways, we can 

 get some idea of the reason why plants must produce a great abun- 

 dance of seeds if their kind is to be perpetuated. 



128. The struggle for existence among plants. But even if 

 seeds finally germinate and get a foothold on the soil, a great many 



FIG. 53. The struggle for existence and the survival of the fittest among 



turnips. 



of the plants thus started will never reach maturity and ripen their 

 seeds. In the first place, each plant is struggling to lift up its leaves 



. l See Bergen's "Essentials of Botany" (1910), p. 202. 



