500 



PEACTICAL BOTANY 



453. Competition and emigration. A little has already been 

 said (Sect. 137) about the severe competition among plants, 

 which often allows only one seed among many thousands to 

 grow into a new plant. This competition frequently tends to 

 cause plants to nourish better in new territory. Other white 

 pines would find it almost impossible to grow up from the 

 seed under adult trees like those of Fig. 321, but seeds blown 



FIG. 373. A wooded river bank invaded by a moving sand dune 

 In the foreground a young cottonwood is being covered by sand 



into neighboring clearings (Fig. 322) or among young birches 

 or other deciduous trees (Fig. 323) may promptly begin their 

 growth into forest trees. In this way there is a constant inva- 

 sion of neighboring species into any territory not already occu- 

 pied by those species or by others which they cannot crowd 

 out of the way. A newly formed island in a river (Fig. 372), 

 the recently drained bed of a lake or a bayou, is promptly 

 populated by the plants which crowd in from adjacent terri- 

 tory. The newcomers may arrive as seeds, or as cuttings, 



