Figure 1-9. Variations in size and shape 

 of cones and in scale characters are found 

 in natural stands of slash pine {Pinus 

 caribaea), the state tree of Alabama. 

 Florida. (U. S. Forest Service.) 





C. pennsylvanica produce more than 60 per cent of their vegetative 

 growth by the end of July. It is evident that during the evolution 

 of grasslands in the Great Plains, natural selection has resulted 

 in producing a high degree of adaptiveness to rigorous and vari- 

 able environmental conditions. 



Within a single stand, even a very homogeneous one, individ- 

 uals of each species probably differ slightly in their minimum re- 

 quirements of water and other substances and in their ranges of 

 ecological amplitude. Moreover, variations in environmental 

 conditions produce microhabitats in even the most uniform 

 stands. The population of a species within a given microhabitat 

 may belong to one genotype, constituting a biotype, while neigh- 

 boring microhabitats contain other biotypes. In this way the 

 various clusters of biotypes, within one species, occupying a par- 

 ticular kind of habitat, constitute an ecotype.^°^ The number and 

 kinds of biotypes vary according to kinds of species and habitats, 

 so a remarkable genetic diversity exists in both natural and cul- 

 tivated populations^^ (Figures 1-9 and 1-10). A number of trans- 

 plant experiments have shown that the relationship of the eco- 

 type to conditions of the original habitat is close, and so long as 



24 



Species azmd PopialatiozKS 



