290 



PLANT PHYSIOLOGY AND ECOLOGY 



together to produce it. Migration carries seeds into the differ- 

 ent areas of a formation or into different formations, with little 

 respect to the physical nature of these. The physical differences 

 between the various areas causes some of the migrants to be 

 established in some places and not in others. Other migrants 

 establish themselves in another series of ike areas, and so on. 

 Competition often hmders or prevents the ecesis of a species in 

 an area in which the physical factors are otherwise favorable, 

 and thus produces alternation independently. Invasion frequently 



Fig. 118. Alternation of spruce forests and gravel slides in Engelmann 



Canon above Manitou. 



carries plastic species into a number of dissimilar habitats, or 

 causes one species to appear in one or more similar areas, and a 

 similar or corresponding species in the remaining areas of the 

 series. In new or denuded areas invasion and physical factors 

 act together to produce alternation, while in closed vegetation 

 competition is a factor which operates independently, or in con- 

 junction with the habitat. 



313. Alternation due to ecesis. When seeds are carried into 

 a number of more or less diverse areas or formations, alternation 



