PLANT ASSOCIATIONS 445 



SUMMARY 



The vegetation of the earth's surface is not homogeneous but is 

 composed of units of varying composition which are termed associa- 

 tions. These units of vegetation have apparently originated, as new 

 associations arise to-day, in regions where the land is partially or 

 wholly deprived of its vegetation by natural or artificial means. 

 Some of the main factors concerned in the origin and development 

 of a new plant association are the migration of plants by means of 

 reproductive parts, the adaptation of plants of different kinds to 

 different environmental conditions, and competition between the 

 individuals of a growing plant population. Plant associations have 

 also been found to be constantly changing, owing to competition and 

 to changes in habitats due to natural and artificial causes. The gen- 

 eral instability of existing vegetation is the natural outcome of these 

 various factors, which are in continual operation over large areas of 

 the earth's surface. 



