284 PLANT PHYSIOLOGY AND ECOLOGY 



mations the reaction of the taller plants in reducing the sun- 

 light to shade, and in modifying humidity, temperature, and 

 wind, determines the final course of the succession. 



306. The laws of succession. The following arrangement of 

 the principles which govern succession furnishes a helpful sum- 

 mary of the course of development. 



L Cause. The initial cause of a succession is the formation or 

 appearance of a new habitat, or a striking change in an 

 existing one. 



II. Reaction. Each stage of a succession reacts upon the 



habitat in such a way as to produce conditions more or 

 less unfavorable to itself, but favorable to the invaders 

 of the next stage. 



III. ^Mobility and nearness 



1. The pioneers of a succession are those species near at 

 hand, or most mobile. 



2. The number of migrants from any formation into a 

 habitat varies inversely as the square of the distance. 



3. The pioneers usually come from two or more different 

 formations, since most formations contain some very 

 mobile species. 



4. The plants of the initial stage are usually algse and 

 fungi, because of their minute spores, composites and 

 grasses on account of their very mobile fruits, or ruderal 

 plants owing to their large seed production. 



IV. Ecesis. 



1. The migrants into a new, denuded, or greatly modified 

 habitat are sorted by ecesis into three groups: (1) those 

 that are unable to germinate or grow, and soon die; 

 (2) those that grow normally under the conditions 

 present; (3) those that pass through one or more of 

 the earlier stages in a dormant state, and appear in a 

 later stage of the succession. 



2. Whenever ruderal plants are present, they furnish a 

 large number of the pioneers, on account of their ready 

 ecesis. In other regions subruderal native species play 

 this part. 



3. Annuals and biennials are characteristic of the earlier 

 stages of secondary successions, on account of their 

 great seed production and ready ecesis. 



