IMVASION AND SUCCESSION 285 



4. In layered formations sun plants appear before shade 

 plants, but yield to them, except when they belong 

 to the primary layer. 



5. Excessive seed production and slight mobility lead to 

 the imperfect ecesis of individuals in dense stands, and 

 hence usually produce groups that are temporary. 



6. Each pioneer produces about itself a tiny area of ecesis 

 and stabilization, which may be entered by its own 

 offspring, by the seeds of its fellows, or by invaders 

 from a distance. 



7. Species that propagate by offshoots or produce immobile 

 fruits in small number usually establish themselves 

 readily, because the offspring appear within the area 

 reacted upon by the parent forms. 



V. Stabilization. 



1. The universal tendency of vegetation is toward stabiliza- 

 tion. 



2. The ultimate stage of a succession is determined by the 

 dominant vegetation of the region. Lichen formations 

 are often final in polar and niveal zones. Grassland 

 is the final vegetation for plains and alpine stretches, 

 and for much prairie, while forest is the last stage for 

 all mesophytic habitats. 



3. Grasslands or forests usually terminate successions, 

 hence they are most frequent in regions showing few 

 physiographic changes. 



4. The end of a succession is largely brought about by the 

 progressive increase in competition, which makes the 

 entrance of invaders more and more difficult. 



5. Stabilization radiates outward from the pioneer plants 

 or masses. The movement of offshoots is away from 

 the parent mass. The chances of ecesis are greatest 

 near its edges in a narrow area in which the reaction 

 is felt, but competition is not so intense. 



VI. General laws. 



1. The stages or formations of a succession are distinguished 



as initial, intermediate, and ultimate. 



2. Initial formations are open, ultimate formations are 

 closed. 



3. The numl^er of species is small in the initial stages. It 



