INVASION AND SUCCESSION 



271 



face offers good conditions for germination, and there is no com- 

 petition to decrease the chances of ecesis. Open formations are 

 readily invaded, since competition is slight, and physical factors 

 rarely extreme. On the other hand, closed formations reduce 

 invasion greatly, owing to the intensity of the competition which 

 newcomers must meet. 



Invasion takes place by the entrance of new individuals. The 

 manner depends largely upon the nature of the seed or fruit. 

 The invasion of ^vind-distributed species with comate or winged 

 seeds or one-seeded fruits is by individuals as a rule. jMigrants 



Fig. 109. Invasion of the shore of a lake, ahnost whollj' by rosette plants 

 which have migrated from the mass in the background. 



in which the disseminule is a many-seeded fruit or jjlant tend 

 to produce a group of invaders. The accidents of migration some- 

 times bring a few separate seeds together into one group, or scatter 

 those of a many-seeded fruit, but these are relatively rare excep- 

 tions to the rule. This difference in the manner of entrance is 

 an important factor in alternation, and is the basis of the two 

 types of abundance, i.e., copious and gregarious. 



