274 



PLANT PHYSIOLOGY AND ECOLOGY 



habitat so that new migrants find increasingly favorable con- 

 ditions. These in turn attain their maximum, and cause the 

 gradual disappearance of the species of the first stage. At the 

 same time they prepare the way for the plants of the succeeding 

 stage. This successive replacement of one set of plants by another, 

 which gives the name to this process, is due partly to the reaction 

 of the plants upon the habitat, and partly to the competition 

 between them. The close competition between individuals of 

 the same or similar species causes them to displace each other, 



'_r",'.^?'t?i r*-.-,-.-. 



Fig. 110. Stages of a primary succession. Owing to the partial breaking 

 down of the rock, four stages which ordinarily follow each other are 

 here found side by side. The solid surface is covered with crustose 

 hchens, the crumbling edge shows foliose lichens, beyond these are 

 mosses, and in the new soil mat and rosette herbs are appearing. 



though they at the same time permit the entrance of invaders 

 of a different character. 



293. Kinds of succession. A particular succession is started 

 by the physical or biological disturbance of a habitat or forma- 

 tion. With respect to the initial cause, successions are normal or 

 anomalous. A normal succession begins with a bare habitat, and 

 ends in a stable formation. An anomalous succession is one in 

 which the facies of the usual final stage are replaced by other 



