SUCCESSION 



261 



often throw back also the sediment that has been deposited in the sea, the 

 marsh vegetation acting as a filter in both cases. Successions of the kind 

 indicated above are regularly mesotropic. Where the soil is sandy, and the 

 filling-up process sufficiently great, or where salts or humus occur in excess, 

 xerophytic formations result. In certain cases, these successions appear to 

 be permanently hydrostatic, changing merely from floating or submerged to 

 amphibious conditions, but this is probably due to the slowness of the reac- 

 tion. As a rule, the accumulation of plant remains is relatively slight, and 



Fig. 67. Pine forest formation (Pinus-xcrohylium) , stage VI of the 

 talus succession. 



plays an unimportant part in the reaction. In peat bogs and other extensive 

 swamps, the amount of organic matter is excessive, and plays an important 

 role in the building up of the swamp bed. 



320. Succession by enriching the soil. This reaction occurs to some de- 

 gree in the great majority of all successions. The relatively insignificant 

 lichens and mosses prodtice this result upon the most barren rocks, while 

 the higher forms of later stages, grasses, herbs, shrubs, and trees, exhibit it 

 in marked progression. The reaction consists chiefly in the incorporation 

 of the decomposed remains of each generation and each stage in the soil. A 

 very important part is played by the mechanical and chemical action of the 



