280 



PLANT PHYSIOLOGY AND ECOLOGY 



300. Succession in flooded soils. Floods are confined largely 

 to the valleys of streams and to coasts. Flood waters spread 

 out over the lowlands of the valley, forming a flood plain. The 

 effect of the overflow is to destroy or to place at a disadvantage 

 all the plants of the flood plain that are not hydrophytes. At the 

 same time a thin layer of fresh silt is deposited upon the floor 

 of the valley. Mesophytic species are washed away by erosion, 

 or are destroyed by immersion in the water. After a flood which 

 destroys the vegetation, algae, liverworts, and mosses come in 



Fig. 115. The fireweed, Chamcenerium angustifolium , which forms the first 

 herbaceous stage of a burn succession in the Rocky Mountains. 



quickly, as pioneers of the succession. These are followed the 

 same year or the next one by weeds, which after a few years are 

 displaced by the original species. 



301. Succession due to man. Man destroys vegetation 

 through fires, lumbering, and cultivation, as well as by drainage 

 and irrigation, though the latter usually modify the water con- 

 tent merely. Such activities are almost universal. The result- 

 ing successions depend not only upon the cause, but also upon 

 the region in which it operates. Man by his activities originates 



