316 Aquatic Societies 



The reason why lenitic societies include practically 

 the entire population of vascular plants has already 

 been stated (p. 145) : the plants have a complexity of 

 organization that cannot withstand the stress of 

 rapidly moving waters. They fringe all shoals, how- 

 ever, and they fill the more sheltered places with growths 

 of extraordinary density. In such places they pro- 

 foundly affect the conditions of life for other organisms : 

 the supplies of food and light and air, and the oppor- 

 tunities for shelter. 



Streams and still waters, inhabited by lenitic societies, 

 may be divided roughly into three categories: 



1. Those that are permanent. 



2. Those that dry up occasionally. 



3. Those that are only occasionally supplied with 

 water. 



These so completely intergrade, and so vary with 

 years of abundance or scarcity of rainfall, that 

 there is no good means of distinguishing between them. 

 Perhaps for the humid Eastern States and for bodies of 

 still water the words pond and pool and puddle convey 

 a sense of their relative permanence. The population 

 of the pond is, like that of the lake, to a large extent 

 perennially active. It will be discussed in succeeding 

 pages. That of the pool is composed of those forms 

 that are adjusted to drouth: forms that can forefend 

 themselves against the withdrawal of the water by 

 migration, by encystment, by dessication, or by bur- 

 rowing, or by sending roots down into the moisture of 

 the bed. Some of these will be mentioned in the dis- 

 cussion of the population of the marshes. The puddles 

 have a scanty population of forms that multiply rapidly 

 and have a brief life cycle. The synthetic forms 

 among them are mainly small flagellates and protococ- 



