AQUATIC VEGETATION. 623 



than in arctic or antarctic regions. At the mouth of rivers and 

 streams, in salt marshes, and in some inland salt seas which 

 have a large influx of fresh water, it is "brackish," i.e., slightly 

 saline. 



lllOa. Movement of water also affects the form and character 

 of aquatic plants. In quiet fresh water the plants are different 

 from those in streams. Ebb and flow of tide show their effect 

 on pelagic forms. The physical condition of the substratum, 

 whether rock or soil, affects the benthonic vegetation. Aside 

 from the effect of salt water, the flora of fresh waters is in- 

 fluenced accordingly as it is rich or poor in calcium carbonate. 



lllOb. Temperature exercises considerable influence on the 

 distribution of water plants, but is not so important a factor as 

 it is on land because it is more equable. Most of the perennial 

 pelagic algae show no period of rest; usually the summer is the 

 growing period and winter the fruiting period, though there 

 are a number of exceptions to this. A greater difference is 

 manifest in the inland lakes and ponds, especially in the cool 

 temperate regions, where the formation of ice interrupts the 

 growth, the perennial benthonic forms passing through a period 

 of rest for their shoots, and the annuals and plankton passing 

 the period through seeds, spores, hormogones, akinetes, or re- 

 sistant vegetation parts. Many bacteria and some algae will 

 live enclosed in ice for considerable periods. Certain benthonic 

 algae (Lemanea) in fresh-water streams pass both the growing 

 and reproductive period in winter often under ice, and rest 

 during the summer period. Warm currents exercise an in- 

 fluence on the distribution of pelagic algae, but not so markedly 

 as they do on land plants. 



1111. Inland salt ponds and lakes. Since, in a number of 

 the inland salt lakes and seas, the percentage of salt is very high, 

 plant life is more restricted than in the ocean, and in some cases 

 is prohibited. But at the intake of fresh-water streams the 

 salinity would be reduced and permit the growth of certain 

 halophytes. The vegetation of the saline waters of the Great 

 Salt Lake in Utah has not been carefully studied with reference 



