THE FRESH- WATER ALGAE 121 



which it exists, one notes ecological relations among the algae 

 quite as much as among higher plants. The forms which may be 

 found are determined very largely by the nature of their environ- 

 ment, and many of them cannot be transferred from one set of 

 conditions to another. A large number of species which live sub- 

 merged in water soon perish if subjected to the atmosphere, while 

 others, such as the common Pleurococcus vulgaris, which normally 

 live exposed to the air, are never found in water. A few forms, 

 such as Chlorella, Stichococcus and Hormidium, may adapt them- 

 selves to either environment, and are very widely distributed 

 under the different conditions where algae are found. 



As all forms are dependent on moisture, the geological formations 

 which determine the amount of moisture must determine the 

 algal flora of any region. In localities where there are large tracts 

 of level land without elevations and depressions, the algal flora is 

 extremely meager; while in a hilly country where the water accu- 

 mulates in depressions of the earth this flora is abundant, certain 

 forms such as Stigeoclonium, Draparnaldia, and Batrachospermum 

 preferring the rapidly-running water of streams, while the larger 

 number of species choose the quieter water of ponds and lakes. 



From early spring to late fall the algae are most numerous, but 

 they are also found in winter, even in the vegetative condition, as 

 many are not injured by freezing. In these cases the chief effect 

 of cold upon them is simply a retarding of growth and of repro- 

 duction. But while some forms are found at all seasons, differ- 

 ent forms predominate at different times, some species being most 

 abundant at one period and others most abundant at another. 

 It does not follow, however, that the same date in successive 

 years will find the same form predominating. Within certain 

 limits the flora of a body of water is constantly changing, due 

 probably to changes in temperature, light, and nutrition, or pos- 

 sibly to other causes too obscure to detect. 



Usually the littoral region supports a number of filamentous 

 algae. Cladophom is one of the most frequent and is especially 

 abundant in regions where wave action is strong and the current 

 great. On the other hand, if the water be shallow and exposed to 

 the direct rays of the sun, such forms as Spirogyra, Zygnema, Oedo- 



