[Chap. XLVII THE ALGAE 611 



pioneer plants on certain kinds of exposed rock surfaces and are primary 

 factors in the formation of the first layers of soil on such surfaces. 



Algal periodicity. Many fresh-water and marine algae have seasonal 

 periods of germination of spores, vegetative growth, reproduction, and 

 dormancy similar to those of flowering plants. Many species in tem- 

 perate climates attain their greatest development in winter, in spring, 

 in summer, or in the autumn; produce reproductive structures, and sub- 

 sequently disappear. These algae are comparable to annual flowering 

 plants. Their dormant period is passed as spores (Fig. 276). 



Fig. 276. Estimated relative abundance of several seasonal assemblages of 

 green algae in central Illinois. Seasonal assemblages would be very different 

 fartfier north and farther south. Ephemerals include plankton species in which 

 a life cycle is completed in a few days. 



There are also many perennial filamentous, or thick and leathery, 

 species in which at least a part, usually the base of the vegetative plant, 

 survives long periods of drought and cold. Each year new vegetative 

 fronds develop. This group includes many seaweeds, certain red algae 

 in fresh-water streams, and green algae such as Cladophora and 

 Vaucheria. 



Plankton algae and some soil algae are very short-lived. A new popu- 

 lation of these extremely abundant and important species may develop 

 every few days. They are the plants that form the so-called "water 

 bloom" so common on ponds and lakes from midsummer to early 

 autumn. Obviously, enormous numbers result from the accumulative 

 reproduction of several generations in a few weeks. 



Algae are more numerous in seasons when the water levels are high. 

 They also reproduce most abundantly under these conditions. Their 



