362 THE ALGAE 



3. Associations of ponds and ditches 



The flora exists under very varied conditions with a regular or 

 irregular periodicity. In the temperate regions Protococcales, 

 Zygnemaceae (dominant in spring) and diatoms (dominant in 

 winter) form the chief elements. There is usually not enough aera- 

 tion to permit the larger filamentous forms to be present, and for 

 this reason the ponds and ditches can be divided into: 



(a) those containing Cladophoraceae, which suggests that the 

 aeration is good; 



(b) those without Cladophoraceae. The substrate and fauna are 

 also important factors in determining the type of vegetation 

 to be encountered. The flora of tropical ponds contrasts 

 sharply with that of temperate regions for there is 



(i) an excess of Mxyophyceae; 



(2) the poor aeration results in a relative scarcity of Clado- 

 phora and Rhizoclonium together with the epiphytes 

 associated with them, and their place is taken by Pitho- 

 phora; 



(3) a scarcity of Vaucheria, Oedogonium, Xanthophyceae 

 andUlotrichales; 



(4) an abimdance of filamentous desmids together with 

 Spirogyra. 



In America Transeau (19 13) concluded that fresh-water pond 

 algae can be divided into seven classes based on abundance, dura- 

 tion and reproductive season, these classes and their periodicity 



Summer annuals 



Autumn annuals 



• -^. Perennials 



JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC 



Fig. 199 Chart showing the estimated relative importance of the 

 different types of algal periodicity throughout the year in the waters 

 of E. lUinois. The irregulars are not depicted. (After Transeau 



from West.) 



