334 FRESH-WATER ECOLOGY 



for hot springs that contain Hving plants {Phormidium laminosum) 

 is 87-5° C. 



In slow-moving rivers there is a definite Potamoplankton 

 divided into : 



(a) Eupotamic, thriving in the stream and its backwaters. 

 {h) Tychopotamic, thriving only in the backwaters. 

 (c) Autopotamic, thriving only in the stream. 



(2) Associations of bogs and swamps. 



These are very mixed associations with little or no periodicity, 

 probably because of the relatively uniform conditions. Zygne- 

 maceae, desmids and diatoms are most frequent, the desmid 

 element changing considerably with altitude and type of substrate, 

 whilst the presence of Utricularia apparently also increases the 

 number and variety of the desmid species. 



(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 

 aeration 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 ; 



[h) 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 Cyanophyceae ; 



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

 and Rhizoclonium together with the epiphytes associated with them, 

 and their place is taken by Pithophora\ 



(3) a scarcity of Vaucheriay Oedogonium, Xanthophyceae and 

 Ulotrichales ; 



(4) an abundance of filamentous desmids together with Spiro- 

 gyra. 



