432 Ecology 



Sometimes members of the Cladophoracese occur, more especially attached 

 to rocks and stones. 



The Zygnemacese are fairly common, Mougeotia being more abundant 

 than either Spirogyra or Zygnema. In the upland and alpine lakes and tarns 

 of the British Islands, in which species of Mougeotia and Zygnema abound, 

 the maximum vegetative abundance of these genera usually occurs in the 

 late summer and early autumn as the temperature is gradually declining 

 (G. S. W. '09 B). In the littoral alga-flora of the alpine lakes of the Pike's 

 Peak region, Colorado, Shantz ('07) also records the maximum abundance of 

 species of Mougeotia and Zygnema in September, when the temperature is 

 falling. In these habitats spores are only rarely produced, the winter season 

 being passed in the form of ' cysts.' In the tropics species of both the above- 

 mentioned genera are less abundant than in temperate areas. On the other 

 hand, certain exclusively tropical genera are known to exist, such as Temno- 

 gametum (fig. 212) and Pyxispora (fig. 216 AG). 



The desmid-flora in pools and lakes is chiefly dependent upon the 

 geological formation. It is very poor in lakes situated on the newer 

 formations and correspondingly rich in those on the older formations. This 

 is strictly true of western Europe and there is much evidence that the same 

 is true of other parts of the world (W. & G. S. W. '09 B ; G. S. W. '09 B). If 

 there is little contamination of the water, that is, if the water is very pure, 

 with only a small amount of dissolved salts, desmids are as a rule fairly 

 numerous, but the abundance of species depends upon the geological formation 

 and the species themselves are dependent to some extent upon the altitude. 

 It would serve no useful purpose to mention specifically any of the desmids 

 which occur in the benthos of lakes. There are about 2000 of them and 

 those of the tropics are for the most part very different from those which 

 occur in subtropical and temperate areas. 



Many of the Protococcales are found amongst the aquatic macrophytes 

 of the benthic region of pools and lakes. Various species of Oocystis, 

 Scenedesmus, Ankistrodesmus, Sorastrum, Pediastrum, etc., are frequent 

 and often abundant. 



The diatoms are largely dependent upon the amount of the dissolved salts 

 in the water and they are in general most abundant in the spring. The 

 zig-zag chains of Tabellaria flocculosa, T. fenestrata and Diatoma elongatum 

 occur sometimes in quantity, and Eunotia pectinalis, E. lunaris, various species 

 of Gocconema, Navicula, Gomphonema, etc., are often abundant. 



Peridinium inconspicuitm has a world-wide distribution. 



Of the Blue-green Algae several species of Anabasna occur among aquatic 

 macrophytes at the margins of pools and lakes, and Merismopedia glauca 

 is frequent. In alpine lakes species of Calothrix and Stigonema are common 

 on the rocks of the shore. Some of the Blue-green Algae, such as species of 



