15] VEGETATIONAL TYPES OF FRESH WATERS 499 



numbers and variety do, remain protected from the current in tufts 

 of Mosses or Algae. 



The nature of the substratum is, as elsewhere, of importance in 

 determining the benthic vegetation of streams and rivers. In 

 general a firm, stony or rocky substratum prevails in rapidly flowing 

 water, because finer particles are washed away, whereas in lentic 

 stretches silty deposits are common and usually copious. Here the 

 rooted vascular plants so important in the hydrosere (as described 

 in the next section) are often prevalent, and the benthos under 

 conditions of slower and slower current approximates more and 

 more closely to that of lake margins. 



Temperature may also have a marked influence on the benthos 

 of running as well as of still waters. Thus, for example, the tempera- 

 ture often varies markedly in difl^erent parts of the same stream, 

 as well as, of course, in the same place at different seasons — com- 

 monly with attendant floristic differences. Accordingly, even in 

 small mountain brooks, the upper part may be dominated (to quote 

 a European example) by Hildenbrandia and the lower reaches by 

 another Red Alga, Lemajiea, while the vernal period may be char- 

 acterized by the dominance of Diatoms and the aestival by Green 

 Algae, followed by a winter reappearance of the Diatoms. In lakes 

 and ponds, also, there are often three or four seasonal aspects to 

 be distinguished. Lastingly cold springs from deep rock-strata are 

 apt to constitute refugia of cold-stenothermal species, while warm 

 springs lack such types and usually include in their flora specific 

 megathermal forms. Thus whereas at the lowest so-called ' thermal ' 

 temperature (30°-35° C.) almost all groups of Algae as well as Mosses 

 and flowering plants are commonly present in favourable alkaline 

 waters, it is only up to about 38° C. that Green Algae survive, and 

 up to 45° C. at the highest that the last Diatoms persist. Above 

 this temperature only Cyanophyceae continue from among these 

 groups, though they persist in fair numbers up to 55° C. A very 

 few species have been found growing above 60° C, but none for 

 certain above 69° C, to which temperature Synechococcus elongatus 

 appears able to survive. Bacteria, however, can withstand much 

 higher temperatures, and have been found living in thermal waters 

 up to at least 77*5° C. (In a dormant state some Bacteria can 

 survive one or two boilings at 100° C., and certain Yeasts are capable 

 of enduring a temperature of 114° C.) 



A little needs to be added on the subject of algal epiphytes (that 

 is, Algae growing attached to other plants). These, in fresh waters, 



