15] VEGETATIONAL TYPES OF FRESH WATERS 489 



vast distances, often involving great changes in ecological conditions 

 and even in climate in different parts of the course. The duration 

 of this transport, depending on speed of current and length of river, 

 determines whether a true potamoplankton is developed ; for most 

 of the planktonic organisms that get carried along in many streams, 

 and particularly in those containing lakes, are merely ' tychoplankton ' 

 washed in from other habitats and doomed to an early death. In 

 effect, a potamoplankton can be developed only when conditions are 

 suitable for the growth and reproduction of ' selected ' species from 

 among those which are washed in. True potamoplankton is thus 

 limited to slowly-flowing rivers of considerable length where the 

 water takes some weeks to reach the sea. On and very near the bottom 

 of these there is often scarcely any current ; consequently conditions 

 and communities approach those of standing water. Even where 

 the flow is rapid it is remarkable how many free-living Algae such as 

 Diatoms and flagellates are to be found among macroscopic benthic 

 vegetation which is able to benefit by improved conditions introduced 

 by the current {see below, especially p. 498). 



The planktonic and other aquatic vegetation of ephemeral pools 

 and puddles is extremely various, depending as it does not only on 

 local conditions but, very largely, on chance dispersal. Quite often 

 an extraordinarily rich and seemingly uninhibited development of a 

 single species is found. In saline lakes, fewer and fewer types of 

 organisms persist as salinity increases above that of the oceans (3-4 

 per cent.), but those which do exist may occur in abundance even 

 at very high concentrations (16-20 per cent.), provided of course no 

 lethal salts are present and the temperature is not too high. 



Cryophytic Communities 



The communities developing on snow and ice are in some ways 

 akin to those found in ephemeral pools, which indeed may result 

 from the melting of snow or ice and, initially at least, often harbour 

 the same species. Thus phases of the common arctic and alpine 

 Red Snow Alga Chlamydomonas {Sphaerella) nivalis are frequently 

 abundant in pools of snow-water. This ' species ' may represent a 

 complex of several different ones which undergo a wide range of 

 intergrading morphological changes through growth under different 

 ecological conditions. 



In general, the ' cryovegetation ' of snow and ice is greatly in- 

 fluenced by the physical and chemical characteristics of the medium, 



