234 Dr. F. E. Fritscb. On the Subaerial and [Jan. 2, 



latter, but from what I have seen I think it probable that certain species are 

 able to nourish better in small isolated pieces of water than in the large 

 expanse of the waters of the tanks, and consequently attain to a degree of 

 development in these pools during the dry season, which is impossible except 

 in such small collections of water. Further, algal spores carried by the wind 

 or other means of dispersal may produce an algal growth in the side-pools, 

 which would never be successful in the main tank. 



The conditions to which the Algae of the pools and ditches are exposed are 

 the same as those above denned for the tanks, although most of them are 

 more emphasised.* Thus, except where shading is afforded by the surrounding 

 trees and bushes, the shallow character of these pieces of water means little 

 or no protection from the strong light, and in the absence of shade algal 

 growth is rarely found. The temperatures attained are very high, and there 

 is a very considerable daily range. The risk of desiccation is great, and 

 drying up or inundation can take place very rapidly. The water is in almost 

 all cases quite stagnant, heavy downpours of rain being the only source of 

 aeration. The conditions of life are on the whole more uniform than in the 

 tanks, the most essential variations lying in the degree of shading and the 

 frequency of desiccation, conditions which are to some extent mutually 

 dependent. 



In the pieces of water under consideration the blue-green element, although 

 still well developed, does not play the same predominant part as in the 

 tanks. The Conjugates are here the dominant group, almost every pool or 

 ditch examined containing a certain and often very considerable amount of 

 Spirogyra,f whilst other Conjugate genera (especially Zygnema, more rarely 

 Mougeotia) are of frequent occurrence, though rarely present in great quantity. 

 The dominance of Spirogyra is very noticeable on comparing the vegetation 

 of these pieces of water with that of the segregated pools round about the 

 tanks, for in the latter Spirogyra only occupies the second place, as in the 

 actual waters of the tanks. It is not easy to understand why the Conjugates 

 play so important a part in the pools and ditches, whilst the Cyanophycese 

 are subsidiary to them. It seems very probable that it is not so much the 

 occurrence of special factors which favours the Conjugate element, but rather 

 the existence of one or more conditions unsuitable to the blue-green element, 

 so that it becomes weakly developed and gives room for the Spirogyra, etc. 



* Some of the smaller shallow tanks come very near to the pieces of water under 

 consideration (cf. footnote on p. 225). 



t The species of Spirogyra in the pools and ditches show the same general features as 

 those of the tanks (viz., simple end-walls and a prevalence of broad forms with many- 

 spirals in each cell). 



