— 80 — 



early autumn months were very rainy, no such autumn-phase 

 occûrred. The same explanalion may apply to 1908, in which 

 however the pond did contract very mueh for a brief period in 

 August ; it is noticeable that in tins year Gloeocystis was pré- 

 sent in November in small quantity (although not observed in 

 October or December), this coinciding with a very low rainfall 

 and a higli température. 



In the case of Ineffigiata there appears to be some évidence 

 that it is not very sensitive to concentration of the water when 

 exposed to summer-temperatures, but that it is more vitally 

 infinenced by such a factor when the température is low. Thus, 

 the complète absence of this genus in the first four months of 

 1907 and 1909 appears to correspond to the low rainfall during 

 this period (considerably less than in 1906 or 1908), indicating 

 a préférence for dilater water. This does not harmonise with 

 the fact that Ineffigiata is little affected by the practical 

 dryingup of the pond in September 1907. There is some évi- 

 dence however, which is considered more in détail on p. 97, 

 that an Alga having attained its maximum under favourable 

 conditions is not ahvays at once adversely affected by unsui table 

 conditions. If Ineffigiata was really favoured by concentration 

 of the water, one would expect some kind of maximum in April 

 or May of 1906 like that shown by Eadorina and Gloeocystis, 

 but this is far from being the case. It may be added that the 

 relative abundance of Ineffigiata in Mardi of 1906 corresponds 

 to exçepiional rainfall in January and February of that year. 



The above considérations on the occurrence of the more im- 

 portant gênera of Protococcales in Barton's pond show a rather 

 marked relation between tlicir periodicity and the prevailing 

 températures, as well as the concentration of the water. Ail 

 of fchem favour the warmer months of the year, but in all(except 

 possibly Ineffigiata) really high summer-temperatures are as 

 unsuitableas low ones. Evidence lias been brought forward to 

 show that Ineffigiata alone requires true summer-temperatures 

 to bring it to a maximum, while the other four gênera are 

 satisfied with spring-temperatures. It is a curious fact that 

 some gênera should prêter relatively dilute, while others (often 

 closely allied) prêter relatively concentrated water, but there 

 appears to be no getting away from this conclusion. It may be 

 well however to draw attention to the fact that the degree of 

 adaptation to concentrated or dilute water is not exactly the 



