— 73 — 



The data Ihus seem to show that Chaetophora is influencecl 

 in ils occurrence by sudden changes in the concentration of the 

 water, which act adversely upon it. It would appear dowever 

 to prefer dilute water, as shown by its longer period of impor- 

 tance in 1908 fchan in 1907; in 1908 there was much heavier 

 rainfall during the two critical montlis (March ami April) than 

 in other years, ami the impetus given by thèse conditions lasted 

 on for some montlis, since there was nothing in the weather of 

 thèse la toi- montlis (May-July) to affect it adversely. 



The scanty représentation of the genus after May 1906 may 

 however also hâve somelhing to do with the high summer-temp- 

 eratures of that year. The decrease in 1907 and 1908 also 

 appears to correspond to the réalisation of summer tempéra- 

 tures. It is probable that Chaetophora, like most of the Pro 

 tococcales, is adapted to médium températures, and that bot h 

 low and high ones are unsuitable. 



G. — The protococcalks. 



In considering the Protococcales, which play a considérable 

 part in the floraof the pond, it bas been thought best todiscuss 

 tlie more important gênera in the first place and to add a few 

 gênerai remarks on the others at the end of tins section. 



Among the Volvocales we need oniy consider Pandorina 

 and Eudorina, as Gonium was very rarely observed. Pan- 

 dorina is more constant than Eudorina, butnever occurred as 

 abundantly as the latter genus did on certain occasions. Botli 

 gênera seem normally to appear in the pond in March or April 

 and remain generally till September (cf. Fritsch, 1903. p. 277); 

 after that they disappear or become very scanty as a rule, and 

 neither was ever observed in January or February. Sphaero- 

 cystis shows a good deal of resemblance to Pandorina in its 

 périodicité (cf. also Chaetophora), but is generaJly consider- 

 ably more abundant. It appears in Mardi, persists in some 

 quantity till about August, then diminishes and is rare or absent 

 during the winter; like Pandorina and Eudorina it was 

 never observed in January and only once in February. Species 

 of Gloeocystis (1) are found nearly ail the year round, attain- 



(1) We liave not noticed aDy striking différence in the frequency of the three 

 species o( Gloeocystis présent in the pond and shall therelbre deal with thein 

 collectivelv. 



