— 74 — 



ing a maximum generally between April and July and fre- 

 quently exhibifing an increase at othei* times ol' the year (e. g. 

 in the autumn of 1906 and 1907). There appears normally to 

 be a break for a brief period in the winter, viz. in December 

 1906 and from December 1908 to Marcli 1909, while in the 

 winter of 1907-08 no such break occurred, although Gloeo- 

 cystis was very rare in January 1908. Tneffigiata shows a 

 rather différent periodicity from tlie other Protococcales. It is 

 rare or completely absent in the first five months of the year 

 and attains its maximum from June to September, sometimes 

 even lasting into November. 



The above remarks and a glance at the periodicity-table on 

 p. 78 are sufficient to indicate that the différent forms not only 

 vary among one another as regards their annual cycle, but that 

 the latter also exhibits important différences in the four years 

 of observation. Thèse différences appear to dépend on adapta- 

 tion to différent concentrations of the water and to différent 

 température-extrêmes. Apart from Ineffïgiata, the gênera 

 under discussion chiefly attain a maximum in the spring and 

 early summer-months, and tins indicates a préférence for warm- 

 er températures (cf. also the chart on p. 76), which is also 

 manifest from the following considérations : 



I. — The présence of Pandorina in some quantity in 

 November 1906 appears to correspond to September, October 

 and November of 1906 being unusually mild, the average mean 

 température being like that of April and May (cf. also Gloeo- 

 cystis in November 1906). 



II. — It was only in 1908 that Sphaerocystis was observed 

 in the month of February, tins being the warmest February 

 during the period of our observations. The persistence of the 

 same genus into Decembei- of 1907 tallies with that month being 

 exceptionally mild. 



III. — In the case of Gloeocystis, which appears to be 

 rather hardier than the other three gênera, the signal for its 

 disappearance or diminution in winter (December 1906, January 

 1908, December 1908-March 1909) appears to hâve been a mark- 

 ed drop in température and to hâve coincided with the coldest 

 period of the year. 



On the other hand it seems as though ail thèse forms were 

 adapted to rather narrow teinperature-limits, and that high 



