- 75 - 



summer-temperatures may operate as adversély as low winter- 

 teflaperatures. In support of this the followiing data may be 

 given : 



I. — The year 1907 was characterised by a more pronounced 

 development of Pandorina than in other years. Eudorina 

 attained to a well-defined maximum in July and August, 

 altliough completely absent at this time in the other three years. 

 The summer of 1907 was characterised by unusually low temp- 

 ératures {cf. p. 38), and this probably accounts for the excep- 

 tional development of the two gênera of Volvocales in that year. 



II. — The absence of Pandorina, Eudorina and Sphaero- 

 cystis in the summer of 1906 corresponds to tins being the 

 warmest summer during the period of observation, July, 

 August and September liaving the highest average maximum 

 températures in the four years. The temporary decrease of 

 Sphaerocystis in July 1908 again tallies with the réalisation 

 of a rather high température. 



III. — Gloeocystis appears less sensitive than tlie other three 

 gênera to high summer-temperatures - 11 , but it may be noticed 

 that in the summer of 1906 it was rarer than in 1907 or 1908. 



In the easeof Ineffigiata, which attains to a maximum later 

 than the other forms 2) , it seeins 1o be the réalisation of high 

 summer-temperatures that gives the stimulus for the flrst pro- 

 nounced development of the genus. This is seen in 1907 

 and 1908; in 1907 it does not become common till August, and 

 this month lias the highest average température for the year; 

 in 1908 it is common already in June with an unusually high 

 average température (equal to that of August 1907). In 1906 

 and 1909, in both of which it was not very abundant, its 

 maximum (September 1906, July 1909) again curiously enough 

 corresponds to months with an average température of 59° F. 

 On the basis of thèse four records, which can hardly be coïnci- 

 dences, it seems that we can conclude that an average temp- 

 érature of 59° F is that which is most suitable for the develop- 



(1) Thus, the relatively high température of July 1908 does not appear to 

 hâve appreciahly affected its development. Its persistance in 1906 prohaldy 

 accounts for its récupération with the lall of température in the autumn, its 

 absence in Neptemher being due to drying up ofthe pond. 



(2) Messrs. W. & G. S. West (11»12, p. 425) record that Botr/jococcus 

 Braunii (which is closely allied, 4 if not identical with Ineffigiata) is a late 

 summer form in the Plankton of Britisli lakes. 



