— 92 — 



filamentous Algae {cf. p. 84) up to the month of August we hâve 

 the following resuit : 



There is also sortie correspondence between the frequency of 

 Navicula from April to July in the first three years and the 

 figures in the table on p. 84, but the évidence is not altogether 

 satisfactory. 1909 is perhaps best left out of considération, as 

 it is difficult to estimate the influence of the ducks in tins con- 

 nection. The winter-maximum of Navicula. would also tally 

 with the enrichment of the water by the decaying Phanerogamic 

 végétation. 



The gênerai préférence for cold water, which is shown by the 

 Diatoras in Barton's pond, is a familiar fact (see also Comère, 

 1906, p. 395; Brunnthaler, 1907, p. 195; Fritsch and Rich, 

 1909, p. 42). Whipple (1894) lias however maintained that 

 the most important condition for the growth of Diatoras is an 

 adéquate supply of nitrates, and that the effect of température is 

 so small that it plays no part in determining their seasonal dis- 

 tribution. We do not feel however that the frequency of 

 Eunotia Arcus for instance can be explained by assuming a 

 variation in the supply of nitrates in the différent years, and we 

 must agrée with Kofoid (1908, p. 54) and Messrs. W. and G. S. 

 West (1912, p. 426) in taking température to be one of the most 

 important controlling factors. We consider it doubtful however 

 whether température opérâtes directly, rather than indirectly 

 in controlling the supply of air dissolved in the water. For, in 

 rivers (Fritsch, 1903a; West & West. 1909. p. 176; Kofoid, 

 1908, p. 54) and large lakes (West, 1909, p. 21 ; West & West, 

 1912, ]>. 425), where the supply of air is more plentiful, Diatoms 

 often exhibit summer-maxima. 



E. — General considération of the periodicity-iJata of 

 the preceding section. 



We think it will be agreed that in the preceding section a pro- 



(1) The brief duration of this maximum may be due to the exceptional temp- 

 ératures of August, 1906, while the relative abundance in 1907 corresponds to 

 a summer cooler than usual. 



