— 48 — 



L9l >9, p. 53). A considérable number of data in support of tliis 

 assumption will be given in the next section of tliis paper. For 

 the présent one or two gênerai cases will be instanced. The 

 efl'ect of the absence of the normal factors at a critical period 

 was well seen in 1907 in which the latter part of spring and the 

 sommer were abnormally cold and rainy. A comparison of the 

 months April-August of tliis year with the corresponding 

 period in 1906 will be instructive, since 1906 may be regarded 

 as a normal year with no great summer-rainfall and the custo- 

 marv bigh températures. It will be noticed that from April to 

 August of 1907 therainfall was uniformly heavy in comparison 

 to the smali amounts (excl. June) for the saine months in 1906 

 (cf. p. 36); with the exception of May the number of hours of 

 sunshine duringthe period in question were înuch fewer in 1907 

 than in 1906 (cf. p. 37); the average maximum températures and 

 the mean températures for thèse months were Jower in 1907 

 than in 1906 (cf. p. 38). The year 1907 therefore lacked the 

 usual pronounced différences in température, amount of sunshine 

 and amount of rainfall between the spring- and summer- 

 phases. The effect of the unusually equable conditions on the 

 more important components of the algal flora will be seen by a 

 glance at the table on the next page, in which the periodicity 

 for 1900 is given for the sake of comparison. Not only was the 

 flora in 1907 until themonth of August well represented both in 

 individuals and species, but owing to the continuance of many 

 forms of the spring-phase the summer-phase was greatly obscur- 

 ed. In June and July tliis fact is perhaps most striking, the 

 dominance of Euglcna being shared by Mougeotia and Mi- 

 crospora, while Zygnema and Chaetophora still persisted. 

 The effect of the peculiar meteorological conditions of 1907 also 

 led to alater appearance or dominance of nearly ail the spring- 

 forms (with the exception of (Jedogonium). Thèse features 

 will be analysed more completely in what follows. 



The effect of sudden changes in the amount of water in the 

 pond was well shown on several occasions. Thus, in thesam- 

 ple collectcd on June î.sth, 1900 twenty-nine species of Algae 

 were observed, some of them in considérable quantity. Two 

 nights before the collection of the next sample (June 30th) there 

 was a déluge of rain, as a resuit of which the water-level rose 

 several inches. The' sample collectée! on June 30th, in spite of 

 careful investigation, revealed only thirteen species, ail of which 



