— 84 — 



Table showing monthly decrease of the six leading 

 filamentous gênera in Barton's pond. 



It will be noticed that the great increase of Euglena in May 



of 1906 corresponds to a great decrease (150, total up to that 

 month 220) in filamentous Algae; in 1907 July is the first month 

 in wliich Euglena becomes common, and tins corresponds to 

 a great decrease of filamentous Algae in that month {riz. 99, 

 total decrease up to that month 180), but the decrease is not as 

 o-reat as in May 1906 and Euglena was not as abundant as in 

 the latter month. In 1908 the month of June(latterhalf)brought 

 the first marked development of Euglena. The actual decrease 

 of Algae in .Tune is only recorded as 50, but it must be noted 

 that the total decrease up to that month was 190, i. e. about ihe 

 same as by July, 1907; moreover the increase of Euglena falls 

 into the second half of .Tune and by that time some of the 

 decrease of 157 recorded for July was already realized. The 

 exceptional sunshine of June 1908 may also bave had something 

 to do with the abundant development of Euglena already in 

 that month. In 1909 the algal flora of the pond was so scanty 

 that no pronounced decrease occurred, nor is there in any way 

 so marked a maximum l'or the Euglena. The rather common 

 of April of that year may correspond to a period of very excep- 

 tional sunshine. 



The mat ter can be pursued further. Thus, if we consider the 

 totaldecrease from June onwards we hâve l'or thethree years : 125 

 (1906), 172 (1907), 212 (1908). In 1906 Euglena became rare in 

 July, in 1907 rather rare in September, in 1908 rather rare 

 only in October. In other words the period of persistence of 

 Euglena in some quantity after ils maximum is in direct rela- 

 tion to the amount of decreaseofthe filamentous Algae. It is also 

 instructive to compare the frequency table of Euglena for the 

 three first years in the months of June, July andAugust with the 



