— 83 — 



in lia rton's pond, being t'ound in some quant ity at nearly ail 

 timesoîtheyea.r.Phacuspleuronectesislessahunda,ntlyrei)res- 

 ented, but appears to show a frequency like that of Euglena. 

 We were for a long time much puzzled to account l'or the 

 varying periodicity of Euglena. Taking its occurrence during 

 the summer-months in the fîrst place, it will be noticed that it 

 attained its maximum in May of 1906, in Jnly of 1907, in June 

 of 1908 and to a very slight extent in April of 1909. There 

 appears lobe no obvions relation to meteorological conditions in 

 this frequency, although the months of abundance are mainly 

 those with a good deai of bright sunshine. Euglena howeveris 

 known to favonr water with a good deal of organic matter in 

 solution, and tlie question therefore arose whether an explana- 

 tion for the varying time of its maximum might be fourni in 

 this feature, and that lias actually proved to be the case. During 

 the late winter and spring months several filamentous Algae, as 

 we hâve seen, play a great part in the liora of the pond; on the 

 approach of the summer the bulk of them dies away, but their 

 disappearancedoesnotoccurat exactly thesame time ineachyear. 

 The dying away of thèse filamentous Algae must add very con- 

 siderably to the organic content of the water. In order to obtain 

 a graphie représentation of thèse features we hâve put together 

 the table below, based on the records of the three gênera of Zyg-- 

 nemaceae, Oedogonium, Microspora and Conferva. The 

 results hâve been calculated as follows : — putting « very com- 

 mun » as 100, « common » as 30, « rather common » as 10, 

 « rather rare » as 5, « rare » as 1, and « very rare » as 0, the 

 amount of decrease lias been reckoned from month to month 

 and summed up as a total for the six gênera, the amount of 

 decrease being entered in the month in which it occurs (1) . 



(1) The numbers chosen for the différent values, though arbitrary, repre- 

 sent roughly the amounts adopted as a criterion for the symbols. An increase 

 in the amount of an Alga has been counted as 0; if in two successive months the 

 frequency of the Alga was the same, the increase would be entered as 0; if the 

 change is from very common in one month to rather rare in the next, the latter 

 month would bave recorde d in it a decrease of 95. Thus, taking the year L906 

 in the Spirogyra-tMe on p. 54, we would hâve the following records : 

 April, 0; May, 0; June, 70; July, 30 ; August, 0. This should sufflciently 

 explain how the table bas been compiled. It may be added that sucb data of 

 Mr. Salisbury"s with référence to the amount of Algae in the pond as are available 

 are quite in agreement with the relative amounts of decrease obtained by our 

 mode of reckoninu-. 



