63 — 



ber). Of the two species of the genus présent in the pond, 

 M. amoena var. gracilis is a common forai and isgenerally 



at ils maximum in winter; the narrower M. stagnorum is 

 mucli rarer and is very poorly represented except in early 

 spring, when in some years (especially 1908) it attains a faint 

 maximum. 



Table to show occurrence of Microspora amoena var. 

 gracilis in Bartons pond, 1906-1910 (1) . 



The following observations refer specially to the commoner 

 species. As the above table shows, tins species attained to a 

 maximum at rather varying times, viz. in December 1906, in 

 Mardi and July of 1907, in the winter and spring of 1907-1908 

 (January to May) and in April and May of 1910, while in 1909 

 it was rare, like ail the other filamentous forais. In view of 

 the fact that Microspora flourishes especially during the 

 winter-months, we may take it that it is a cold water form (2 >, 

 a fact which is not striking when we consider that the thick 

 cell-membrane will render such an Alga very sensitive to a rise 

 of température of the water, involving a réduction in the volume 

 of dissolved gases. We may therefore in the first place seek an 

 explanation of the varied occurrence of the genus in the temp- 

 erature-data. The resuit is as follows : 



I- — December, 1906 : Lowest températures of winter of 

 1906-1907; sudden drop of température, as compared with 

 November 1906; lowest December-temperatures for the period 

 of observation. 



(1) Regarding the symbole, see footnote 1 on p. 54. 



(i 1 ) Mr. Salishury records that in the second week of January 1908 the 

 Microspora was frozen into the solid ice, hut was in a perl'ectly healthy condi- 

 tion; this indicates that it is eminently suited to low températures. 



