— 58 — 



fall), the period of maximum development oi' the Desmids 

 probably dépends in the main on increasing sunshine and 

 température (1) {cf. also West and West, 1898, pp. 35, 36). 

 So that a heavy previous rainfall, although it would lead to 

 a late attainment of the concentration favourable to the Zygne- 

 maceae {cf. pp. 52, 53), would owing to the lesser concentration 

 of the water in June (as compared with a year in which the 

 previous rainfall had not been so heavy) lead to a more pro- 

 nounced development of the Desmids at the time of their 

 normal maximum. We could therefore imagine a year in which 

 there had been very heavy previous rainfall followed by a very 

 warm and sunny May giving us the Zygnemaceous-and Desmid- 

 phases at the same time. 



Unicellular forms such as the Desmids may well hâve occasion- 

 ally escaped the collector, and therefore we do not désire to 

 draw any conclusions from the relative amounts of the Desmids 

 from month to montli. Nor, in view of the scanty représentation 

 of the Desmids as a whole in Barton's pond, is it possible to give 

 many détails as to the relative occurrence of the différent species. 

 Of the speciesof Cosmarium, C. obtusatum proved to be much 

 hardier than the others, as it was never absent when the genus 

 was to be found in the sample. The next species most commonly 

 encountered were C. Thwaitesii and C. venustum, while 

 C. renifor?ne and more especially the remaining species men- 

 tioned on p. 43 were distinctly rarer. The species of Closterium 

 are named in order of abundance on pp. 42, 43. 



C. — The Oedogoniaceae. 



Oedogonium is to be met witli in Barton's pond practically 

 ail the year round, although in varying abundance. Its period 

 of maximum development falls into the months Mardi to June ; 

 after that it is generally of little importance. It was most abun- 

 dantly represented in May of 1910. There do not appear to be 

 any very appréciable différences in the time of occurrence of 

 the différent species ; Oedogonium fragile is the form most 

 frequently met with. 



It will be noticed from the table on the next page that Oedo- 

 gonium reached its maximum much earlier in 1907 {viz. in 



(1) Note that Cosmarium in 1906 and 1907 appears with the first month 

 having plenty of sunshine, cf. also Fritsch, 1903, pp. 275-27(3. Even in the 

 hothouses at Kew Desmids are very rare before April. 



