208 
William J. Hodgetts 
degrees on August 22nd, 1919); and the decrease at the end of June 
1921 was doubtless due to the same cause. 
From a consideration of the sunshine-data it appears that only 
moderate amounts of bright sunshine are sufficient for good vege¬ 
tative development, but that abundant bright sunshine is essential 
for conjugation. Zygospores were observed (locally common) during 
May and early June 1919, and May 1919 was a record month for 
sunshine (Fig. 2). It is true that in 1918 the species attained its 
maximum in a sunny month (July), and no zygospores were observed 
in that year; perhaps they may have been scarce and were overlooked. 
It is probable, therefore, that this Desmid agrees with Closterium 
acerosum, Mongeotia scalaris, Sfiirogyra spp., and doubtless many 
other Conjugates, in requiring abundant bright sunshine for zygo¬ 
spore-production . 
Cosmarium Regnellii ga ve a frequency-curve of almost exactly the 
same form as that of C. Botrytis, but. was never represented by so 
many individuals as the latter species. Cosmarium reniforme was 
frequently observed in the samples from the pond but was generally 
very rare, although it showed a slight increase during the warmer 
months of the year. 
Staurastrum brevispinum was one of the commonest Desmids 
observed in the pond, and gave a frequency-curve (in Fig. 7) the 
maxima of which tend to fall rather later than those of Cos. Botrytis , 
namely some time between July and earty September, when the 
monthly mean temperature was between 13° and 16*5° C. The 
species, however, tends to persist in some quantity into the middle 
(1919) or even to the end (1918) of autumn, while it remained 
“ rather rare ” during the winter 1918-1919. It is thus a fairly hardy 
form, but attains its best development under summer-conditions. 
The most favourable degree of concentration of the water appears to 
lie between 14 and 21 degrees, and while lower ones are tolerated 
the higher concentrations apparently acted adversely in August and 
again in October 1919 (cf. with data in Fig. 3). There seems no 
relation between bright sunshine and vegetative growth. 
Zygospores 1 of this species were observed in the pond during 
June—July 1919, and again in August—September 1920, and, rather 
contrary to what might have been expected (cf. Closterium acerosum 
and Cosmarium Botrytis above), there seems little relation between 
bright sunshine and zygospore-production in this Desmid. The 1920 
1 Apparently never recorded before since W. and G. S. West in their 
Monograph of the British Desmidiacece state ( 4 , p. 146) that the zygospores of 
this Desmid are unknown. They are 50-53/x diam., and have a thick, smooth, 
colourless wall. 
