206 William J. Hodgetts 
observed-—conditions were favourable only for vegetative growth; 
but in May 1919 and also 1920, both months with very abundant 
sunshine (cf. Fig. 2), zygospores of this species were locally common. 
A certain amount of vegetative growth generally precedes zygo¬ 
spore-production in all the Conjugates, and in April 1918 and 1921 
this requisite degree of development in the case of the present 
species apparently was not attained. Cl. acerosum, in requiring 
abundant bright sunshine for conjugation, agrees with certain other 
Conjugates which have produced zygospores in the present pond (cf. 
Spirogyra spp., Mougeotia scalaris, Cosmarium Botrytis), although, 
as will be shown below, certain Desmids of the pond conjugated 
without the stimulus of bright sunshine. 
Closterium striolatum (Fig. 5), as already mentioned, rather tends 
to form a connecting-link between the two broad groups into which 
the Desmids of the pond have been divided. The maximum of Cl. 
striolatum tends to fall when the average temperature-conditions fall 
between 7 0 and 14-5° C. (usually in spring and early summer, but 
throughout the whole of the mild summer of 1920), while the most 
favourable concentrations of the water lie between 12 and 20 degrees. 
Thus the species is adapted to rather higher temperatures, and 
distinctly higher concentrations than Cl. acerosum , although it can 
hardly be termed a summer-species. High temperature was probably 
the cause of the decrease of this Desmid during July—August 1918 
(since the concentration remained favourable), but the decline during 
June—July 1919 was probably due more to the high concentration 
of the water than the temperature. The long-continued maximum 
of 1920 (up to September) was due to the fact that both temperature 
and concentration remained favourable during the cool summer of 
this year (cf. Figs. 2 and 3). 
The Desmids of the second group—those whose development is 
favoured by the higher temperatures of summer and early autumn— 
now come under consideration. The periodicity-curves of three of 
these are given in Fig. 7. Cosmarium Botrytis (Fig. 7) shows a 
comparatively regular curve the maximum heights of wdiich fall 
generally between May and September. The relation to the tem¬ 
perature (cf. Fig. 2) is obvious, monthly mean temperatures between 
io° and 15-5° C. being necessary for good development. There is also 
a distinct relation to the concentration-curve (Fig. 3), the most 
favourable concentrations lying between 15 and 20 degrees. The 
rather early decline of this species towards the end of July 1919, 
at a time when temperature-conditions remained favourable, was 
obviously due to the high concentration of the water (rising to 27 
