Periodicity of Freshwater Algce in Nature 209 
maximum, especially, occurred after a month (July) poor in sunshine 
(cf. Fig. 2), yet zygospores were noted at the time to be “locally 
rather abundant,” It is true that conjugation took place during the 
maximum of 1919 after two months of very sunny weather (cf. data 
in Fig. 2), but on that occasion zygospores were recorded as “very 
rare.” The evidence therefore points to abundant bright sunshine 
not being essential for the conjugation of this species. It is interesting 
to note that West (30) was led to conclude that the precise conditions 
resulting in zygospore-formation in Desmids “are probably widely 
different for various species ” ( l.c . p. 35); and the present observations 
rather tend to confirm this view. 
Two other species of Staurastrum (5. Dickiei, S . Jurcigerum) were 
present in the pond, but since their frequency-curves tend to follow 
very closely that of S. brevispinum they have not been reproduced 
here. Both tend to arrive at a maximum towards the end of summer, 
persisting as “rather common” until the middle of autumn, when 
they always decrease and remain “ rare ” (or “ very rare ”) during the 
winter and early spring. All three species of Staurastrum were very 
tolerant of the high concentration (27 degrees) attained in August 
1919, and all three remained “rather common” during August and 
September of that year. In this respect they are distinctly more 
hardy than the species of Cosmarium in the pond. This is quite the 
reverse of the conclusion arrived at by Fritsch and Rich (18) in the 
case of the Desmids of Barton’s pond, since these authors concluded 
that the two species of Staurastrum (S. tricorne , S. brevispinum) were 
apparently much more sensitive to high concentrations of the 
water than the Cosmarium spp. and Closterium spp. present in that 
pond. 
Zygospores of Staurastrum Dickiei were observed in the pond, but 
were scarce, on one occasion only, namely on August 7th, 1920 (when 
S. brevispinum was also in conjugation), and this was after a rather 
dull July, so that S. Dickiei appears to agree with the very closely 
allied S. brevispinum in not requiring abundant bright sunshine for 
conjugation to take place. 
Xantkidium antilopceum (Fig. 7) is a perennial constituent of the 
algal flora of the pond, and is more abundant in the warmer months. 
A monthly mean temperature between io° and 15-5° C. is essential 
for this species to become really common, although it may persist 
as “rather rare” at temperatures rather lower than these. The low 
temperatures of winter are decidedly harmful. The relation to the 
concentration of the water is again obvious (cf. Figs. 3 and 7), the 
most favourable concentration lying between 14 and 20 degrees. An 
5—13 
