Periodicity of Freshwater Algce in Nature 205 
low concentration of water possibly acted as a limiting factor daring 
the early part of 1920.. 
Sphcerozosma granulatum (Fig. 6) is a form decidedly more 
sensitive than Hyalotheca, as regards concentration of the water, but 
it may withstand somewhat higher temperatures, a monthly mean 
temperature between 6° and 13 0 C. apparently being most favourable. 
Higher summer-temperatures appear to have been harmful in 1918, 
but less so in 1919. The greatest observed development (October 
1918) fell with the concentration rising gradually from 12 to 18 
degrees, while the increase of this species during April—May 1919 
also took place when the concentration was rising approximately 
between 10 and 18 degrees. The high concentrations attained in the 
latter half of 1919, and the sudden variations in the concentration 
during April—May 1920 (see Fig. 3), were much more unfavourable 
for this Desmid than they were for Hyalotheca, and caused the slight 
maximum of 1920 to fall late in the year, namely during August— 
October. It may be noted that the temperature-conditions during 
the cool summer of 1920 were never very unfavourable towards this 
species. No explanation can be offered for the non-appearance of 
Sphcerozosma in the spring of 1921. There seems no relation to 
sunshine unless it is that continued bright sunshine is harmful (May 
1918 and 1919), but it is difficult to separate the influence of this 
factor from that of the higher temperature. 
Closterium acerosum (Tig. 6), in contrast to almost all the other 
Desinids of the pond, tends to thrive best in very shallow water at 
the margin of the pond. It is a typical vernal Desmid, although in 
1918 it showed two maxima, one in spring and another during 
September—October. Vegetative growth was most active with a 
monthly mean temperature (of the air) between 7 0 and 13 0 C. (either 
falling or rising), and when the concentration of the water was 
between 10 and 16 degrees. During the autumn of 1919 the tem¬ 
perature was favourable (see Fig. 2), but the high concentration 
(cf. Fig. 3) was a limiting factor which prevented growth; and again 
the moderately high concentrations which prevailed during the dry 
spring of 1921, were doubtless the cause of the “maximum” of that 
season being very slight—in fact only in one month (April) did the 
concentration fall to a favourable degree, and this is the month in 
which the “maximum” occurred. 
With reference to the conjugation of this species (which occurred 
in May 1919 and 1920), it is obvious that bright sunshine is a very 
important factor in bringing this about. During the maximum of 
October 1918, a month poor in sunshine, no conjugation was 
