20 
William J. Hodgetts 
oospore-production in this species (cf. Spirogyra, Mougeotia scalaris, 
Closterium acerosum, Cosmarium Botrytis, (Edogonium spp.). 
There seems some evidence that abundant aeration of the water 
may be essential for the growth of Volvox aureus, since the various 
maxima generally come after periods of heavy rainfall, when the 
pond would receive a fresh supply of dissolved gases from the 
atmosphere; but it is difficult to disentangle the influence of this 
factor from that of the low concentration of the water, which 
undoubtedly is a very favourable condition. Perhaps both factors 
(low concentration and good aeration) are concerned in the develop¬ 
ment of the species. It may be noted in this connection that in early 
May 1920 some puddles of almost pure rain-water (resulting from 
the heavy rains of April), lying in the marshy region between the 
pond and the fence (cf. Fig. 1), were bright green in colour owing 
to an almost pure growth of Volvox in them; and the water of these 
shallow puddles would certainly be better aerated than that of the 
main body of the pond, where the species was less abundant. 
XVIII. THE CYANOFHYCE2E 
The most abundant Blue-green Algae represented in the pond 
during the period of observation were Anabcena oscillarioides, 
Cylindrospermum majus, Oscillatoria tenuis, 0 . splendida, Phor- 
midium uncinatum, P. molle, P. tenue and Arthrospira Jenneri. The 
frequencies of the first two species, and of the species of Oscillatoria 
and Phormidium (grouped together and considered as a whole) are 
given in Fig. 11. 
Anabcena oscillarioides 1 is a typical summer-form, its period of 
maximum development being from June to September. The species 
during these months is abundant not only in the shallower water at 
the margin of the pond (where it is generally mixed with more or 
less Cylindrospermum majus), but also amongst the filaments of the 
wide species of (Edogonium which predominate in the deeper central 
parts of the pond during the summer (cf. Section VI). A . oscillarioides 
requires the higher summer-temperatures (a monthly mean between 
12 0 and 16*5° C.) before it becomes common, and it commences 
development during the first really hot month of the year (generally 
May or June), its maximum being attained some time during June— 
August. The persistence of the species into the autumn-months is 
1 Other species of AnabcBna were possibly present in the pond at times, 
but were always rare and unimportant. 
