Periodicity of Freshwater Algce in Nature 217 
1920, February 1921.) Generally two rounded aplanospores were 
formed in each cell and the filaments broke up into H-shaped pieces 
in order to set them free. In addition to forming these aplanospores, 
however, the species generally persisted through unfavourable 
periods by means of isolated vegetative filaments. 
T. bomhycinuni /. minor was quite as frequent in the pond as the 
type form, and its periodicity was worked out separately, but since 
the resulting curve is almost identical with that of the type (given 
in Fig. 9) it has not been reproduced here. Undoubtedly this form 
is influenced by external conditions exactly in the same way as the 
type, and the two were generally found intermixed, so that the above 
remarks concerning T. bombycinum apply with equal force to its 
forma minor. Doubtless between such closely allied forms growing 
together competition must be very active. No aplanospores were 
ever observed in the case of T. bombycinum f. minor , the Alga 
always tiding over unfavourable seasons by means of isolated 
filaments. 
T. affine was in general decidedly less abundant that T. bomby¬ 
cinum, with which it was usually mixed. Its frequency in the pond 
(given in Fig. 9) very closely resembles that of T. bombycinum , 
activity commencing during the falling temperature of early autumn, 
and a maximum reached in October or November. Other factors, 
apart from temperature, however, caused the behaviour of this 
species to vary somewhat during the winter months, the most 
important probably being the concentration of the water. While 
T. affine apparently agrees closely with T. bombycinum in the matter 
of temperature and also sunshine, it reacts somewhat differently in 
regard to the concentration of the water. On two occasions (Novem¬ 
ber 1918 and 1919) the position of the maximum on the T. affine 
curve coincides with the highest concentration reached during the 
autumn and winter months (cf. data in Fig. 3), and on one of these 
occasions (November 1919) the concentration was the highest (30 
degrees) observed during the period of observation. In T. affine, 
therefore, we have an Alga which has its period of development 
(autumn—winter) determined mainly by the low temperature and 
scarcity of bright sunshine, although the actual position of maximum 
growth in this period is conditioned by the concentration of the 
water, the "maximum” coinciding with the highest concentration 
attained during the season. 
There seems little doubt from the above observations that T. 
affine is able to utilise directly the organic peaty compounds which 
were abundant in the pond during times of high concentrations (cf. 
