226 
William J. Hodgetts 
Dictyosph&rium, Ehrenbergianum (Fig. io) is a perennial con¬ 
stituent of the algal flora of the pond, its maxima falling in the middle 
or towards the end of summer (June—July 1918, July—August 1919, 
August 1920). The most favourable temperature appears to be a 
monthly mean between 12 0 and 15 0 C. (cf. data in Fig. 2), and higher 
temperatures appear to be rather harmful, as shown by the decline 
of the species in July 1918, at a time when other conditions seemed 
favourable. From a comparison with the concentration-curve (in 
Fig. 3), it is apparent that gradually rising concentrations between 
16 and 22 degrees are those most suitable for this species in the 
present pond, while a decreasing concentration—even between these 
limits—is distinctly unfavourable. The sudden decline of the species 
at the end of August 1919 (cf. Fig. 10) was due either to the high 
concentration of the water (27 degrees) attained in this month, or 
to the sudden decrease in the concentration during August—Sep¬ 
tember, or again to the high temperature of August, or, more 
probably, was the result of more than one of these factors. 
Ankistrodesmus falcatus 1 (cf. above table) is also adapted to 
moderately high temperatures (a monthly mean between n° and 
14 0 C.); and again higher temperatures apparently caused a decrease 
of the species in July 1918. In 1920, probably on account of the 
favourable temperature-conditions (see Fig. 2), the maximum per¬ 
sisted well into the autumn. The relation towards the concentration 
of the water is much the same as with the last species considered, 
the most favourable concentration for growth lying between 16 and 
22 degrees, although the species may persist in fair quantity with 
concentrations outside these limits. A falling concentration does not 
appear to be so harmful to this species as it was with Dictyospharium . 
Scenedesmus denticulatus var. linearis closely followed Dictyo- 
sphcerium in its occurrence, although it is a decidedly more sensitive 
form. A monthly mean temperature between 12-5° and 15*5° C. 
appears to supply the best temperature-conditions for growth; and 
again higher summer-temperatures were concerned in bringing about 
a decrease of the species (in August 1918, and perhaps August 1919) 
after the July maximum, although the concentration of the water 
probably also played a part. The species increased to a maximum 
generally when the concentration was rising and between 16 and 21 
degrees. The slight lowering of the concentration during June—July 
1920 (cf. Fig. 3) apparently did not have any adverse influence on 
1 The vars. acicularis and spirilliformis of this species showed a periodicity 
in the pond similar to that of the type, but they were always much less abundant 
than the latter 
