16 William J. Hodgetts 
hottest month (August) of the year. A falling temperature (between 
16-5° and 13 0 C.—mean monthly temperatures) appears to be more 
favourable than a rising one. 
The species also appears to prefer a falling concentration of the 
water, growth usually taking place with a high or rather high 
concentration, decreasing between 27 and 18 degrees; and in this 
respect the slight development during October—November 1919, 
when the concentration was extremely high (see Fig. 3), is note¬ 
worthy. It seems probable (as certain workers have demonstrated 
by cultural experiments in the case of other species of Chlamydo- 
monas 1 ) that C. Reinhardi is partly able to utilise directly soluble 
organic compounds, such as were abundant in this pond (cf. Section 
IV) more especially during times of high concentrations—in other 
words that the species prefers a mixotrophic mode of nutrition. 
There is no evidence that abundant sunshine is necessary for the 
development of this species. 
C. Debaryana was observed in the pond only in November and 
December 1919, when it was “rather rare,” becoming very rare in 
January 1920. Its development coincided with a low average 
temperature, but with a very high concentration of the water (cf. 
Fig. 3), since between November 22nd and December 13th 1919 the 
concentration was falling from 30 to 24 degrees. This species, there¬ 
fore, like C. Reinhardi, is probably adapted to a mixotrophic nutrition 
in nature. The same applies to C. monadina which was also noted in 
the pond in November (very rare) and December (rare) 1919, but 
not at any other time. C. monadina is undoubtedly a cold-water 
form, and has frequently been observed in small ponds and puddles 
in the midland counties, and always shows a pronounced development 
during late autumn and winter; while it appears to be favoured by 
the presence of organic matter in the water. 
Pandorina Morum (Fig. 10) commences its development in the 
pond during April or May, with the rising spring-temperature, 
persists in more or less abundance throughout the summer, finally 
becoming very rare or disappearing altogether during September. 
Even the high summer-temperatures do not appear to affect this 
alga adversely (July 1918). The fluctuation of the species between 
May and August, however, is rather puzzling, and does not appear 
to be directly related either to the temperature or sunlight, although 
1 Cf. in this connection the work of Artari on Chlamydomonas Ehrenbergii 
(Jahrb. f. wiss. Bot. 52, 1913, pp. 410-466). Artari found that this species 
preferred a mixotrophic mode of nutrition in cultures. 
