Periodicity of Freshwater Algce in Nature 21 
rather variable (most pronounced in 1919), and is probably deter¬ 
mined by a variety of factors, including competition with other species. 
With regard to the influence of the concentration of the water 
the higher concentrations lying between 17 and 27 degrees (cf. data 
in Fig. 3) are apparently requisite for pronounced growth of A. 
oscillarioides, so that the species is doubtless favoured by abundance 
of dissolved organic matter in the water (cf. Section IV)— i.e. its 
nutrition is probably mixotrophic. In August 1919, when the very 
high concentration of 27 degrees was reached, the species was 
common, but apparently the sudden dilution of the pond which 
followed in September resulted in adverse conditions for this alga, 
since it decreased in the latter month (although temperature in 
September was rather favourable). The adverse infl uence of a sudden 
decrease in the concentration (when temperature remains favourable) 
is also apparent during August—September 1918 and the same 
months of 1920. 
Fritsch and Rich ( 18 ) from their observations on Anabcena catenula 
in Barton’s pond concluded that this species is “ apparently adversely 
affected by exposure to prolonged summer-sunshine,” but there does 
not seem much evidence to support this in the case of A . oscillarioides 
in Hawkesley Hall pond. In 1918, for instance, the three sunniest 
months were May, June and July (cf. Fig. 2), and the species had 
its maximum from the middle of June to the end of July, showing 
at least that bright sunshine did not appear to affect its development 
adversely, while the month (August) in which it commenced to 
decline was rather poor in sunshine. Bright sunshine is certainly not 
essential for the growth of this species, as shown in 1919 and 1920. 
The formation of spores in Anabcena always followed immediately 
after any noteworthy development had taken place, and it does 
not seem to require any special set of conditions apart from those 
influencing vegetative growth. 
Cylindrospermum majus (Fig. 11) is also a summer-form but 
appears to be adapted to rather lower temperatures than the 
Anabcena since its maximum growth never occurred in the hottest 
months, but in those with a mean temperature between 12 0 and 
I4°C.; while it was best developed in the pond during the cool 
summer of 1920. It is a form which was undoubtedly adversely 
affected at times by the growth of Anabcena, in company with which 
it was usually found, and this may explain its curious absence in 
the summer of 1919. Like Anabcena it prefers high or rather high 
concentrations of the water, but a rapid fall in the concentration (as 
