23 
Periodicity of Freshwater Algce in Nature 
(18 to 20 degrees), while the two remaining maxima occurred with 
moderate concentrations. These results are sufficient to show that 
high concentrations of dissolved organic matter in the water (cf. 
Section IV) are particularly favourable for the growth of these two 
genera; in fact one would be fairly safe in predicting that the 
development of one or more of their species would result from the at¬ 
tainment of a high concentration of the water of this pond during the 
autumn of any particular year (other conditions being fairly normal). 
Temperature is undoubtedly an important factor with these two 
genera, and high or moderately high temperatures (a monthly mean 
between 8° and i6° C.) are best suited for their growth. Development 
almost always commenced during the warmest months of the year 
although the maxima generally occurred with the temperature falling 
in the autumn. Temperature appeared to be concerned in the slight 
local development in January 1921 (an extremely mild month—see 
Fig. 2), and it was also the cause (together with the favourable con¬ 
centration-conditions) of the early growth of 0 . tenuis and Ph. molle 
in May 1921. No relation is shown towards bright sunshine. 
A few words may be added on the occurrence of Arthrospira 
Jenneri. It showed a slight local development on three occasions, 
namely August 1919, June—July 1920, and June 1921, and was also 
noted, but was very rare, during several other months. It appears 
to be a summer-form, adapted to the higher temperatures, and 
undoubtedly prefers high concentrations of dissolved organic matter 
in the water, since on each of the three occasions mentioned above 
the concentration was high (very high in August 1919—cf. Fig. 3). 
This species was always observed in company with species of 
Oscillatoria or Phormidium, and it seems to be influenced by external 
conditions much in the same way as these two genera, and like them 
it also appears to have a mixotrophic mode of nutrition. 
XIX. THE FLAGELLATES 
Only three species belonging to this group were at all abundant 
in Hawkesley Hall pond, namely Cryptomonas ovata, Trachelomonas 
volvocina and T. hispida, and their occurrence is represented in the 
table on p. 24. 
Cryptomonas ovata is a hardy form but generally most abundant 
in late summer and early autumn (July—September) but although 
it appears to require summer-temperatures (a monthly mean between 
13 0 and 16-5° C.) for development in any quantity, it may become 
