Periodicity of Freshwater Algce in Nature 225 
Table showing the Occurrence of Four Members of the Protococcales 
in Hawkesley Hall Pond during 1918-1921. 
• 
Jan. 
Feb. 
Mar. 
Apr. 
a 
June 
July 
bb 
P 3 
Sept. 
Oct. 
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(1918 
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Ankistrodesmus J 
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falcatus j 
11920 
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Scenedesmus 1 
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(1918 
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11919 
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var. linearis 
11920 
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U921 
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Pediastrum \ 
11919 
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Boryanum 1 
11920 
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[1921 
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Pediastrum j 
11919 
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— 
PBH 
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tetras j 
11920 
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[1921 
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All these forms exhibit a fairly uniform periodicity, their develop¬ 
ment being most pronounced during the warmer months, the maxi¬ 
mum generally occurring at the end of summer or in early autumn. 
In general moderately high temperatures are preferred 1 , although it 
appears that with certain of the species the higher summer-tem¬ 
peratures are distinctly unfavourable 2 . The next most important 
factor is the concentration of the water, and in the present pond a 
moderately high concentration between 16 and 22 degrees appears 
to supply the best conditions as far as this factor is concerned (cf. 
however Pediastrum below). No relation to bright sunshine is 
apparent. 
The aquatic animal life of the pond is another factor of importance 
influencing the development of these algal species, since, of course, 
many aquatic animals feed voraciously upon them. In Hawkesley 
Hall pond animal life was fairly well developed throughout the 
greater part of the year, although rather more abundant during 
spring and early summer; and this may have caused the various 
members of the Protococcales to attain their maximum development 
rather later in the year than they otherwise would have done. 
1 This agrees with the observations of West (31) who states that the great 
majority of the Tetrasporine and Chlorococcine Protococcales are late spring 
and summer forms, often occurring far into the autumn. 
2 Fritsch and Rich (18) found that Sphcerocystis Schrceteri was adversely 
affected by high summer-temperatures in Barton’s pond. 
