Periodicity of Freshwater Algce in Nature 223 
Table showing the Occurrence of Nitella flexilis and Coleochcete 
Nitellarum in Hawkesley Hall Pond during 1919-1921. 
d 
aj 
23 
A 
a 
u 
Oh 
May 
June 
July 
Aug. 
Sept. 
-M 
O 
O 
Nov. 
Dec. 
Nitella 
flexilis 
( 1919 
— 
— 
— 
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r 
rc! 
rc! 
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rr 
rr 
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1920 
vr 
vr 
vr 
rr! 
rr! 
r! 
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vr 
rr! 
rc! 
rc! 
rc 
1 1921 
rc 
rc 
rr 
rr 
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r 
Coleochcete 
(1919 
— 
— 
— 
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vr 
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rc! 
rr! 
rr! 
r 
vr 
. _ 
Nitellarum 
1 1920 
— 
— 
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— 
r 
r 
r 
rr 
( 1921 
rr 
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Nitella flexilis developed rather irregularly in the pond, and in 
the form of more or less local patches growing from the bottom in 
places where the water was only moderately deep. Its growth was 
undoubtedly greatly hindered by the extensive development of 
Potamogeton natans during the summer (cf. Section II), probably 
owing to the fact that the floating leaves of this plant cut oh 
considerable light from the bottom of the pond; and it was observed 
that the patches of Nitella were always confined to those parts of 
the water where the floating leaves of Potamogeton happened to be 
poorly developed or absent altogether. No very close relation to the 
temperature is apparent; the first maximum occurred when the 
monthly mean temperature fell between 13*5° and 14*5° C., the 
second with a rising spring-temperature, while the rise to the third 
maximum coincided with a falling temperature in autumn, although 
the species persisted in quantity throughout the succeeding mild 
winter. Apparently a moderately high temperature (a monthly mean 
between 9 0 and 14 0 C.) is best for good vegetative growth, although 
somewhat lower ones are tolerated. 
A rather close relation is shown between the vegetative growth 
of Nitella and the sunshine-data, while abundant bright sunshine is 
particularly essential for the development of sexual organs in this 
species. Thus the increase to the first maximum, in 1919, was 
preceded by two months of very sunny weather, while development 
towards the end of March 1920 coincided with a sunny March (see 
data in Fig. 2). Sexual organs were formed in abundance in the case 
of the maximum of 1919 and the first maximum of 1920, when the 
ispecies had the stimulus of abundant sunshine (see Fig. 2), but were 
scarce during September—November 1920 (dull months), and absent 
altogether during the remaining months (which were dull, but mild) 
of this long-continued third maximum. There is no doubt therefore 
that oospore-production in Nitella flexilis is favoured by abundant 
bright sunshine. 
