24 
William J. Hodgetts 
Table showing the Occurrence of Three Flagellates in Hawkesley 
Hall Pond during 1918-1921. 
d 
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Feb. 
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June 
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Aug. 
Sept. 
Oct. 
Nov. 
d 
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f 1918 
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Cryptomonas 
1919 
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ovata 
1920 
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[1921 
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(1918 
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Trachelomonas 
IT919 
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volvocina 
1920 
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i.1921 
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f 1918 
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_ 
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Trachelomonas 
1919 
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— 
— 
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rc 
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— 
— 
— 
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hispida 
1920 
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— 
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U921 
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“rather rare” without showing much relation to temperature (cf. 
its slight development November—December 1919). The species was 
always poorly represented during the winter and early spring. High 
or rather high concentrations of the water are decidedly favourable, 
rapid growth always coinciding with a concentration between 17 and 
21 degrees, and still higher concentrations (August and November 
1919—cf. Fig. 3) are far less unfavourable than lower ones, in fact 
low concentrations, especially when combined with low temperatures, 
are obviously very antagonistic towards the growth of this species. 
No relation to bright sunshine is apparent. 
Trachelomonas volvocina (cf. table above) exhibits a rather 
puzzling periodicity, which is not closely related to temperature- 
conditions, although the lower temperatures of winter are clearly 
unfavourable. Growth commences earlier in the year than with 
Cryptomonas, and the maximum is also reached earlier (often in 
spring), and sometimes there is a secondary maximum in August 
(1919, 1920), while the species is generally rare during the autumn 1 . 
Its occurrence in the pond appears to be best explained by reference 
to bright sunshine (Fig. 2) and the concentration-data (Fig. 3). Of 
the five maxima observed, three (May—June 1918, August 1919, 
June 1920) occurred in very sunny months, and also when the 
1 Kofoid (22) found that T. volvocina was best developed in the plankton 
during the warmer summer months. On the other hand Fritsch and Rich (18) 
found that this species had its maxima frequently at the end of winter or 
during early spring, although at other times the maximum of T. volvocina and 
T. hispida coincided with the warmer months of the year. 
