Periodicity of Freshwater Algce in Nature 25 
concentration was high (18 to 27 degrees), one (April—May 1919) 
took place with a low concentration (rising to 18 degrees by the 
end of May), while the fifth (August 1920) coincided with a rather 
dull month, although the concentration was moderately high (18 
degrees). Thus although the occurrence of T. volvocina cannot be 
explained by either bright sunshine or the concentration alone, the 
conjunction of abundant sunshine with a high (or rather high) 
concentration of the water, and during the warmer months of the 
year, appears to supply the best conditions for growth. Probably 
some other factor (perhaps competition) has controlled the occurrence 
of this species in the pond. 
It may be suggested that T. volvocina can to some extent vary 
its mode of nutrition in nature, being at times mainly (or wholly) 
autotrophic, when supplied with abundant bright sunshine 1 , and 
without any considerable amount of organic matter in the water, 
while at other times, when supplied with large quantities of organic 
matter, it may have a mixotrophic method of nutrition and thus do 
away with the necessity for bright sunlight. This tentative suggestion 
would perhaps best explain the occurrence of the species in the 
present pond. Naumann 2 has suggested that the probable cause of 
the abundant development of T. volvocina, forming a chocolate- 
coloured water-bloom in a tank in the Botanic Gardens at Lund, 
was the supply of organic matter in the water; in the case of Haw- 
kesley Hall pond, however, this factor—although probably an 
important one—cannot completely explain the occurrence of the 
species. 
Trachelomonas hispida was generally less abundant in the pond 
than the preceding species, but its periodicity (cf. table above) does 
not differ in any important respect from that of T. volvocina. None 
of its “maxima,” however, falls with a concentration of the water 
above 18 degrees, and the slight development in April 1920 occurred 
with an extremely low concentration, so that perhaps this species 
may not be able to utilise organic matter in the water to the same 
extent as T. volvocina. The two maxima of T. hispida in 1918 and 
1919 coincide with periods of very abundant bright sunshine, which 
is further evidence that the species is mainly autotrophic in its mode 
of nutrition. 
1 The translucent brown shell would explain the necessity for this condition 
if the species is to any extent autotrophic. 
2 Bot. Notiser, Lund, 1913, p. 249, etc. 
