220 
William J. Hodgetts 
V. aversa tends to develop best in fairly shallow water near the 
sides of the pond, and generally forms more or less definite com¬ 
munities, often almost unmixed with other filamentous species, 
although sometimes growing in company with Tribonema bomby- 
cinum. It is a form which prefers cold or moderately cold water, 
the most favourable temperature apparently being a monthly mean 
between 4*5° and 8° C., and thus development tends to fall in autumn 
or winter, while the species may persist into the following spring 
(see table above). It was most abundant during late December 1918 
and early January 1919, when the concentration of the water was 
rather low, namely 13 degrees (cf. Fig. 3); while a smaller maximum 
occurred during January—February 1920, with the concentration 
in the neighbourhood of 20 degrees. The evidence, although not 
conclusive, points to moderate or rather low concentrations being 
preferred. During the very high concentrations at the end of 1919 
(Fig. 3) the species was very rare, and did not commence development 
until late December, when the concentration was falling very rapidly. 
Bright sunshine is certainly not necessary either for good vege¬ 
tative growth or for the development of sexual organs in V. aversa t 
since both the maxima coincided with periods very deficient in 
sunshine. V . aversa thus differs from many species of Spirogyra and 
(Edogonium (and apparently also from V. repens, according to the 
experimental work of Klebs) in fruiting without the stimulus of 
bright sunshine 1 . 
From the usual mode of occurrence of Vaucheria it appears that 
abundant aeration of the water is essential for good development, 
and this is borne out by the present observations on V. aversa, since 
the various maxima of this species always followed a month of 
heavy rainfall, when there would be a marked increase in the gaseous 
content of the water, since the rain of course would bring in a supply 
of dissolved gases from the atmosphere, and the prevailing low 
temperature would favour the continuance of this condition. The 
slight vegetative development of V. aversa which occurred during 
September—October 1918 (cf. above table) coincided with a pheno¬ 
menally wet September (see rainfall-data in Fig. 3); while December 
1918 was the second wettest month of the year, and the species 
again increased during that month. December 1919 was also the 
second wettest month of 1919 and V. aversa showed increased 
development during that month. 
1 Fruiting specimens of V. aversa (and V. geminata) have been collected on 
several occasions from small pools near Birmingham, and always in the colder 
months of the year when sunshine is deficient. 
