46 
STUDIES OF BRITISH FRESHWATER ALG^E. 
emphasised the marked vernal reproduction of Spirogyra in ponds 
of S. England, and suggested as possible influencing factors “ the 
increased intensity of light, the small percentage of dissolved salts in 
the water, the rising temperature, and (?) the decrease in amount of 
dissolved gases.” It is interesting to note that G. S. West^ in his 
recent paper on the “ Alg« of the Yan Yean Reservoir ” finds that 
the Zygnemacese are dominant and almost all in a fruiting condition 
in November, and to a less extent in December, and that “ this com- 
mencement of the fructiferous state simultaneously with a rising 
temperature is strictly comparable with what occurs in the southern 
and south-western counties of England.” We are not inclined to 
attribute this commencement of sexual reproduction solely to a rising 
temperature, but believe that the other factors above-mentioned also 
play an important part ; such evidence as is available for this view is 
stated on pp. 431, 432 of our paper. West finds the (Edogoniacese 
reproducing simultaneously with the Zygnemacese, a feature which we 
have noticed in several other ponds, ^ V>ut which, as above-mentioned, is 
not the case in the pond wo are considering. This indicates that the 
sexual reproduction of Zygnemace^ and (Edogoniaceffi is not influenced 
by quite the same set of factors, and that in the case of Abbot’s Pool 
the factors necessary for such reproduction in (Edogoniurn have not 
been realised during the period of observation. 
As stated in our previous paper the prevalent reproduction of 
Spirogyra during the vernal phase might be due either to an inherent 
tendency or to the regular occurrence of certain combinations of 
external conditions in spring. The former suggestion was considered 
unlikely in view of what is known about algal reproduction in general, 
but the possibility of some of the species being biennial, as far as their 
reproduction was concerned, made it impossible to disprove absolutely 
the existence of such an inherent tendency.^ Since then, however, we 
have additional data, which show that a biennial condition does not 
obtain. Thus in the case of Spirogyra affinis and S. Weber% both of 
which formed zygospores in the spring of 1906, but not in 1905 (in 1904 
there was no Spirogyra, cf. above, p. 37), it seemed possible that 
zygospore-formation took place only every two years ; since then however 
both have been found with zygospores in the spring of 1907, and this of 
course absolutely disproves the biennial theory in the case of these two 
species. Under these circumstances we are therefore confronted with 
the fact that some species oi Spirogyra (like S. varians) form zygospores 
whenever they are present in spring, whilst others (like S. affinis and 
S. Weberi ) do so in some years, but not in others ; others again (like S. 
1 Loc. cit. , p. 32. 
'^e.g. in a pond at Telsoombe, near Newhaven, in 1904 ; cf. Fritscli, in New. 
Phytol. Vol. V. (1906), p. 163 et sq. 
'®cf. Fritsch and Rich, loc. cit., pp. 430, 431. 
