196 William J. Hodgetts 
of all the species of the genus noted in the pond, S. inflata more 
particularly tending to be a perennial form. It will be seen that the 
general trend of the curves for 5 . inflata and S. protecta are the same; 
for instance, the maximum in October 1918 carries over both species 
through the winter into the following vernal phase, and a similar 
behaviour is shown by 5 . colligata in the very mild winter 1920-1921. 
There is little doubt that the vernal maximum of Spirogyra (as 
Fritsch and Rich ( 16 ) concluded) is the result of the periodic re¬ 
currence of a group of certain factors, of which temperature is 
undoubtedly important. All the species (except S. bellis) of the genus 
noted in the pond appear to flourish best when the monthly mean 
temperature falls between 6° and 12 0 C., although the hardier species 
are able to tolerate much lower temperatures—for example, S. 
protecta was observed in January 1919 in a state of conjugation under 
a cover of ice 3-5 cm. thick. The higher summer temperatures are 
distinctly unfavourable, except perhaps in the case of S. bellis, which 
occurred locally during June—August, 1919) when the mean tem¬ 
perature of the air was 14 0 to 17 0 C. The autumnal maximum always 
falls when the temperature-conditions again become favourable, but 
of course some other condition may play the part of a limiting factor 
and prevent the development of this phase (cf. case of autumn 1919, 
discussed below). 
Sunshine is another important factor, and a comparison of the 
curves of Fig. 4 with the sunshine-data (Fig. 2) is interesting. The 
sunniest month in each of the three complete years was May, and 
this is the month in which Spirogyra was at its best each year. In 
the very sunny month May 1919 all the species of the genus recorded 
for the pond (except 5 . bellis and S. Grevilleana) were present and 
produced zygospores; and of these S. inflata, nitida, protecta, mirabilis, 
longata, colligata and also Grevilleana were again present (and in 
conjugation) in May 1920; while in 1921 the vernal phase was fairly 
well developed (S. colligata, protecta, inflata, mirabilis again con¬ 
jugating) and reached a maximum in the very sunny May of that 
year. The relation to sunshine is most obvious in the case of the 
more sensitive species ( 5 . catenceforniis, condensata, Jurgensii, mira¬ 
bilis, longata, maxima, Weberi) which were confined to the vernal 
phase (maximum in May); while in the case of the hardier species 
( 5 . inflata, S. protecta, and in a lesser degree S. colligata and S. 
varians), as an examination of the curves shows, the relation is much 
less obvious. Thus the autumnal phase of 5 . inflata and S. protecta 
in 1918 was at a maximum in a dull month (October) although 
growth was most marked in the fairly sunny September; and in 
