198 William J. Hodgetts 
1920 S. inflata reached its maximum and conjugated earlier than the 
other species, namely in February, and there was not much bright 
sunshine during Januaty— February 1920. 
An obvious relation is shown between the growth of Spirogyra in 
the pond and the amount of dissolved matter in the water (lowest 
curve Fig. 3). The greatest development of this genus coincided with 
a moderately low concentration of the water of 12 to 18 degrees, 
and undoubtedly this is the concentration which is most favourable, 
at any rate for the more sensitive species of Spirogyra (except 
perhaps S. bellis). During the spring of 1918 the concentration of 
the water was not determined, but in the autumn the concentration 
rose from 12 (September 21st) to 18 degrees (November 16th), and 
this corresponded to a pronounced autumnal phase in October (cf. 
Figs. 3 and 4). During the very pronounced Spirogyra maximum of 
May 1919 the concentration rose from 15 (May 17th) to 18 degrees 
(May 28th), but the further increase in the concentration during 
June resulted in the genus becoming rare. 
An explanation is now forthcoming of the absence of an autumnal 
development of Spirogyra in 1919. During the summer of this year 
the water-level fell very low (see Fig. 3), and a very high concentra¬ 
tion (reaching 30 degrees on November 22nd) resulted in the autumn, 
and there is every reason to believe that this was the particular 
factor which prevented the development of the genus during this 
season, especially as the meteorological conditions were favourable 
at that time. 5 . inflata is rather more tolerant in the matter of 
concentration (and also temperature and sunshine) than the rest of 
the species noted; the February maximum of 1920, for example, 
falling when the concentration was about 20 degrees, while the 
pronounced dilution of the pond during February—April 1920 
affected this species but little (Fig. 4); and again in the early part 
of 1919 a similar disregard of a low concentration is shown by 
5 . inflata, and also the hardy S. protecta . 
The concentration-conditions during May 1920 were rather 
unfavourable, since after a fall to the very low concentration of 8 
degrees on April 10th, and 9 degrees on May 8th, there was a very 
sudden increase to 18 degrees on May 29th 1 . Now of the six species 
of Spirogyra present during April—May 1920 (the most abundant 
being 5 . longata , protecta, colligata and inflata), by the end of May 
two ( nitida, niirabilis) had disappeared, two {protecta, inflata) had 
become very rare, while 5 . longata and S. colligata each remained 
"rather rare.” It thus appears that a sudden increase in the con- 
1 See p. 159 for explanation of this sudden$increase. 
