STUDIES OF BRITISH FRESHWATER ALGJ5^. 
39 
concentration was still realised in time (though later than usual) so 
that the appearance of the vernal phase was merely delayed. An 
alternative possibility, which would, however, operate simultaneously 
with the other, is that the excessive rainfall during the last months of 
the previous year leads to such a rise of the water-level that the 
amount of light reaching the bottom of the water in the following 
spring is below the minimum intensity necessary for the germination 
of the zj'gospores. We have no record of a considerable change 
of water-level, although slight changes must certainly occur ; in view of 
the steep sides of the pond, however, they might not be easily noticed. 
In connection with this second suggestion it may be pointed out that 
most of the species of Spirogyra (S. affinis, S. nitida^ S. rividaris) 
commence their vernal phase already in January or February, whilst 
some (e.g. 8. varians) only appear in March or April (cf. table on p. 
47). This appearance of different species at diflferent times may also be 
due to their zygospores requiring different light-intensities for 
germination. 
It seems fairly certain, therefore, that a heavy autumnal rainfall has 
a direct bearing on the elimination of the vernal iS^^^iro^ym-phase in 
the following year, and it is not improbable that this may be due to 
the dilution of the water, or to decreased intensity of illumination at 
the bottom of the pond owing to higher water-level, or to both. This 
is very suggestive as to the conditions which determine the otherwise 
normally recurring vernal phase ; they would include the realisation of 
a certain concentration of the water and of a certain light-intensity, 
both of which, in a normal year, would come as a matter of course 
during the spring-months. A certain temperature is no doubt also 
necessary, but inasmuch as this varies little during the spring-months of 
the five years of observation, we have no data to go upon ; temperature 
is certainly not accountable for the almost complete absence of Spirogyra 
in spring of 1904 and 1908. 
The autumnal Spirogyra is generally rather insignificant as 
compared with the vernal one, which is further as a rule the only 
period of zygospore-formation. As there is usually a period of almost 
complete disappearance or anyhow of great scarcity of Spirogyra 
during the summer-months (J uly, August), its reappearance in the 
autumn must be due either to the germination of some of the zygospores 
formed in the spring or to the persistence of a certain number of 
filaments in the dormant condition at the bottom of the pond during 
the period of apparent absence. The latter seems to be more likely,^ 
for it is difficult to understand why, if some of the zygospores germinate, 
all do not do so. Vegetative reproduction is moreover at times so 
prolific in Spirogyra that persistence of a few filaments would easily 
account for an autumnal phase. The conditions necessary for the 
development of the autumnal phase are by no means always realised, 
and a species present in the autumn of one year may be absent in that 
of the next, e,g. S. jugalis present in autumn of 1905 and 1907, but 
1 See, however, Fritsch & Rich, loc. cit., p. 425, and footnote 1. Further con- 
sideration has led us to modify the view there expressed. 
