48 
STUDIES OF BRITISH FRESHWATER ALG^. 
Having thus disposed of the possibility of vernal reproduction in 
Spirogyra being due to an inherent tendency, we may briefly consider 
the conditions influencing the reproductive process. In the first place 
it is important to note that the necessary combination of conditions is 
almost without exception realised only in the spring, and that, when 
(as in 1904 and 1908) there is no vernal *S'/?^Voyym-phase, zygospore- 
formation does not take place at all,^ even in species, like *S'. varians 
and S. neglecta, which seem to reproduce with the greatest ease in a 
normal year. This seems to us to indicate that change of the factors 
in the direction of intensification is the essential stimulus for conjugation 
in Spirogyra^ and such a change is in a normal year likely to occur 
only in spring.^ If therefore the genus is not present at that time 
zygospore-formation does not occur in that particular year. The only 
exception to the exclusive reproduction of Spirogyra in spring in 
Abbot’s Pool is furnished by S. affinis, which was found with zygospores 
in October of 1907. In considering this case attention may be drawn 
to the fact that S. affinis (the same applies to S. neglecta) is a species 
in which the reproductive period tends to be of rather longer duration 
than in most other species of the genus. This may well indicate that 
the reproductive process in this case is not dependent on so restricted 
a range of factors as in other species, and there is therefore more 
likelihood of these conditions being realized at other times of the 3 ^ear. 
The autumn of 1907 was in many respects like a second spring ; after 
a very dull and cold summer there came a spell of three or four weeks 
of almost uninterrupted sunshine and high temperature with little rain- 
fall. As a result there must have been in September, 1907 a general 
intensification of influencing conditions (viz. increased light and 
temperature, increased concentration of water owing to evaporation, 
probably also relatively less dissolved gases than in the previous months) 
rather similar to that ordinarily taking place in spring, and this is in 
our opinion very probably the explanation for the reproduction of 
Spirogyra affinis in the autumn of 1907. In our previous paper on 
Spirogyra we dealt with another abnormal case of reproductiorl of this 
genus in the autumn (in a pond at Telscombe, near Newhaven) ^ ; there 
were two species concerned, one of them again being iS'. affinis (the other 
was S. catceniformis (Hass.), Kiitz.), so that this species certainly seems 
to have a tendency towards autumnal reproduction. In this case full 
meteorological data were available, showing that in the months preced- 
ing the autumnal reproduction of these species there was considerably 
less rainfall than usual and considerably more hours of sunshine than 
the average, so that there was no doubt again a general intensification 
of all the factors concerned. 
A study of these two abnormal cases thus gives a good deal of 
justification for the view that it is the general intensification of those 
^ cf. however tlie remarks on S. affinis below. 
“Fritsch and Rich, loc. cit., p. 431. 
■H^oc. cit., p. 433. 
