STUDIES OP BRITISH FRESHWATER ALG^. 
45 
it is only during the colder portion of the year that they ever occur in 
any noticeable quantity. They are represented chiefly by species of 
Oscillaria, the few other genera present being very scanty in point of 
individuals. The normal increase in amount during the winter months 
was apparently not exhibited in 1904 and in 1906 for some at present 
incomprehensible reason. Species of Oscillaria are known to get on 
best when there is a considerable amount of organic matter dissolved 
in the water, and this is no doubt the case in the late autumn and 
during the winter months (cf. p. 30) ; possibly some change in this factor 
was responsible for the small amount of Cyanophycece in the winter of 
1904 and 1906. 
^._general consideration of reproduction 
IN THE POND. 
Although Abbot’s Pool has furnished abundant material for the 
study of reproduction in Spirogyra, it has not been favourable for 
similar observations on other forms. Reproduction in Cladophora, 
(Edoqonium^ Mougeotia^ and Microspora seems to have taken place 
only asexually during the period of observation, although it is possible 
that the occurrence of sexual reproduction in the first of these genera 
has escaped our notice. But even of asexual i-eproduction by zoospores 
we have seen very little in Cladophora, and it almost seems as though 
the reproduction of this genus in Abbot’s Pool were almost wholly 
vegetative, i.e. by a process of breaking up of the filaments. The same 
seems to apply to the Microspora, while the Mougeotia (since no 
zygospores were observed) must have reproduced wholly by vegetative 
means. The apparently complete absence of sexual reproduction in the 
different species of CEdogonium is ver}^ striking, as this genus is so 
commonly to be found with oogonia and oospores. Asexual reproduc- 
tion by zoospores (of which the abundant young plants bear testimony) 
takes place freely at certain times of the year, but for some reason or 
other the stimulus necessary for the sexual process is never given. We 
are inclined to associate this with the rather uniform conditions^ in the 
pond, which lead to the uniform occurrence of species of the genus all 
the year round, but we do this with reserve, and hope on some future 
occasion to be able to cast more light on the matter. The periods of 
vegetative and asexual reproduction in the genera just considered 
naturally correspond to, or immediately precede, their periods of chief 
abundance (e.g.^ numerous young plants of CEdogonium in the autumn 
of 1905); we have however been able to make so few observations on 
possible determining conditions that we prefer to leave any discussion 
of this matter to a later communication. 
In every year in which the normal Spirogyra-^o^^Q occurred in the 
spring, reproduction of diverse (not all) of the species of this genus was 
a great feature in Abbot’s Pool ; and we may now proceed to consider 
this important point. In the first paper of this series ^ we have 
^ cf. Oltmanns, Morphologic u. Biologic d, Algcn. Vol. II. 1905, p. 248. 
^ Loc. cit. , p. 429 et sq. 
