426 Frit sch and Rich. — Studies on the Occurrence and 
alien Jahreszeiten in Slimpfen und Teichen,’ although his own subsequent 
remarks do not point to an occurrence at alt times of the year. Regarding 
inflata , Vauch. (p. 233), he states that he found it (in the sterile condition) 
in spring, 1894 (apparently present already in February), and that the first 
indications of conjugation were observed at the beginning of May. wS. 
Weberi , Kiitz. (p. 241), appeared regularly in a pond in the Botanic 
Gardens at Basle in March and attained an abundant development in the 
course of April ; at the end of April or the beginning of May conjugation 
took place, after which the Alga disappeared. Klebs found this species 
again in the autumn, and remarks upon it as follows: 4 Erst im Herbst, 
sei es aus iiberlebenden, einzelnen Faden, sei es aus Zygoten, entwickelt 
sich die Alge noch einmal, kommt aber nicht mehr zur Konj ligation.’ This 
is in complete accord with the observations we have made on some species 
of Spirogyra , although in the case of S. Weberi our data testifying to 
an autumnal occurrence are rather meagre. These few statements quoted 
from the literature indicate that the general occurrence of Spirogyra is 
much as we have described it in the preceding paragraph, but that minor 
modifications of the ordinary scheme are frequent (as indeed our own 
observations show). It is not at all likely that the periodicity of Spirogyra 
(or of any other Alga) will be quite the same even in the same latitudes as 
numerous modifying climatic factors must come into play. 
The different species of Spirogyra certainly as a rule attain their 
maximum development in the vernal phase. The autumnal phase is on 
the whole quite subsidiary to the vernal phase — a fact which is already 
evidenced by the lack of reproduction in the autumn. The reappearance 
of species of Spirogyra in the autumn probably depends on the realization 
of certain external conditions which stimulate a certain number of the 
zygospores to germination. In the absence of the necessary conditions the 
autumnal phase may be far less evident, or even completely suppressed, 
and species which are ordinarily present both in spring and autumn may 
appear to have a vernal phase only (cf. S.jugalis , present in Abbot’s Pool 
in autumn, 1905, but absent in autumn, 1906 ; see also .S'. affinis). On the 
other hand, it is quite possible that those species, which we have been led 
to regard as purely vernal ones (e. g. S. various ), do under exceptional 
circumstances exhibit an autumnal phase as well. We have no data at 
present in support of this latter view, but they may be forthcoming in the 
course of further investigation. All species of Spirogyra would then have 
the tendency to be present both in spring and autumn, but in the case of 
some of them, the necessary conditions for an autumnal appearance would 
only very rarely be realized. The causes for the disappearance of Spirogyras 
after the vernal phase will probably be found in some or all of the factors 
connected with the advent of summer. These factors may be enumerated 
as follows : — 
