Reproduction of British Freshwater Algae in Nature . 435 
give some idea of the different appearance of the zygospores according to 
their position in the cell ; in some cases the ends of the spores were quite 
pointed, at other times blunt and almost square. For a time we were 
inclined to regard this form as a distinct species, but we found so many 
transitions between it and the typical 5. neglecta that we have come to the 
conclusion that it cannot be separated from that species ; it may possibly 
prove to be a definite variety. 
We may add a few notes regarding other species. S'. various (Hass.), 
Kiitz. (Fig. 1), is mentioned by G. S. West 1 as frequently exhibiting 
lateral conjugation. Although, as will be seen by a glance at the table, 
we have often found this species in the reproductive condition, we have 
noticed only scalariform conjugation. In a pond at Hendon, near 
London (sample collected on April 25, 1905), a long filament of S. varia 7 is 
was observed, which was doubled back and conjugating with itself. In 
Fig. 3 vS. affinis is shown with both scalariform and lateral conjugation in 
the same filament. Figs. 6 and 7 show the two types of conjugation in 
S. longata. 
E. Summary. 
The species of Spirogyra , which we have examined, are either purely 
vernal or exhibit both a vernal and an autumnal phase with an intervening 
period of scarcity or complete disappearance ; it seems possible that there 
is also a period of disappearance in midwinter, but this is not certainly 
established. The autumnal reappearance of certain species of Spirogyra is 
no doubt due to the influence of certain combinations of external factors 
causing a small number of zygospores to germinate ; in the absence of 
these conditions there may be no autumnal phase. Abnormal meteoro- 
logical conditions may bring about abnormal absence or occurrence of 
Spirogyra. 
Reproduction takes place ordinarily in the vernal phase, and is most 
probably the result of certain periodically recurring combinations of 
factors, which vary for different species. A considerable number of data 
are advanced in support of this view. The nature of the stimulus causing 
vernal reproduction is probably an intensification of those conditions, 
which are liable to change in spring. Such intensification may excep- 
tionally take place at other times of the year and lead to reproduction at 
other times than in spring. 
1 A treatise on the British Freshwater Algae. Cambridge, 1904, p. 125. 
