430 Fritsch and Rich . — Studies on the Occurrence and 
and 1907 ; vS\ quadrata (Hass.), Petit, and .S', cataeniformis (Hass.), Kiitz., 
formed zygospores only in 1906, 5 . Hassallii (Jenn.), Petit, only in 1907. 
Although the period of reproduction may be roughly limited to April- 
June, reproduction takes place at different times in the individual species 
during this period (cf. the table). Thus in Barton’s Pond a considerable 
amount of sterile Spirogyra was present on April 9, 1906 ; on April 21, 
S'. quadrata (Hass.), Petit, and 5 . tenuissima (Hass.), Kiitz., were observed 
with zygospores, while 5 . cataeniformis (Hass.), Kiitz., was beginning to 
conjugate ; on May 5 matters were unaltered, and 5 . cataeniformis was 
only observed with zygospores on May 19. By June 18 all Spirogyras 
had disappeared. Similarly, in Abbot’s Pool in 1905, 5 . varians and 
S'. jugalis were found in the reproductive condition in April ; while .S. 
neglecta and 5 . nitida , although previously present, did not begin to repro- 
duce before May. In some species the period of reproduction is remarkably 
long (e. g. S', affinis (Hass.), Petit, S', neglecta (Hass.), Kiitz.), in others short 
(e. g. S', vaidans (Hass.), Kiitz., 5 . Weberi , Kiitz., S. jugalis (Dillw.), Kiitz.). 
As a rule, if the same species is found in two or more ponds, and is 
reproductive in one of them, it is found to reproduce in all of them, though 
not always at exactly the same time (e. g. S', varians , S', neglecta var. ternata , 
S', quadrata , S', jugalis in 1905) ; this is, however, not without exception 
(cf. S'. Weberi , S. jugalis in 1906). In the case of some species (e. g. 
S', varians , S', tenuissima , S’, condensata , S', affinis) the conditions causing 
reproduction affect nearly all the filaments, and with the formation of 
zygospores the species disappears ; in other cases, however (e. g. S', neglecta 
var. ternata , .S', nitida , S'. Weberi ), by no means all the filaments become 
involved in conjugation, and these sterile filaments may persist for some 
time after zygospore-formation. This phenomenon is no doubt also 
dependent on the degree of development of the external factors influencing 
reproduction in Spirogyra , and one and the same species may in some cases 
disappear completely after reproduction, in other cases still persist for a time 
(cf. S', affinis , S', neglecta , &c.). Another feature illustrating the varying 
effect of external conditions on reproduction in Spirogyra, is the occasional 
conjugation of a species without the process coming to an end (i. e. without 
formation of zygospores), cf. S', jtigalis (Dilhv.), Kiitz., in Abbot’s Pool in 
1905, and at Tiltham’s Pond in 1906. 
The facts detailed in the preceding paragraphs suffice to show the 
complexity of the conditions influencing vernal reproduction in Spirogyra . 
The prevalent occurrence of reproduction in the vernal phase may be due 
to an inherent tendency, or to certain combinations of external conditions, 
which occur more or less regularly every spring (seasonal factors). If vernal 
reproduction is the result of inherent tendency, then it is difficult to under- 
stand why a species reproduces in the spring of one year and not (although 
present) in the spring of another year (cf. the data given above) ; moreover, 
