22 BOTANICAL GAZETTE [january 



where several closely allied species are associated, this is practically 

 impossible. Fortunately this did not occur except in a few places. 



may 



and 



conclusions relative to a given species during its period of vegetative 



ermine with some 



the different species of Spirogyra have any definite or fixed periods 

 at which they conjugate, or whether this phenomenon takes place only 

 under certain conditions of environment. In all cases notes were 

 taken at each visit to a pond, and the determination of the species 



my 



life-history could be worked out with some certainty. Leaving out 

 of account uncertain conditions such as the vegetative activities, and 

 placing special importance on the period at which conjugation takes 



^ m _ jfe 



nomena (vegetative and reproductive activities) in the life-history of 

 this alga appears periodically, not due exclusively to seasonal condi- 

 tions nor to environment. 



No one can say with absolute certainty whether the natural forces 

 that produce conjugation are internal or external. It is a fact, 

 however, that whenever a condition was present, in any part of a 

 mass, which was able to initiate and control reproductive activity, 

 there was at the same time and place either the same or some other 

 condition which brought about the destruction of all vegetative 

 filaments. In this study no exception was found to this rule, whether 

 under laboratory or under natural environment. 



Up to the time when conjugation was abundant in ponds, brooks, 

 and ditches, I still had over 300 aquaria in the laboratory. A small 



percentage of them contained fruiting Spirogyra. The 



for any given species began about one week earlier and lasted about 

 one week longer in the laboratory than that of the field. The disap- 





time 



filament 



It was 



May they began to decline and were kept only with increasing care 

 until mid-summer, when out of 700 only one remained. 



As to the different sizes of aquaria, it ought to be said that some 



ary test-tubes; some contained 25-30 liters; and the 



one to ten liters. The results were not constant 



were 



majority ranged from 



