Sex in the Conjugatae and the relative frequency of the different 
types of conjugation 
H. W. Tuurston, Jr. 
It has been much disputed in the past whether there is a true 
sexual differentiation between the cells that fuse in the process 
of zygospore formation as found in the filamentous Conjugatae. 
Bessey (2) and others have argued that the process probably is 
sexual in cases of scalariform conjugation at least. Bennett (1) 
says: ‘‘I do not myself entertain any doubt that even in lateral 
conjugation there is an incipient differentiation of sex, although 
this differentiation extends only as far as the individual cells.’ 
The Wests (6) have gone even farther, and say, ‘‘against the 
sexuality of the Zygnemeae only two plausible objections can be 
raised; these are the phenomena of /ateral and cross-conjugation,”’ 
but they conclude that there is no reason even on these grounds 
to regard the Zygnemeae as other than sexual. Regarding the 
Mesocarpeae however, they say further that ‘indications of sexu- 
ality . . . are much less marked than in the Zygnemeae,”’ and, ‘‘as 
these scarcely appreciable indications of sexuality are often absent, 
we may regard the Mesocarpeae as having lost almost all traces 
of differentiation into male and female gametes.”’ Still later, G. S. 
West (7, p. 135) discusses the question as follows: “‘ The term ‘sex- 
ual’ is often used to embrace all forms of gamogenesis, and is in this 
sense firmly established in botanical literature. It would, how- 
ever, be more scientifically accurate to distinguish between gamo- 
genesis (or the mere fusion of gametes) and sexual reproduction 
in the narrower sense (which should be restricted to those cases 
where there is a fusion of clearly differentiated 7 and 9 gametes). 
From this point of view, therefore, gamogenesis although including 
sexual reproduction is not identical with it. It must be remarked, 
however, that the gradation is so fine, especially in the Chloro- 
phyceae, that the distinction is scarcely worth making. Some- 
times, as in many of the Zygnemaceae, the gametes are morpho- 
logically’ indistinguishable but physiologically differentiated, 
441. 
