SEXUAL REPRODUCTION (CONJUGATE) 227 



In some species of Zygnema (e.g. Z. pectinatum), however, the 

 events leading up to conjugation take place simultaneously in 

 the two filaments, and the fusing protoplasts meet in the middle 

 of the conjugation canal ; in such forms there is no outward 

 differentiation of sex, the gametes being isogamous as in Chlamy- 

 domonas and Ulothrix (Fig. 122, D). But even in the case first 



FIG. 122. Sexual .reproduction in various Conjugatae. A, Spirogyra 

 weberi, showing lateral conjugation (after Petit). B and C, Spirogyra 

 bellis. B, successive stages (a and b) in conjugation ; C, completed 

 conjugation. D, ladder-like, and E, lateral conjugation in Zygnema 

 pectinatum. F, small part of filament of same, c.c., conjugation 

 canal ; n., nucleus ; p., pyrenoid ; z., zygospore ; $ = male cells. 



described the gametes are merely distinguished physiologically, 

 although a difference of size occurs in one of the related genera. 

 The sexual process of the^Conjugatae therefore, whilst'peculiar 

 in itself, shows the same analogous series of stages' to that 

 observed in other Algae. 



In some species, both of Spirogyra and Zygnema, sexual union 

 may take place between adjacent cells of the same filament 



