344 Conjugate 



often twisted at the middle, each half having a central pyrenoid (fig. 215 D 

 and E}. Moreover, in this species the two halves of the chloroplast, although 

 irregularly lobed, are not produced into slender branched processes. 



Multiplication by fragmentation of the vegetative filaments is very unusual 

 in Zygnema, 



Asexual reproduction by aplanospores is more frequent in Zygnema than 

 in any other Conjugates except those which have been placed in the genus 

 Gonatonema. One species, Z. spontaneum, is habitually reproduced by aplano- 

 spores, although scalariform conjugation does occur (consult Nordstedt, '78 ; 

 W. & G. S. W., '97 ; '07 ; G. S. W., '09). It is not unlikely that the aplano- 

 spores are in some cases parthenospores. They are usually globular and 

 rather smaller than the zygospores. 



Conjugation in Zygnema is almost exclusively of the scalariform type and 

 the zygospore is sometimes lodged in the conjugation-canal, species in which 

 this occurs having been placed by Kiitzing ('43 ; '49) in a separate genus, 

 Zygogonium. This character is, however, of no generic importance, since in 

 Z. spontaneum, a tropical and subtropical species, all stages occur between the 

 complete inclusion of the zygospore within the female gametangium and its 

 lodgment in the middle of the conjugation-tube (G. S. W., '09). Each gamete 

 is formed from an entire protoplast which contracts away from the cell- wall 

 and becomes a somewhat ellipsoidal mass. The gametes then glide slowly 

 into the conjugation-tube or one of them glides through that tube into the 

 opposite gametangium. In a few species of Zygnema the female gametangium 

 is considerably swollen and in some the ripe zygospores assume a dark blue 

 colour. The zygospore undergoes a period of rest and on germination the 

 young plant shows no appreciable differentiation into base and apex. 



During conjugation the male and female gametes behave differently. The 

 axis of the male gamete rotates through 90 so that a line through the two 

 chloroplasts and the nucleus is at right-angles to the axis of the filament. 

 This position is retained while the male gamete advances into the conjugation- 

 canal (Dangeard, '09; Kurssanow, '11), whereas the orientation of the female 

 gamete remains normal all through the process of conjugation. On the fusion 

 of the gametes the anterior chloroplast of the male gamete moves to one side 

 so that the male and female nuclei come into contact. The time elapsing before 

 the fusion of the nuclei is apparently variable, being much shorter in some 

 species than in others. According to Kurssanow (11) the male chloroplasts 

 disintegrate immediately after the formation of the zygote- walls. The nucleus 

 (fusion nucleus) of the zygote thereupon divides twice, three of the nuclei 

 degenerating and one assuming the characters of the true zygote-nucleus. 

 Kurssanow states that there is no conjugation of two of the secondary nuclei 

 (i.e. two of the four produced by division of the fusion-nucleus of the zygote) 

 such as Chmielewski describes in Spirogyra. It must be confessed, however, 



