1 24 ChlorophycecK 



an asexual one 1 . The aplanospores (or, as they are frequently 

 termed, ' parthenospores ') of Zygnema are generally globular and 

 rather smaller than the zygospores, but those of Spirogyra are as 

 a rule very similar in form to the zygospores. Klebs has stated 

 that parthenogenetic resting-spores can be produced in filaments 

 with long conjugating- tubes by placing them in a strong solution 

 of sugar. Spirogyra mirabilis (Hass.) Petit is reproduced solely 

 by spores resembling aplanospores, but produced by a degenerate 

 form of conjugation'-. Many of the upland forms of Zygnema 

 habitually form solitary resting-cells or ' cysts ' in order to survive 

 the winter. 



Sexual reproduction takes place by conjugation in all the 

 genera of this sub-family. The usual type of conjugation is scala- 

 riform, between the cells of two distinct filaments. The conju- 

 gating-tube is formed exactly as in Mougeotia, and in all except 

 species of the genus Debarya and certain species of Zygnema 

 belonging to the section Zygogonium, the fusion of the gametes 

 takes place in one of the gametangia. In the formation of a 

 gamete the protoplasmic mass contracts away from the cell-wall, 

 the chloroplast loses much of its original form, and the mass 

 becomes of an ellipsoidal shape. The whole of this mass then 

 glides gradually from its gametangium through the conjugating- 

 tube into the opposite gametangium. The gametes may coalesce 

 immediately on contact and before the one has completely passed 

 through the conjugating-tube, or they may lie side by side in the 

 gametangium before fusing. On the coalescence of the gametes 

 and the fusion of their nuclei the mass generally assumes a spheri- 

 cal or ellipsoidal form, develops a thick cell-wall and is known as 

 the zygospore (or zygote). 



The cell in which the fusion of the gametes takes place and 

 therefore the one in which the zygospore is formed, is known as 

 the female cell, and the one which is emptied as the male cell ; 

 and that part of the conjugating-tube developed from the female 



1 Nordst. 'Alg. et Char. Sandvic.' 1878, p. 17, t. 1, f. 23, 24. 



- Petit in his ' Hpirogyra des envir. de Paris,' p. 14, writes concerning S. mira- 

 bilis : "Cette tres curieuse espece ne conjugue pas et ne laisse voir aucun tube 

 copulateur; a une certaine epoque de la vie de la plante, les cellules renflent vers le 

 milieu, 1'endochrome se partage en deux parties qui se concentrent sous forme de 

 globule aux deux extremities de la cellule ; il se forme ainsi une differentiation entre 

 les parties de 1'endochrome. Bientot les deux globules se rapprochent vers la partie 

 reuflee de la cellule et finissent par se reunir en constituent ainsi la zygospore." 

 This is a similar phenomenon to that which occasionally occurs in the formation of 

 the aplanospores of Gonatonema. (Consult page 118.) 



