170 



STRUCTURAL BOTANY 



grows in from the outside of the cell, first appearing as 

 a mere ring, and gradually advancing towards the middle, 

 until a complete disc of cellulose is formed across the 

 cell, dividing its contents into two parts and separating 

 the newly-formed daughter-nuclei (see Fig. 71). The 

 growth of the cell-wall is entirely due to additions from 

 the protoplasm. This mode of cell-division by ingrowth 

 from the outer wall is common among the lower plants, 

 while in the rest of the vegetable kingdom the new wall 

 is as a rule formed simultaneously over its whole area. 



FIG. 71. Cell of Spirogyra nitida during division. There are 

 several spiral chloroplasts, with numerous pyrenoids. The 

 two daughter-nuclei have already separated ; at W W the 

 new cell-wall is growing in between them. Protoplasm con- 

 centrated at the growing edge of the cell-wall. Magnified 

 about 200. (R. S.) 



2. REPRODUCTION 



Except for the occasional breaking up of a filament 

 into its constituent cells, Spirogyra possesses no asexual 

 means of propagation. Its normal reproduction is 

 always sexual, and is a form of conjugation, consisting 

 in the union of like cells. As we shall see, however, it 

 is quite different from the mode of conjugation in 

 Ulothrix, for in Spirogyra and its allies it is the 

 ordinary vegetative cells which unite. Conjugation, in 

 most cases, takes place between two filaments lying side 



