PRINCIPAL GROUPS OF PLANTS. 19 



bands (Fig. 9, //), which extend from one end of the cell to the 

 other. In these bands are embedded protein bodies known as 

 pyrenoids. The nucleus lies in the centre of the cell and is con- 

 nected with the cytoplasmic layer lining the walls of the cell by 

 delicate threads of cytoplasm. 



Spirogyra may be propagated vegetatively by one or more 

 cells of a filament breaking off and forming new individuals by 



FIG. 9. II. Spirogyra stictica, showing parts of two filaments with band-like chroma- 

 tophores (chloroplasts), in which are embedded spherical pyrenoids. Nuclei are shown 

 in some of the cells with delicate threads of cytoplasm radiating from them. Two of the 

 cells (a, a) of the adjoining filaments (A, B) are beginning conjugation. I, 5. Heeriana, 

 showing different stages of conjugation. In the upper cells, the contents have rounded off 

 previous to the rupture of the adjoining walls of the two filaments. The two middle cells 

 show the contents passing from one cell into the opposite cell. In the lower cell to the 

 right the zygospore is shown. After De Bary. 



cell division. The plant is also reproduced by means of zygo- 

 spores, as follows: The cells of two adjoining filaments each 

 send out processes (Fig. 9, II, a, a}, which meet; the end walls 

 are absorbed, forming a tube through which the contents from one 

 cell pass over into the other (Fig. 9, /) ; the contents of the two 

 cells then fuse, after which the mass becomes surrounded by a 

 cellulose wall. The spore thus formed may remain dormant over 

 winter, and the following spring germinate and form a new Spiro- 



