232 



PART III. THE CLASSIFICATION OF PLANTS. 



it simply breaks up into a number of portions each of which 

 becomes a new individual. In the filamentous forms, vegetative 



FIG. 163. Glceocapsa (x 300) in 

 various stages. A becomes B C D E 

 by repeated division. (From Sachs.) 



FIG. 164. A filament of Nostoc ; the large 

 unshaded cell is a heterocyst; B portion of 

 a filament of Oscillaria (x 300). 



propagation is effected by the breaking up of the filament into 

 lengths, each such portion being termed a hormogonium in most 

 of them (except Oscillariaceae) the limits of the hormogonia are 

 indicated by large inert cells, heterocysts (Fig. 164 -4), which differ 

 both in size and colour from the living cells of the filament. The 

 hormogonia are motile, though the mechanism of their movements 

 is not understood ; they eventually separate, and, escaping from 



FIG. 165. (After Thuret: x 330). A and B development of a filament from a hormo- 

 gonium of Nostoc vesicarium. A cells of hormogonium dividing at right angles to its long 

 axis ; B rows of cells formed as in A uniting at alternate ends, so as to constitute a Nostoc- 

 filament ; z heterocysts ; C germinating spores of Anabcena licheniformis. 



the common mucilaginous cell-wall of the filament, they develope 

 by growth and cell-division into new filaments (Fig. 165 A B). 

 In many cases special reproductive cells, spores, are produced. 



