GROUP I. THALLOPHYTA : ALGLE. 



245- 



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

 indicated by large inert cells, heterocysts (Fig. 135 ^4), which differ 

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



Pis. 131. Gkeocapsa (x 300) in 

 various stages : A becomes H CD E 

 by repeated division. (From Sachs.) 



PIG. 135. A Filament of Nostoc ; the large 

 unshaded cell is a heterocyst. B Portion of 

 a filament of Oscillaria (+ 300). 



hormogonia are motile, though the mechanism of their movements : 

 is not understood ; they eventually separate, and escaping from 

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

 by growth and cell-division into new filaments (Fig. 136 A B}. 



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

 Each spore is formed from a single cell of the body, which sur- 

 rounds itself with a thick firm exospore ; the spore germinates 



s* t ~ ** 



PIG. 136. (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 B uniting at alternate ends, so as to constitute a Nostoc- 

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



under favourable conditions, the exospore being ruptured (Fig. 

 136 C). 



It is possible that zoospores are produced in some forms, but 



