412 



Heterotrichales 



apical cell may have a conical extremity or, as in T. affinis, it may be 

 apiculate. The cell-wall is very firm and has a definite structure, readily 

 breaking up into H -pieces in the genus Tribonema, but to a much less 

 marked degree in Bumilleria. Each H -piece consists of a transverse wall 

 with a cylindrical piece on either side, and the whole is composed of a number 

 of apposed layers of pectic compounds (fig. 263 6r). Each cell is thus 

 bounded by the halves of two H -pieces. The cells are uninucleate or very 

 rarely binucleate. The chromatophores are parietal discs in T. bombycina 



Fig. 263. A G, Tribonema bombycina (Ag.) Derb. & Sol.; A, part of vegetative filament; 

 Z>, showing aplanospores (ap); (7, zoogonidium ; DF, germination of zoogonidium and 

 formation of young plants ; G, showing structure of cell-wall after treatment with potassium 

 hydrate. H and I, T. bombycina forma minor (Wille) G. S. West. G, x 570 (after Bohlin) ; 

 A F, H and 7, x 450. 



and most other species (consult fig. 263 A and E). It is this discoidal 

 character of the parietal chromatophores which at once distinguishes the 

 common species of Tribonema from all other Green Algse. In T. affinis the 

 chromatophores, although parietal, are few in number and very irregular 

 in shape. The stored reserve is oil, which is scattered in small globules 

 through the cell. 



Asexual reproduction takes place by the formation of globular or ellipsoid 

 aplanospores, which escape by the breaking up of the filament (fig. 263 B 



