26 BRITISH DESMIDIACE.E. 



+ + Chloro])last plane, axile. 

 + Cells solitary. 



Mesotasnium. 

 + + Cells united into short filaments. 



Ancylonennt . 

 Two chloroplasts in each cell. 



f Chloroplasts star-shaped, radiating froni a central 

 pyrenoid. 



Cylindrocyslis. 

 ft Chloroplasts ridged with longitudinal ridges; edges of 



ridges notched. 



Netrinin. 



Sub-family II. PLACODERM.E. 



Cell-wall segmented, with a differentiated outer layer. Cell- 

 division following a fixed type and the younger half -cells interpo- 

 lated between the old ones. The younger portions of the cell- wall 

 are not continuous with the older portions but are joined obliquely 

 to them. 



A. Point of division of cells variable or sometimes fixed (at 



the isthmus). 



Tribe 3. Penieae. Cells of moderate length, straight and more 

 or less cylindrical, sometimes with a slight central constriction. 



" . 



Points of division often variable, following no law. Cell- wall with 

 or without pores. There is frequently a periodical growth of the 

 cell until maturity is reached. 



Penium. 



Tribe 4. Closteriese. Cells elongate, generally curved. Points 

 of division regularly placed in the middle region of the cell. Cell- 

 wall commonly with pores. 



' Cells almost cylindrical, scarcely attenuated. Ghloroplast 

 single. Nucleus in a lateral position. Apical vacuoles 

 absent, 



Roy a. 



** Cells strongly attenuated towards each end. Two chloro- 

 plasts in each cell. Apical vacuoles with moving 

 granules present. 



Closteriwm. 



B. Point of division always fixed (at the isthmus). 



Tribe 5. Cosmariese. The cells exhibit great diversity of form, 

 and the cell-wall consists of two thin, firm layers with pores. The 

 cell becomes adult soon after division by the mature growth of the 

 young SBiiiici'H. There is no periodical growth. 



