Desn iidiacece 149 



is too scanty to allow of splitting them up into a number of separate 

 genera. One cannot see how, in the present state of our know- 

 ledge, such genera could be denned, and yet some of these plants 

 are certainly not nearly related. The plant commonly known as 

 Penium minutum (Ralfs) Cleve is the most puzzling of all. 



I have accepted the two sub-families proposed by Liitkemuller 

 and also his five tribes, but have placed the " Gonatozygae " first 

 and altered the definition of the " Penieaa " in order to include the 

 heterogynous collection of Desmids embraced in the present genus 

 Penium. 



The following is a synopsis of all the known genera of Desmids, 

 four of which (Ichtkyocercus, Triploceras, Phymatodocis and Strep- 

 tonema) are exclusively tropical and one (Ancylonema) is exclusively 

 arctic. 



Sub-family I. SACCODERM.B. 



Cell-wall uusegmented and without pores. Point of division of cells 

 indefinite and unknown previous to the actual division. The young half of 

 the cell is developed obliquely and its walls are absolutely continuous with . 

 the walls of the older half. 



Tribe 1. G-onatozygae. Cells elongate, cylindrical and unconstricted, 

 forming loose filaments. Cell-wall with a differentiated outer layer of which 

 the small roughnesses and spines form a part. 



* Chloroplasts axile 1. Gonatozygon. 



** Chloroplasts- parietal and spirally twisted ... 2. Genicularia. 



Tribe 2. Spirotaeniese. Cells solitary, relatively short and mostly un- 

 constricted. Cell-wall a simple sac, without a differentiated outer layer. 

 The cell becomes adult by periodical growth. 



* One chloroplast in each cell. 



t Chloroplast .spirally twisted, axile or parietal 3. Spirotcenia. 

 tt Chloroplast plane, axile. 



Cells solitary 4. Mesotcenium. 



Cells forming short filaments f>. Ancylonema. 



** Two chloroplasts in each cell. 



t Chloroplasts star-shaped, radiating from a 



central pyrenoid 6. Cylindrocystis. 



tt Chloroplasts ridged with longitudinal 



serrated ridges . 7. Netriwm. 



V-v 



Sub-family II. PLACODERM.E. 



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

 a fixed type, with the interpolation of the younger halves between the old 

 ones. The younger portions of the cell-wall are joined to the older portions 

 by an oblique surface. 



