48 BRITISH DESMTDIACE^E. 



Genus 4. MESOTJENIUM Nag. 184'.. 



Niig. Gatt. einz. Alg. 1849, p. 108. 



De Bary, Conj. 1858, p. 20, 30, 74. 



Arch, in Proc. Dubl. Nat. Hist. Soc. 1864, p. 20. 



Cooke, Brit. Desm. 1886, p. 47. 



De Toni, Syll. Alg., 1889, p. 811. 



Cells cylindrical or subcylindrical, usually straight, 

 often slightly curved, not constricted, apices rounded 

 or subtruncate ; with a solitary chloroplast (more rarely 

 with two chloroplasts), which is axile, flattened, and 

 plate-like ; pyrenoids one or several ; nucleus frequently 

 excentric. 



Of the ten British species of this genus, only three occur 

 as free-floating Desmicls in quiet water. The remaining 

 seven are found in gelatinous masses amongst mosses, princi- 

 pally on wet rocks. 



The cells are frequently filled with large quantities of 

 reserve products which completely obscure the choloroplast. 



The British species are best arranged in the following 

 manner : 



A. Cells embedded in mucilaginous masses. 



^ Cell-sap uncoloured. 



t Cells cylindrical or ellipsoidal, poles rounded- 

 Cells large, often curved, 3-5 times longer than 

 broad. 



1. M. De Greyi. 



Cells of medium size, rarely curved, l-2 times 

 longer than broad. 



2. M. mirificum. 

 Cells small, cylindrical, 2-2i times longer than 



broad; diam. 15/u. 



3. M. macrococcum. 

 Cells small, cylindrical, 2-2^ times longer than 



broad ; diam. 11-12 ^. 



4. M. clilamydosporum. 

 ft Cells subcylindrical, poles attenuated. 



5. M. caldariornm. 

 ttt Cells cylindrical, poles truncate. 



6. M. trimcatum. 

 ' f Cell-sap normally coloured violet. 



7. M. violascens. 



