Desmidiacece 161 



the measurements of a species, the diameter across the middle of 

 the cell should be given, the distance between the apices, and the 

 curvature of the outer margin expressed in degrees of arc. 



The zygospores are generally globose and smooth, but the 

 spherical zygospore of Cl. co.losporum Wittr. is furnished with 

 conical papillations. The zygospores of certain species, such as 

 Cl. rostratum Ehrenb. (fig. 56 G) and Cl. Kutzingii Breb., are 

 flattened and quadrate with truncate angles ; that of Cl. Venus 

 Klitz. is also angular and sometimes twisted. 



There are about 60 British species of the genus, of which Cl. turgidum 

 Ehrenb. (length 476 940 p. ; breadth 43 80 /*) is the largest and Cl. pusillum 

 Hantzsch var. monolithum Wittr. (length 29'8 40'4 ,u. ; breadth 7'5 8'6 //,) 

 the smallest. Cl. aciculare Tuffen West and C. pronum Breb. are the most 

 elongate species of the genus, and certain forms of Cl. acutum Breb. (fig. 56 E) 

 are the narrowest. The commonest and most widely distributed species are 

 Cl. parvulum Nag. (fig. 06 F), Cl. Venus Kiitz., Cl. Leibleinii Kiitz. (fig. 51 B), 

 Cl. moniliferum (Bory) Ehrenb., Cl. Ehreribergii Menegh. (fig. 51 A) and Cl. 

 acerosum (Schrank) Ehrenb. Cl. gracile Breb. is often abundant in Sphagnum 

 bogs. The commonest striated species are Cl. striolatum Ehrenb. and Cl. 

 rostratum Ehrenb. 



Liitkemiiller has divided the genus and I think quite correctly into two 

 groups, the first one being characterized by the possession of an elongated 

 median girdle, which is an intercalation of a cylindrical piece of cell-wall 

 resulting from a growth to maturity subsequent to cell-division. The second 

 group is characterized by the absence of this girdle, the two daughter-cells 

 becoming adult immediately after cell-division. 



B. Point of division of cells always fixed at the isthmus. 



Tribe 5. Cosmariece. 



The great majority of Desmids are included in this tribe and 

 amongst them there is great diversit/ of form and size. They are 

 solitary or filamentous, or grouped in spherical colonies, and most 

 of them possess at least three planes of symmetry. The cell-wall 

 consists of two thin, very firm layers, with pores, and is frequently 

 ornamented with spines, warts and excrescences of all kinds. 

 There is no periodical growth, the cell becoming adult immediately 

 after division by the mature growth of the younger semicell. 



Series a. The oblique junction of the new and old halves of 

 the cell-wall (at the region of the isthmus) remains quite plane 

 during division. The plants are solitary or colonial. 



W. A. 1 1 



