Desmididcece 135 



off in the TemnogametaceEe. Gray's description of Choaspis 1 is 

 a very good one and is twenty-two years previous to Klitzing's 

 description of Sirogonium. 



Ch. stictica (Eng. Bot.) O. Kunze [ = Sirogonium sticticum Kiitz.] is the only 

 British species (fig. 50 A C). The cells are 2 6 times longer than their 

 diameter (which is 40 50 /*), and there are several more or less longitudinal 

 chloroplasts. The zygospores are ellipsoidal, about 75 /* in length and 42 p. in 

 breadth. The plants usually occur attached to stones over which the water 

 is running fairly fast ; they are also said to occur in stagnant water. 



Family 2. DESMIDIACE.EJ. 



The plants included in this family are remarkable for their 

 great diversity of form and their wonderful symmetry. Indeed, 

 the group includes some of the most beautiful of microscopic 

 objects. They are unicellular plants and the major portion of 

 them lead a solitary existence. Certain of them are, however, 

 associated in colonies and others are more or less closely united 

 into long filaments. They are essentially free-floating Algae and 

 frequently occur in enormous abundance in small ponds, in the 

 quiet margins of lakes, and in other favourable localities. 



Most Desmids exhibit a more or less distinct constriction into 

 two perfectly symmetrical halves ; each half is termed a semicell 

 and the narrower part connecting the two semicells is known as 

 the isthmus. The angle resulting on either side from the con- 

 striction or narrowing of the cell is known as the sinus. 



One of the most striking features of the family is the extra- 

 ordinary complexity of the cell-outlines. The cell is often deeply 

 lobed or incised, and the exterior of the cell-wall is frequently 

 covered with granules, spines, wart-like thickenings, or other pro- 

 tuberances, most of which have a definite pattern of arrangement. 

 This firmer portion of the cell-wall consists chiefly of cellulose, and 

 external to it are layers of gelatinous pectose compounds. The 

 latter often form a thick mucilaginous coat completely surrounding 

 the individual, or, as in the case of colonial forms, entirely envelop- 

 ing the colony. It is by means of this mucilaginous envelope 

 that Desmids adhere to other larger aquatic plants, -and sometimes, 

 when the conditions have been favourable for rapid multiplication, 

 enormous numbers of individuals occur embedded in masses of 

 jelly. Sometimes the mucus is very tough. In the genus Spon- 



1 S. F. Gray, Arrang. Brit. Plants, 1821, vol. i, p. 299. 



