354 Conjugate 



Family Desmidiacese. 



The Alga3 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. Desmids are unicellular plants 

 and the greater number of them lead a solitary existence. Certain of them 

 are, however, associated in colonies and others are more or less closely united 

 into longer or shorter filaments. They are essentially free-floating Alga? and 

 frequently occur in great abundance in small ponds, in the quiet margins of 

 rocky lakes, in Sphagnum-laogB,' 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 constriction or narrowing of the cell is known as the 

 sinus. The depth of the constriction varies very much even in different 

 species of the same genus (compare fig. 219 A and fig. 219 D) and most of the 

 very deeply constricted forms have a linear sinus. None of the true Sacco- 

 derm Desmids are constricted, but of the vast majority of Desmids, which 

 belong to the Placoderma?, few are without a median constriction of greater 

 or less depth 1 . 



One of the most striking features of the family is the extraordinary com- 

 plexity of the cell-outlines. The margin of 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 protuberances, most of which are 

 arranged in some definite pattern. The greatest complexity of cell-outline is 

 met with in species of Micrasterias (fig. 223), Euastrum (fig. 220) and Staur- 

 astrum (figs. 221 and 222). Some Desmids are more or less cylindrical 

 (consult figs. 227, 230 and 238), many others are compressed in the plane of 

 the front view (consult figs. 219, 220 and 223), and those of the genus 

 Staurastrum, when seen from the end view, are triangular, quadrangular, or 

 with radiating processes (consult figs. 221 and 222). In Closterium the cells 

 are subcylindrical, attenuated from the middle towards each end, and curved 

 or sublimate (fig. 229.4 and J9; fig. 231-4 E). In some species of this 

 genus the cell-wall is longitudinally striated, the striations representing 

 internal thickenings of the wall. External to the firmer portion of the cell- 

 wall, which consists chiefly of cellulose, are layers of mucilaginous pectose 

 compounds. The latter often form a thick mucilaginous coat completely 

 surrounding the individual (fig. 228 B E), or, in the case of some colonial 

 forms, entirely enveloping the colony. It is by means of this mucilaginous 

 envelope that many Desmids adhere to other larger aquatic plants and that 



1 With the exception of one or two species, such as Closterium subcompactum W. & G. S. West 

 ('02), the species of Closterium are destitute of a median constriction. Several species of Penium 

 are likewise unconstricted. 



