Desu iidiacece 165 



there is generally a well-marked protuberance in the middle of 

 each side of the semicells. There is one axile chloroplast in each 

 semicell, which is often very irregular; occasionally it possesses 

 irregularly disposed, longitudinal plates. There is a single central 

 pyrenoid in the small species and several scattered ones in the 

 large species. The zygospores are globose or ellipsoid, and are 

 furnished either with numerous simple spines or with conical 

 papilla?. 



There are 43 British species of the genus, about half of which are widely 

 distributed. The smallest and perhaps the most abundant species of the 

 genus is E. binale (Turp.) Ralfs; length 10 20 /z; breadth 9 16 ju.; thickness 

 5-5 7 /A; fig. 60 C. E. elegans Breb. (fig. 60 A and B), E. ansatum Ehrenb., 

 E. Didelta (Turp.) Ralfs and E. oblongum (Grev.) Ralfs (fig. 60 D) are widely 

 distributed all over the country, but in some districts they are by no means 

 common. E. insigne Hass. (length 98 110 /j. ; breadth 49 60 ^) is an upland 

 form found abundantly amongst Sphagnum. E. verrucosum Ehrenb. and 

 E. gemmatum Breb. are two of the prettiest species, and E. crassum (Breb.) 

 Kiitz. (length 167 185 ^; breadth 87 97 p) and E. oblongum (Grev.) Ralfs 

 (length 144 167 // ; breadth 80 83 /i) are the largest species found in Britain. 

 E. pectinatum Breb. is more frequently found with zygospores than any other 

 species of the genus. 



Genus Micrasterias Ag., 1827. [Holocystis Hass., 1845 ; 

 Tetrachastrum Dixon, 1859.] This genus contains some of the 

 largest and prettiest of Desmids. The cells are flattened, in many 

 cases almost disc-shaped, and they are circular or broadly elliptical 

 in outline. The semicells are divided by deep incisions into either 

 three or five lobes, of which the polar lobe may be entire or 

 furnished with a wide excavation at the apex. The lateral lobes 

 are sometimes narrow and attenuated, but more frequently they 

 widen from the base outwards and are divided by incisions of 

 variable depth into lobules. The flat surfaces of the cells are 

 occasionally furnished with spines or wart-like projections, and in 

 side or vertical view the cells are generally narrowly fusiform. 

 There is a central, plate-like chloroplast in each semicell, often 

 exhibiting irregular thickenings or ridges, and containing many 

 scattered pyrenoids. The zygospores are globose and furnished 

 with strong spines, simple or furcate at the apex. 



There are 16 British species of the genus, none of which are really abundant, 

 although J/. denticulata Breb. (length 205 350 JJL ; breadth 185 276 p. ; 

 fig. 61 C) and M. truncata (Corda) Breb. (length 100138 ^ ; breadth 90 

 129^; fig. 61 B) are widely distributed. N. rotata (Grev.) Ralfs (length 

 220240 n ; breadth 195220 M ) and J/. papillifera Breb. (length 135145 p ; 



