THALLOPHYTA. 



655 



Desmidioidece. — The Desmids are a large family of fresh-water forms numbering 

 over a thousand species. Perhaps their most favourite habitat in this country is 

 the water which collects and stands between mosses and similar plants on imper- 

 vious soils. Many of the Desmids are among the most beautiful of algal forms. 



The great characteristic of the Desmid-cell is its almost invariable division into 

 two symmetrical halves, often separated by a circular constriction (cf. fig. 372). 

 The cell-membrane usually consists in fact of two distinct valves whose edges meet 

 in this median plane. In cell-division these two valves are forced apart, a new 

 cylindrical piece of me nbrane being intercalated between them. A transverse wall 



..^T^'^^, 



-4 





Fig. 372. Desmids, 



' ilicrasterias papilU/era. s Micrasterias morsa. 3 Cosmarium polygonum. * Xanthidinm acuteatum. 6 Staurastrum 

 /urcatuin. ^ Euastrum oblongum. ' Penimn Brebissonii. 8 Ctosterium Lunula. ^ Xanthidium octocorne. lo Staur- 

 astruut alternans (two views). " Cosjnarium Utraophthalmuin. la Aptogonum Desmidium. All the figures magnilied 

 abuut t^UD times. 



is then formed at the equator, and each half of the new piece of wall gradually 

 assumes the characters of the old half-cell to which it belongs. 



The actual form of the cell is very various; it is often lobed, and its wall 

 sculptured in various ways. A good idea of the shapes of some of the commoner 

 types can be obtained by an inspection of fig. 372; see also Plate I., i, k. 



The chromatophores are also extremely various in form. A common type is an 

 axile rod bearing longitudinal plates which radiate in all directions. Each chroma- 

 tophore contains one or more pyrenoids. 



The cell-membrane is usually perforated by series of regularly arranged, very 

 minute pores which give exit to extremely delicate filaments of protoplasm. The 

 projecting end of each filament is surrounded by a mass of mucilage, and these 

 masses together form a complete sheath covering the entire cell, and sometimes even 



