THALLOPHYTA. 



655 



DesTmdioidecB. — 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 {of. 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 menabrane being intercalated between them. A transverse wall 



Fig. S72. Desmids. 



1 Micrasterias papillifera. 2 Mwrasterias morsa. 



* XanthidiuTn aculeaium. 



5 Staurastrum 

 i» Staur- 



3 Cosmarmm polygonum, 

 furcatum. ' EvMStrum dblongum. i Penium Brebisscmii. « Closterium Lunula. ' Xwnthidium octocorne. 

 astrwm altemans (two views). " Cosmarium tetraophthalmum. " Aptogonum Besmidium. AU the figures magnifled 

 about 200 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 



