268 ALGsE 



ORDER 3. DESMIDIACE^:. 



The Desmids are unicellular organisms, for the most part solitary, 

 and inhabiting almost exclusively fresh water, especially stagnant, where 

 they occur in very large numbers; a very few species are brackish. 

 They always float free without any attachment to the bottom or to other 

 algae," and many species possess a power of apparently spontaneous 

 motion through the water similar to that of diatoms, though not so 

 strongly marked. In several genera, as Desmidium (Ag.) and Hyalotheca 

 (Ehrb.), the individuals are united into long filaments ; and either the 

 separate individuals or the filaments are invested by a more or less 

 dense mucilaginous envelope, species of Desmidium and Hyalotheca 

 frequently forming a green slime on the surface of moor pools. The 

 origin and structure of the mucilaginous sheath appear to be the same 

 as in the Zygnemacese. According to Hauptfleisch, the cell-wall of 

 desmids always consists of two distinct layers, sometimes of more, which 

 are then provided with a girdle-band similar to that of diatoms. He 

 also states that in nearly all species the cell-membrane is perforated, 

 and that through these pores proceed threads of protoplasm, connecting 

 the protoplasm in the interior of the cell with the gelatinous envelope 

 which is excreted through the pores from the cell-contents. In the 

 filamentous species the protoplasm is probably in connection throughout 

 the filament. 



The cells vary greatly in size and form in different species, the 

 largest (Cosmarium, Cord., Micrasterias, Ag.) being just visible to the 

 naked eye. The individual is usually divided by a deep constriction into 

 two symmetrical halves ; and even where this is not the case, the cell- 

 contents chlorophyll-bodies and starch-grains are symmetrically 

 arranged in the two halves of the cell. The cell-wall is smooth, or 

 punctated, warty, or even elevated into spines, but has no (or very little) 

 deposit of silica. The cells contain a large quantity of chlorophyll of a 

 bright green colour, never concealed by any pigment, often arranged in 

 bands or stars, and containing much starch. Each genus has its own 

 general form of cell, often of very great beauty. In Docidium (Breb.), 

 Penium (Breb.), and Tetmemorus (Ralfs) the individual or ' frustule ' 

 is elongated, cylindrical, and usually divided by a constriction into two 

 halves placed end to end; in Closterium (Nitzsch) it is crescent- shaped; 

 in Micrasterias it is very thin and flat, usually with a more or less orbi- 

 cular or elliptical outline, deeply divided into two symmetrical halves, 

 and each half more or less deeply lobed ; in Euastrum (Ehrb.) the indi- 

 vidual is usually smaller than in Micrasterias, often very minute, and 



