356 



Conjugate 



in the case of S. vertebratum and 8. Aubertianum are the sole means of 

 attachment of the cells. Colonial species sometimes occur in genera in 

 which the species are normally solitary, such as Pleurotsenium coronulatum, 

 PL perlongum, and Micrasterias foliacea (fig. 239 B}. The latter is a tropical 

 and subtropical species with remarkable tooth-like apical connections, which 

 result in very rigid filaments of cells. The apical attachment of this extra- 

 ordinary species was first accurately described and figured by Johnson ('94). 

 In Cosmocladium the Cosmarium-\ike cells are held together by stalks of 

 mucilage, and the entire colony is quite small, branched in character, and 

 free-floating (fig. 226 A and B). In Oocardium, the most curious of all the 

 genera of Desmids, the colony is hemispherical in shape, 1 to 2 mm. in 

 diameter, and occurs attached to calcareous rocks in the beds of streams. It 

 consists of a number of more or less parallel, radiating strands of mucus of 



Fig. 220. Three species of Euastrum. A, E. aspenim Borge, x 468 ; B. E. serratum Joshua, 

 x 468 ; C, E. inermius (Nordst.) Turn. var. burmense W. & G. S. West, x 468. a, front view 

 of cell ; b, vertical or end view; c, side view. 



considerable thickness, each strand widening out towards the surface of the 

 colony and occasionally branching. In the free end of each mucous strand is 

 lodged a single cell, so arranged that its longitudinal axis is at right angles 

 to the axis of the mucous strand (fig. 226 D F). 



The minute structure of the cell-wall was first studied by Klebs ('85) who 

 examined the nature of the mucilaginous outer coat and also demonstrated in 

 certain species the presence of pores passing right through the wall. Haupt- 

 fleisch ('88) also contributed some information on this subject, more especially 

 with regard to the extent and structure of the mucilaginous envelope. Some 

 years later Liitkemliller ('94) described very minutely the pores in the cell- 

 wall of Closterium, and eight years later (Liitkemiiller, '02) published an 

 important account of the general structure of the cell-wall in Desmids. A 

 further paper by the same author emphasized the importance of the structure 

 of the cell-wall in formulating any sound scheme of classification of Desmids 

 (vide Liitkemiiller, '05). 



