Desmidiacese 



359 



the pores are evenly distributed over the whole surface of the wall, except at 

 the isthmus, where they are always absent, and in many cases there are 

 numerous smaller pores between the large ones. It is highly probable that 

 pores are present in the cell-wall 

 of all Placoderm Desmids. In the 

 striated species of Closterium the 

 pores are in longitudinal rows 

 between the striations. In granu- 

 lated and verrucose species of 

 Cosmarium the pores are in many 

 instances arranged in groups 

 around the individual granules, 

 and in some genera there are 

 often larger pores at the ex- 

 tremities of the cell, which are 

 usually situated in local thicken- 

 ings of the cell-wall. In Cosmo- 

 cladium and in many of the 

 filamentous genera of Desmids, 

 such as Hyalotheca, Gymnozyya, 

 Onychonema, etc., the pores are 

 not evenly distributed, but have 

 a definite disposition in certain 

 parts of the wall (consult figs. 

 225 B and 226 G). 



External to the firm layers of 

 the cell-wall is the mucus which 

 is secreted through the pores. In 

 the great majority of Desmids this mucus is small in amount and is easily 

 diffluent, but in some it is copious and forms a mucous mass entirely 

 enveloping the individual (fig. 228 B E), or, in some filamentous species, 

 it forms a cylindrical mucous sheath of variable width surrounding and 

 enclosing the filament. In most filamentous forms of Desmids the apical 

 attachment of the cells is by a tough mucus which acts as a cementing 

 substance. The outer mucilaginous coat often exhibits a prismatic or fibrillar 

 structure (fig. 228 G') and is frequently the home of rod-shaped bacteria or of 

 minute epiphytic Flagellates. Schroder ('02) investigated the nature and 

 amount of the mucus secreted by certain Desmids, obtaining much informa- 

 tion by placing living Desmids in indian ink or sepia, the exact limitations of 

 the secreted mucus being thus revealed. 



It has been known for a long time that some Desmids exhibit irregular 

 and jerky movements. These movements were first investigated by Stahl 



Fig. 223. Micrasterias Thomasiana Arch. var. 

 pulcherrima G. S. West, a, front view of cell; 

 b, end or vertical view, x 315. 



