DESM1DIUM. 241 



often extremely depressed, usually much broader than 

 long, with a distinct but only moderately deep 

 constriction ; in vertical view either elliptical, usually 

 with mamillate poles, or 3- or 4-angled ; chloroplasts 

 axile, one in each semicell, with a massive lobe, con- 

 taining a pyrenoid, radiating from the centre into each 

 angle, or sometimes opposite to each face, and with a 

 pair of plates extending into each angle ; in forms with 

 an elliptical vertical view the number of pyrenoids is 

 more variable. The cells are attached to each other 

 in forms with an elliptical vertical view merely by the 

 close apposition of ridge-like thickenings on adjacent 

 apices, and in the angular forms by short truncate 

 processes projecting from the apices of the cell, one 

 in each angle. In the latter case there is often a 

 space of varying size between the apices of adjacent 

 cells. 



Zygospores rounded or ellipsoidal, smooth or some- 

 times with short flattened conical papilla?. 



The genus Desmidium, together with the closely allied follow- 

 ing genus Gymnozyga and the tropical genus Streptonema, differ 

 from all other Desmids in their method of cell-division. In 

 these genera, when a cell is dividing, that part of the cell-wall 

 where the new and old walls abut on each other develops a ring- 

 like thickening, which is transformed by further growth into a 

 sort of imagination of the wall projecting into the old semicell. 

 As the new semicell develops this plication straightens itself 

 out. Where the cells are united by their whole apical surface 

 only one such imagination is formed during cell-division. But 

 where the union is effected by means of short apical processes 

 as many imaginations are formed as there will eventually be 

 processes. The projecting ridges of the new cell- wall are very 

 conspicuous during cell-division. 



There are eight British species of the genus, of which none is 

 really abundant. These may be arranged as follows : 



* Seniicells with well-developed apical feet and cavities, easily 

 seen, between adjacent cells. 



1. D. Aptogonum. 



2. D, pseudostreptonema. 



VOL. V. 16 



