Desmidiacece 157 



the cell-wall not being continuous with the older portions, but 

 joined to them by a narrow, oblique or bevelled surface. The cell- 

 division is of a fixed type, taking place strictly in the manner of 

 previous cell-divisions, and the younger semicells are interpolated 

 between the older ones. The sub-family can be divided into two 

 sections, one of which includes those Desmids in which cell-division 

 does not always take place at a fixed point and those in which it 

 does always take place at a fixed point known as the isthmus. 



A. Point of division of cells variable or sometimes fixed 



at the isthmus. 



Tribe 3. Peniece. 



This tribe only includes one genus, in which the cells are 

 solitary, of moderate length, straight and generally cylindrical. 

 Sometimes, but not always, there is a slight median constriction. 

 The points of division are often variable, although the actual cell- 

 division is of the same type. The cell-wall may be with or without 

 pores, and the cell often grows periodically until it becomes adult. 



Genus Penium Breb., 1844. This genus is difficult to strictly 

 define, as it undoubtedly contains many species which will ulti- 

 mately have to find a resting-place elsewhere. The cells are 

 straight, cylindrical, subcylindrical, ellipsoidal, or fusiform, and the 

 apices may be rounded or truncate. The vertical view is always 

 circular. The inner layer of the cell-wall is in many forms orna- 

 mented with striations, punctulations or granulations, but in others 

 the cell-wall is quite smooth. There is one axile chloroplast in 

 each semicell, with radiating longitudinal plates which have the 

 free margin entire. The pyrenoids are one or many and uniseriate. 

 Sometimes there is a prominent vacuole near each extremity of 

 the cell containing moving granules. 



There are about 27 British species of Penium, of which P. Libellula (Focke) 

 Nordst. (length 250354 p, ; breadth 3851 p. ; fig. 55 D) is one of the largest 

 and most striking. P. margaritaceum (Ehrenb.) Brt-b., P. CyUndrus (Ehrenb.) 

 Breb. (fig. 55 A and B) and others, are granulated species, and P. spirostriolatu/n 

 Barker (length 123274 p. ; breadth 23 26 p. ; fig. 55 C) is a species with spiral 

 striations on the cell-wall, which often anastomose or become broken up into 

 dot-like thickenings. P. polymorpltum Perty (length 55 58 p.; breadth 25 

 27 p.) is one of the most general of the upland, moorland species, and some of 

 the smallest species of the genus are P. iiiconspieuum West (length 14'5 19 p.; 

 breadth 5 5'7 p.), P. minutissimum Nordst. (length 12 - 2 13 p.; breadth 



