PEXIUM. 71 



Tribe 3. PENIK i:. 



The cells are of moderate length, straight and usually 

 cylindrical, sometimes with a slight median constriction. 

 The points of division are often variable, although the 

 actual divisions conform to the same type. The cell- 

 wall may be with or without pores, and the cell 

 frequently grows periodically by the interpolation of new 

 cylindrical pieces of cell-wall until maturity is reached. 



There is only one genus in the tribe. 



Genus 7. PENIUM Breb., 1844. 



Bivb. in Diet. univ. hist. nat. 1844, vol. iv, p. 513 ; in Kalfs' Brit. Desm. 



1S4S p. 14S. 



Kiitz. Spec. Alg. 1849. p. 167. 

 Arch, in Pritch. Infus. 1861, p. 720 and 750. 

 Eabeuh. Flor. Europ. Alg. 1868, III, p. 118. 

 Gay, Mon.^r. loc. Conj. 1884, p. 38. 

 Cooke, Brit. Desm. 1886, p. 38. 

 De Toni, Syll. Alg. 1889, p. 855. 



Cells straight, cylindrical, subcylindrical, ellipsoidal 

 or fusiform, uiiconstricted or with a slight median con- 

 striction, apices rounded, subtruncate or truncate ; with 

 one axile chloroplast in each semicell, consisting of a 

 central mass with several radiating longitudinal plates 

 which are entire at their free edges ; pyrenoids one or 

 more in each chloroplast, arranged in an axile series ; 

 cell-wall commonly with pores. 



The genus Pen!, am as now constituted is undoubtedly an 

 artificial one, including Desmids of widely different affinities. 

 As our knowledge is at present insufficient to relegate these 

 plants to their true positions we have still retained them in the 

 genus Pen him. 



We have so framed both the definition of the tribe Penieae 

 and of the genus Penium as to include all these plants, 

 although a certain section of them represented by Penium 

 inargaritaceum, P. cylindnis, and others, would form a well- 

 marked genus clearly differentiated from the rest of the 

 Placoderm Desmids. 



For the present the British species may be arranged as 

 follows : 



SECTION A. (Holopenium Gray). Cells unconstricted, usually 

 with no distinct demarcation between the old and the young 

 semicells. Cell-wall smooth. 



