Desn lidiacece 159 



median portion of the cell there is a slight lateral excavation in 

 the chloroplast for the lodgement of the nucleus, which is thus 

 asymmetrical. There are from four to thirteen pyrenoids in the 

 chloroplast, arranged in a single series. The cell-wall is relatively 

 thin and without pores. 



The genus is readily distinguished from Glosterium by the 

 absence of any marked attenuation towards the apices and by the 

 single chloroplast with convex extremities, the latter being in 

 such close proximity to the ends of the cell that there is no room 

 for an apical vacuole. The lateral position of the nucleus is also 

 characteristic. 



There are only three species of the genus, all of which occur in Britain. 

 R. obtusa (Breb.) W. & G. S. West (and its var. montana; fig. 55 G and H) 

 is not an uncommon Desmid in mountainous districts; length 48 117 /x; 

 breadth 5'5 12'5 p.. R. Pseudoclosterium (Roy) W. & G. S. West is a very 

 narrow species of rare occurrence; length 98 192 p.; breadth 2 - 6 3 p. ; 

 fig. 55 J and K. R. Cambrica W. & G. S. West is only known from N. Wales; 

 length 173 177 p.; breadth (5'2 6 - 7 p.; fig. 55 I. 



Genus Clostermm Nitzsch, 1817 l . The cells are elongate, 

 generally distinctly curved, and often markedly lunate or arcuate. 

 In most cases they are strongly attenuated towards the poles, the 

 latter being obtuse, truncate, rostrate, or drawn out into long 

 needle-like points. In most of these plants the cell-wall possesses 

 pores and in a large number of species it is striated, the striations 

 being internal thickenings of the cell-wall running from pole to 

 pole and disposed at regular intervals. The number and strength 

 of the striations varies very much in different species of the 

 genus. There are two chloroplasts, one in each semicell, and 

 there may be one or many pyrenoids in each chloroplast. The 

 extremities of the chloroplasts are concave and they do not reach 

 the apices of the cell, thus affording room for prominent apical 

 vacuoles which contain one or many moving granules of variable 

 size and shape. In those species in which the apices are greatly 

 produced the chloroplasts dd not extend into the prolongations, 



1 The name "Arthrodia" (Raf. in Desv. Journ. 1813, i, p. 235) cannot possibly 

 be utilized as a generic name in place of Clo&terium. Rafiuesque's diagnosis 

 applies equally to Aiikixtrodesmus, Penium, Docidiuin, Pleurotanium, Cylindroeystis, 

 and Ti'tmemorus. Nordstedt (in Hedwigia 1893, Heft 3, p. 148) has clearly pointed 

 out that " Arthrodia" must always remain a "genus ignotum " and a " nomen deleu- 

 durn." The same author's remarks should also be consulted concerning "(;////>.-, 

 Ehreub.," "HeUerella Bory," "Ursinella Turpin," " Prolifera Vauch." and"Cora- 

 jiujata Vauch." 



