Desmidiacece 171 



ing one or two pyrenoids. The zygospores are globular, and the 

 outer surface may be smooth or clothed with simple, conical or 

 subulate spines. 



Of the eleven British species, A. Incus (Breb.) Hass. is the only one that is 

 general and abundant. It is a small species, frequently met with in the con- 

 jugating state, and is one of the most variable of Desmids ; length without 

 spines 13 33 p. ; breadth without spines 13 28 p, ; length of spines 5 -32 p ; 

 fig. 64 A C. A. convergens Ehrenb. and A. octocornis Ehrenb. (fig. 64 F and 

 G) are not uncommon, but the other species are all rare. 



Genus Staurastrum Meyen, 1829. This is the second largest 

 genus of Desmids and contains some hundreds of species of very 

 varied character, the majority of which possess a deep median 

 constriction. The semicells are elliptical, semicircular, oblong, or 

 cyathiform in outline, and the vertical view is generally triangular, 

 but may be quadrangular or polygonal. The angles are sometimes 

 rounded, sometimes acute, or they may be produced into processes 

 of considerable length. In many cases the angles of one semicell 

 alternate with those of the other. The cell-wall is occasionally 

 smooth, but it is more frequently adorned with a variety of spines 

 or wart-like excrescences, usually symmetrically disposed. In 

 those species in which the angles are produced into processes the 

 latter are generally furnished with two or three strong diverging 

 spines at their extremities. 



There is usually one chloroplast in each semicell, consisting of 

 a central mass with a number of radiating plates, varying from five 

 to eight (usually six; two in each angle). One pyrenoid is present 

 in each chloroplast. In a few species there are several chloroplasts 

 in each semicell disposed in the manner of parietal cushions, but 

 intermediate states between this condition and a central mass are 

 not infrequent. This indefinite character was utilized by Lundell 1 

 for the formation of a sub-genus, Pleurenterium, which has recently 

 been proposed as a genus 2 . The absurdity of such a genus is 

 realized on considering the few incongruous forms it would have to 

 include. It should be borne in mind that the parietal condition 

 of the chloroplasts has been arrived at quite independently by a 

 few widely different species of the genus Staurastrum. 



The zygospores are globose or angular, rarely winged, but more 

 commonly clothed with long spines, which are simple or furcate at 



1 Lundell in Nov. Acta Keg. Soc. Scient. Upsala, 1871, ser. 3, vol. viii, p. 72. 



2 Wille in Engler & Prantl's Natiirl. Pflanzenfam. 1890, p. 11. 



