The New England species of Closterium 



Joseph Augustine Cushman 



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Staurastrum, the genus Closterium is usually the most noticeable 

 in number of species and individuals of any genus of desmids. 

 Oftentimes pure collections may be obtained of certain species in 

 ditches or small pools. The genus is an interesting one in show- 

 ing the many changes that may take place in a simple form with- 

 out a complication of lobes and about a single axis. 



In its generalized form Closterium consists of an arcuate cell 

 tapering from the middle toward the ends and generally circular 

 in cross-section. Upon this simple plan of structure all the vari- 

 ous differences in shape, size, and ornamentation take place. All 

 gradations exist between species in which the axis is a straight 

 line to those in which it forms a semicircle. The cell-wall may 

 be colorless or in older specimens various shades of yellowish or 

 reddish-brown. The surface may be smooth or have delicate st- 

 ations, costae, or even longitudinal markings made up of a series of 

 granules arranged in a longitudinal manner. The apices of the 

 cell form one of the constant features in the species. In the genus 

 they range from squarely truncate to the finest of acute points or 

 even may be larger than the adjacent portions of the cell. In a tew 

 of the species the ends of the cell are drawn out into long colorless 



setaceous beaks. 



There are two distinct groups of ClcsUrium, according to the 

 manner of division. In one, the cell divides in the m.ddle and the 

 new halves are of the usnal size, the new cells being of pracucally 

 the same length as the first. In another group, divB.cn may take 

 Place and then be followed by an elongation of the old cell by the 

 insertion of a median section between the original semi-cells ne 

 limits of this are shown by distinct transverse lines across the cell, 

 each suture really representing a cell division. . „ if „ 



The semi-cells each contain a single chloroplast w,th long.tu- 

 dinal ridges in most cases ; at the distal end of the chloroplast « a 



