16 DESMIDS OF THE UNITED STATES. 



The cell, that is to say, the plant, is multiform, varying from 

 the simple cylindrical and fusiform, to the denticulate, crenuhite. 

 lobed and otherwise much indented margins of the equal parts 

 into which the cell is with few exceptions, beautifully and sym- 

 metrically divided. The division of the cell into two equal 

 parts is effected by a transverse constriction, usually so deep 

 as to leave about a third of the diameter of the constricted cell 

 for a connecting link between the two parts; sometimes, how 

 ever, the constriction is so slight as to produce merely an obtu>e 

 angled sinus on each side of the cell ; a few have no constriction. 



The wall -.cytiodernii of the cells is not constituted alike for all 

 the genera ; in some cases it appears to be of a silicious character, 

 judging by the appearance of the empty case, and by the firm- 

 ness with which it retains the contour of the cell, even years 

 after the Desmid had been collected ; but usually the wall is more 

 fragile aud easily liable to collapse or rupture, unless the speci- 

 men is kept in a preservative fluid. 



One of the characteristics of Desmids which confirmed- many 

 of the earlier microscopists in the belief of their animal char 

 acter, was their seemingly voluntary movements. These move- 

 ments are most apparent in the larger forms of Closterium and 

 Cosmarium, but are more or less evident in all of them ; they 

 consist of slow, steady oscillations, and sometimes they go for- 

 ward aud backward, similar to the movement of diatoms, but- 

 slower, therefore less observable. By means of this locomotive 

 power, they work themselves to the surface when gathered into 

 a vessel with dirt and other foreign matter, and collect in such 

 positions as are most exposed to the light. Many theories have 

 been advanced to account for this phenomenon, but hitherto all 

 have failed to establish a satisfactory explanation. 



Another movement belonging to the Desmids is that of the 

 granules within the cells. This is always present in good living 

 specimens, and consists in a circulation of granules in the watery 

 fluid next to the cell walls ; they appear to be constantly passing 

 to and fro between the center and the ends of the cells. 



A third motion belongs to the so-called vacuoles of CJoateriums, 

 Docidiums and Peniums. In the end of many of these cells is a 

 well-defined globular, transparent space r<i<-in>lc filled with a 

 number of small granules, significantly called, by the Germans. 

 tanzenden kornchen, or dancing granules, because of their sprightly 

 activity. A similar behavior of the granules in the bodies of 

 smaller Desmids. and in the cells of filamentous AI</<I . is often 

 observable, particularly in unhealthy plants. No satisfactory 



