BRITISH DESMIDIACE.E, 



INTRODUCTION. 



DESMIDS are unicellular plants of extremely varied 

 form. Most of them are zygomorphically symmetrical 

 in three different planes at right angles to each other. 

 They are nearly always constricted in the middle, the 

 constriction varying from a slight narrowing in the 

 central portion of the cell to a relatively deep incision. 

 The portion on each side of the constriction is termed 

 a semicell, and that portion which connects the semi- 

 cells is called the istlinni*. The incision on each side 

 of the isthmus between the semicells is known as the 

 xinns, and the apices of the semicells are often termed 

 the poles. In a few genera (such as Closterium, 

 Mesotaenium, Gonatozygon, and Spirotsenia) in which the 

 cell itself is not constricted, that part of the protoplasm 

 which contains the chloroplasts is almost invariably 

 divided into two symmetrical portions. 



Before the time of Ralfs, who advocated the view 

 that Desmids are unicellular, nearly every author with 

 the exception of Kiitzing considered that they con- 

 sisted of two cells, even Meneghini and Ehrenberg 

 being under the impression that they were bicellular. 



Desmids belong to the green Algas (or Chlorophycese) 

 of which by far the greater proportion are inhabitants 



