104 PLANT LIFE. 



present work, in accordance with Bennett's idea of their 

 affinities, are by some writers included in the present 

 group, with which they agree in the sexual mode of 

 reproduction. 



The Desmids are all unicellular, and in most instances 

 free, but some species are united by a gelatinous sheath 

 into long filaments. In the majority of species the cell is 

 divided by a deep constriction into two symmetrical halves, 

 united by a narrow band called the "isthmus." Generic 

 characters are founded to a great extent on the form of the 

 cell, that is, of the individual, which varies in outline from 

 circular through elliptical and crescent-shaped, to straight 

 and linear. The cell-wall is usually transparent, but in many 

 species contains a little silica, but never to the extent met 

 with in the cell-wall of Diatoms. The surface may be 

 perfectly even, warted, or furnished with prominent spines, 

 usually symmetrically arranged. The chlorophyll is bright 

 green, and often arranged in bands or stars, and even in 

 those cases where the cell has no median constriction the 

 chlorophyll is arranged in two halves of the cell. 



Many of the free species exhibit spontaneous movements, 

 considered to be due to a film of protoplasm outside the 

 cell-wall, which protrudes from the interior through perfor- 

 ations in the wall. 



Asexual or vegetative reproduction takes place by the 

 division of the cell in the following manner. The isthmus 

 or neck-like constriction increases in size and becomes 

 divided by a septum or cross wall into tn-o equal parts, 

 one of which is continuous with each original half-cell ; the 

 half-cells of the isthmus receive a certain amount of en- 

 dochrome from the original half-cells of the desmid, and 

 rapidly grow and assume all the structural features of the 



