CH. X] 



DESMIDS AND DIATOMS 



427 



the nucleus. Their large green chromatophores are often, 

 compound and elaborate. They reproduce chiefly by di- 

 vision, the two halves separating at the constriction and 

 each regenerating the missing part. The sexual reproduction 

 occurs solely by conjugation. In course thereof, two cells 

 come together and surround themselves with a gelatinous 

 envelope, within which they leave their firm walls, move 

 together, and fuse, forming thick-walled resting zygospores 

 which are often themselves elaborately decorated. They 



FIG. 296. Diatoms, of various forms ; X 50-300. 



The pairs, of identical length, represent in each instance the face and 

 profile views of the same individual. (From Kerner.) 



have power of limited locomotion, by aid of projecting threads 

 worked against the ground, but are spread chiefly by inci- 

 dental movements of the water. Some 1000 species are 

 known, but none have economic importance, unless their 

 beauty be rated as a utility. 



Diatomacece (Bacillariacece) : the Diatoms. These differ 

 in some features from the Desmids, but resemble them in 

 method of sexual reproduction, in unicellular construction, 

 and in the variety and symmetry of form and sculpturing of 

 the walls, which reach a degree to place the Diatoms among 



