240 PHARMACEUTICAL BOTANY 



fission but this is peculiar for these plants. The cell-contents within 

 the siliceous case separate into two distinct masses and the valves 

 separate slightly from each other. As the two daughter-masses 

 become more and more developed, the valves of the parent-cell are 

 pushed more widely apart. Each of the two massses secretes for 

 itself a new valve on the side opposite to the original valve. When 

 the process is completed the girdle of the parent-diatom separates 



FIG. 117. Fossil diatoms: a, a, a, Gaillonella procera and G. granulata; b, b, 

 Surirella plicata; c, Surirella craticula; d, d, d, Gaillonella (Melosira) biseriata 

 (side view) ; e, Gomphonema gracile; f, Cocconema fusidium; g, Tabellaria vulgaris; 

 h, Pinnularia dactylus; i, Pinnularia nobilis; k, Surirella caledonica; I, Synedra 

 ulna. (After Carpenter.) 



and the two daughter-diatoms thus become independent plants. 

 Each of these possesses one of the parent valves and a second, 

 which it has formed itself more or less parallel to the first. 



In a number of species, repeated fission results in the formation 

 of succeedingly smaller and weaker individuals. This process, 

 however, goes on only for a certain number of generations until 

 the decrease of size has reached a limit for the species, when the 



