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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. ii 7.—Fossil diatoms: a, a, a, Gaillonella procera and G. granuiata; 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; l, 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 
