350 PHARMACEUTICAL BOTANY 
seas, often in such abundance as to form Diatomaceous earths 
or Kieselguhrs (Siliceous Earths). Huge geological deposits 
of this material have been found in various parts ‘of the world. 
One of the most remarkable for extent as well as for the number 
and beauty of the species contained in it is that at Richmond, 
Virginia. It is in many places 24 to 40 feet in depth and extends 
for many miles. Another deposit at Lompoo, California, is 
claimed to cover an area of 12. 
square miles with an average 
depth of over 100 feet. Many 
of the diatomaceous earths are 
useful as absorbent and polish- 
ing powders. 
| Terra Silicea Purificata (Puri- 
fied Siliceous Earth) is a powder 
consisting of the frustules and 
fragments of diatoms which has 
been purified by boiling with 
diluted hydrochloric acid, 
washed and calcined. 
Many diatoms exhibit a 
Fic. 254.—Two mag Diatoms. gilding or jerky motion. This 
To left, Diatoma vulgare, a, side view of 15 stated to be caused by the. 
frustule; 4, frustule undergoing division. streaming movement of their 
To right, Grammatophora serpentina: a, cytoplasm. 
front and side views of single frustule; : bs 
b, 6, front and end views of divided Diatoms exhibit two modes 
frustule; c, frustule about to undergo Of reproduction, viz., fission 
division; d, frustule completely divided. and formation of an auxospore. 
Bet CORR The more common method is 
that of fission but this: is peculiar for these plants. Thecell-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 
masses 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 and the two daughter-diatoms thus 
become independent plants. ‘Each of these possesses one of the 
