Page:Pharmaceutical botany.djvu/265

Rh ends of stalks, while others are held in gelatinous masses. Their siliceous skeleta are deposited constantly on the floor of ponds, rivers, lakes and 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. 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 25 to 40 feet in depth and extends for many miles.

Many of the diatomaceous earths are useful as absorbent and polishing powders. The United States Pharmacopoeia IX recognizes, under the name of Terra Silicea Purificata (Purified Siliceous Earth), a powder consisting of the frustules and fragments of diatoms which has been purified by boiling with diluted hydrochloric acid, washed and calcined.

Diatoms exhibit two modes of reproduction, viz., fission and formation of an auxospore. The more common method is that of