190 
PHARMACEUTICAL BOTANY 
cells. Each protrudes a process or conjugation tube; these unite 
and the protoplasm from one cell passes over and coalesces with that 
in the cell opposite. The result of the process is a new cell called a 
zygospore or zygote. This is set free by decay of the walls of the old 
cell and falls to the bottom of the water, there to undergo a resting 
stage until favorable conditions for growth arise. 
4. Order Diatomales.- — Family Diatomaceae. — This family com- 
prises several thousand species of unicellular plants called Diatoms 
which are found in fresh, brackish and salt water forming much of the 
diet of small animals. While unicellular they frequently are united 
in colonies. They all possess chromatophores containing chloro- 
phyll but this green pigment is often obscured by the presence also 
of a brown pigment. 
The most striking peculiarity of the group is the structure of the 
enclosing cell wall. This is in the form of a siliceous case consisting 
of two valves which fit into each other like the halves of a pill box. 
The valves, which are beautifully sculptured, are similar except that 
one is slightly larger than the other so as to fit over it. Diatoms 
vary in form being either circular, linear, elliptical, cylindrical 
rhomboidal, triangular or fan-shaped, etc. Some are borne on the 
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 polish- 
ing 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 for- 
mation of an auxospore. The more common method is that of 
fission but this is peculiar for these plants. The cell-contents within 
