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PHARMACEUTICAL BOTANY 
cell and falls to the bottom of the water, there to undergo a resting 
stage until favorable conditions for growth arise. 
5. 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 
Fig. 115.—Two species of Diatoms. To left, Diatoma vulgare; a, side view of 
frustule; b, frustule undergoing division. To right, Grammatophora serpentina: 
a, front and side views of single frustule; b, b, front and end views of divided 
frustule; c, frustule about to undergo division; d, frustule completely divided. 
(After Carpenter .) 
chlorophyll 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 
