14 



BOTANY AND PHARMACOGNOSY. 



source of food of small animal forms inhabiting these waters. 

 One of the distinguishing characters of the group is that 

 the cell wall is incrusted with silica. For this reason they 

 are practically indestructible and form marls and strata in 

 the earth. They occur either singly or grouped in bands or 

 chains. They are very variable in shape, being boat-shaped, 



Fig. io. Diatoms: A, Pleurosigma attenuatum as seen from above; B, Pleurasigma 

 baliicum as seen from the girdle side; C, D, E, Fragilaria virescens showing colonies 

 attached to an alga in C, a view of a single diatom from above at D, and a chain of 

 diatoms viewed from the girdle side at E; F, G, two views' of Navicula viridis; H, I, the 

 formation of auxospores in Navicula firma, H showing the exit of the protoplasts and the 

 throwing off of the original valves. — A, B, D, after Van Heurck; C, E, after W. Smith; 

 F-I, after Pfitzer. ' 



ellipsoidal, spherical, or peculiarly curved in some forms. They 

 are either free or attached to a substratum, as stones, water plants, 

 etc., those which are free having an active movement (Fig. lo). 

 The cell-wall of Diatoms practically consists of two halves, one 

 fitting over the other like the lid of a box. These are known 

 as " valves " or " theca." The manner in which the two valves 



