PRINCIPAL GROUPS OF PLANTS. 



37 



the old one. In each case the valve formed fits into the old one, 

 and hence in the case of the smaller valve the new cell or plant 

 becomes smaller than the parent plant, the walls not being able 

 to expand on account of the siliceous composition. In this way 

 the cells of one series gradually becomes smaller and smaller until 

 a certain minimum is reached, when the plant rejuvenates itself 



FIG. 19. Diatoms: A, Pleurosigma attenuatum as seen from above; B, Pleurosigma 

 baUicum 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 

 frorr the girdle side at E; F, G, two views of Navicula viridis; H, I, the formation of auxo- 

 spores 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. 



by the production of spores (auxospores). These are formed in 

 two ways: In one case the valves separate from each other, the 

 protoplast escapes, grows larger and develops a new wall ; in the 

 other case, of which there are several types, two individuals come 

 together, and envelop themselves in a mucilaginous covering. 

 They then throw off their siliceous walls and the protoplasts 

 unite to form a zygospore, which grows until it is three times the 



