282 



PART IV. VEGETABLE TAXONOMY. 



process only goes on for a certain number of generations, when 

 it is interrupted by the formation of what is called a.n.lauxospore. 

 This may be formed asexually, simply by rejuvenation or the 

 escape of the protoplasm from the old valves, or it may be the 

 result of the conjugation of the protoplasm of two plants and 

 the discarding of the old valves. In either case the new valves 

 which are secreted are of the same size as those with which the 

 first generation started. 



Diatoms are exceedingly abundant plants, both in individuals 

 and in species, being found in nearly all waters, both salt and 





Fig. 475. 



Fig. 477- 



Fig. 478. 



Fig. 474. — Side view of valves of Pinnularia dactylus, magnified about 300 diameters. 



Fig. 475. — Front view of the same diameter, showing the way the valves fit together. 



Fig. 476. — Valve of Triceratium intermedium, magnified about 500 diameters. After 

 Grove and Sturt. 



Fig. 477. — Triceratium favus, magnified about 150 diameters. 



Fig. 478. — Cosmiodiscus Normanianus, magnified about 600 diameters. After Grove 

 & Sturt. 



fresh, that are reasonably free from putrid matters. They occur 

 in the tropics, in springs where waters are so hot that few other 

 forms of life are able to survive, and in the ice-cold waters of 

 the polar seas. Their shapes are also exceedingly various. Some 

 of the different forms are represented in Figs. 474 to 478, 

 inclusive. 



