196 



THE ALG^E 



The diatoms resemble the desmids in the similar halves of 

 the cell and in the development of a peculiar type of spore called 

 an auxospore, winch probably corresponds to a zygospore. Some 



auxospores are formed by 

 the fusion of two gamete 

 protoplasts which leave 

 the diatom shells at one 

 side (Fig. 188, B}, and 

 these are true zygospores 

 very similar to those of 

 the desmids (Fig. 186, A). 

 Other auxospores are de- 

 veloped without proto- 

 plasmic fusions and are 

 probably examples of sex- 

 ual degeneration ; that is, 

 cells which were origi- 

 nally gametes now develop 

 directly into auxospores. 

 The auxospores are rest- 

 ing spores and appear to 

 be formed after long 

 periods of vegetative cell 

 divisions to stimulate or 

 rejuvenate the protoplasm 

 for further activities. 



The shells of diatoms, 

 being composed of silica, 

 resist solution in water 

 and are constantly being 



FIG. 188. Diatoms 



A, Navicula, the boat diatom, the side view at 

 the right showing the two overlapping shells 

 or valves ; B, Acnanth.es, an auxospore with 

 the four empty shells of the^ two diatoms 

 whose contents united to produce this sex- 

 ually formed spore similar to the zygospore 

 of the desmids (see Closterium (Fig. 186, A) ; 

 C, Tabellaria, groups of cells united with 

 one another to form a zigzag filament; D, 

 Licmophora, groups of cells borne on gelati- 

 nous stalks ; E, Epithemia ; F, Triceratium. 

 B, after West 



deposited at the bottom 

 of seas, lakes, ponds, and marshes, sometimes in such quantities 

 as to form so-called siliceous or diatomaceous earths. There are 

 some geological deposits (Tertiary) of diatomaceous earth many 

 feet in thickness, as that at Eichmond, Virginia. Some of these 



