196 



THE ALGJE 



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, which 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. 18 6, -4). 

 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, 



D 



FIG. 188. Diatoms 



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

 the right showing the two overlapping shells 

 or valves ; -B, Acnanthes, 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 being Composed OI Silica, 

 one another to form a zigzag filament; D, res ist Solution in water 

 Licmophora, groups of cells borne on gelati- . 



nous stalks; E, Epithemia; F, Triceratium. and are constantly being 



-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 



