350 



BOTANY 



PART II 



Class VII 



Diatomeae (Diatoms) (^' -^) 



The Diatomeae (Bacillariaceae) constitute a very large class of 

 unicellular Algae. They occur, usually associated together in large 

 numbers, in both fresh and salt water, and also on damp soil. 



The cells are either solitary or 

 form colonies ; they are free-floating, 

 iii//////;.v^;\ or are attached by means of gelatin- 



ous stalks, excreted .by the cells 

 themselves (Fig. 264). Sometimes 

 the cells remain connected and 

 form bands or zigzag chains, or, 

 on the other hand, they are at- 



Fi(i. 2iii.—Licimophora flahellata. 

 Colony of Diatiims with bniiiclHid 

 gelatinous stalks. (After Smith, 

 froniiGoEBEL's Oryaiwgraphie.) 



Fig. 265. — Formation of auxospores in Navicula 

 viridula. A, Cell seen from the valve side. B, 

 Two cells lying alongside one another ; their 

 contents have divided into two daughter cells, 

 each of which possesses two nuclei. C, D, 

 Conjugation in pairs of the daughter cells to 

 form the auxospores, which at lirst contain 

 four nuclei. K, The two full-grown auxospores ; 

 the two larger nuclei in each have fused into 

 one, the two smaller ones have disintegrated. 

 (After Karsten, x 500.) 



tached and enclosed in gelatinous tubes, Avhile in the case of the 

 marine genus Scldzonenia they lie embedded in large numbers in a 



