352 



BOTANY 



PART II 



characterised by peculiar creeping movements, resulting from the streaming 

 protoplasm in the longitudinal slit of the raphe. 



The formation of the auxospores, which correspond to the zygospores of the 

 Conjugatae, is accomplished in various ways. The process in Navicula (Fig. 265), 

 Pleurosigma, etc., may be compared with the zygospore formation of S'pirolacnia ; 

 two cells lay themselves side by side, the protojjlast of each undergoes division 

 into two, and the resulting cells emerge from the valves of the cell wall and con- 

 jugate in pairs. By division of the nucleus each gamete was provided with a 

 larger and a smaller nucleus ; in the auxospores the two large nuclei fuse, wliile 

 the two small nuclei disintegrate. 



In SurireUa and Cocconeis (Fig. 266) the conjugating cells do not undergo 

 division, but unite directly with one another. Tlie nuclei, however, divide ; in 



the former genus two 

 nuclear divisions occur in 

 each of the conjugating 

 cells, resulting in one large 

 and three small nuclei : in 

 the latter genus there is 

 only a single nuclear divi- 

 sion giving one large and 

 one small nucleus. The 

 large nuclei fuse, the small 

 ones degenerate. 



In Achnantltcs suhscs- 

 silis the cell contents of a 

 single cell divides into two 



daughter 



Pio. 2(56. — Cocconf.'is placcntula. 1, Vegetative cell 4 



cells, which 

 escape, and then fuse to- 

 gether to form the auxo- 

 spore. 



In some Pennatae the 

 sexuality is lost and tlie 

 auxospores arise apogam- 

 ously. In S'l/itcdra the 

 pair of n;iother cell divides into 



cells before conjugation; 3 U, cells in process of conjnga- ^^^.^ ^^^,^ ^^.^^.^^^ .^^^^ 



tion. gk. Large nucleus ; kk, small nucleus ; f/, gelatinous ° 



substance. (After Karstkn, from Oltmanns' Algae.) auxospores ; the nuclei of 



the daughter cells undergo 

 a division, but the resulting nuclei again fuse. In Ehahdonema arcuutv.m the 

 process is similar, but the second nuclear division does not occur. 



Ilhabdonema adriatieum goes a stej) further ; the nucleus divides, but one of 

 the daughter nuclei is extruded from the protoplast. The undivided mother cell 

 develops into the auxospore. 



Many Diatoms occur in places v»diere decomposing substances are present in 

 abundance. Such species can assume a saprojjhytic mode of life, their chromato- 

 pjiores becoming colourless and reduced in size. It has been shown that some 

 colourless species of Nitzschia which occur in the sea are exclusively dei)endent 

 on organic substances for food, the reduction of their chromatophores and pigment 

 being complete (^"). 



Navicula ostrcaria is a Diatom occurring in the oyster-beds of the French coast, 

 which serves as food for the oysters ; its protoplasm contains a sky-blue pigment 



