3. r ,0 



BOTANY 



transformation into AUXOSPORES. The auxospores are usually two 

 or three times larger than the cells from which they arise, and by 

 their further development they re-establish the original size of the 

 cells. 



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

 Conjugatae, is accomplished in various ways. The process in Navicula (Fig. 275), 

 Pleurosigma, etc. , may be compared with the zygospore formation of Spirotacnia ; 

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

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

 conjugate 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, while 

 the two small nuclei disintegrate. 



In Surirella and Cocconds (Fig. 276) the conjugating cells do not, undergo 

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

 the former genus two nuclear divisions occur in each of the conjugating cells, 



Fio." 277. Planktoniella sol. 'A disc-shaped 

 Diatom from the Plankton. A parachute-like, 

 hollow [membranous wing springs from the 

 girdle. (After SCHUTT. x 190.) 



Fio. 278. Bacti *i-ifti -i varians. From the 

 Plankton. A chain of cells which Ix-ar 

 bristles as a floating apparatus. (After 

 Scathr. x 200.) 



resulting in one large and three small nuclei ; in the latter genus there is only 

 a single nuclear division giving one large and one small nucleus. The large 

 nuclei fuse, the small ones degenerate. 



In Achnanthes the cell contents of a single cell divides into two daughter cells, 

 which escape, and then fuse together to form the auxospore. 



In many genera sexuality is not obvious in the process of forming the auxospores, 

 which arise asexually from cells that do not conjugate. This is the case in 

 Synedra and Rhabdonema, in which a cell divides and the two daughter cells 

 emerge and develop into auxospores. In Melosira there is no longer any cell 

 division, the protoplast enlarges, bursts the cell wall, and becomes directly trans- 

 formed into an auxospore. The original preliminary divisions are, however, still 

 frequently indicated by nuclear divisions, in Melosira only by the appearance of 

 two nucleoli in the nucleus of the auxospore. 



Some plankton Diatoms exhibit a breaking up of the protoplast by repeated 



