390 



BOTANY 



The auxospore formation in the Centricae does not take place by the conjugation 

 of two gametes but by the protoplasmic body of a cell becoming free from the cell 

 walls arid increasing in size ; the enlarged cell is first surrounded by a weakly 



FIG. 3-2l.Corethron Valdiviae. From the Antarctic plankton, a, Cell with floating bristles and 

 tentacles ; b, Auxospore formation ; the protoplast after casting off one valve has emerged from 

 the other and, surrounded by the perizonium, has become four times its original size ; c, the 

 protoplast contracted within the perizonium and forming the new upper valve ; (/, the peri- 

 zonium having disappeared above, the auxospore forms the new lower valve and escapes from 

 the perizonium. (After KARSTEN.) 



silicified membrane (perizonium), and in this the new valves are formed (Figs. 

 321, 322 ). 



The Centricae further differ from the Pennatae by possessing a special method of 



FIG. 322. Biddulphia mdbiliensis. A, View from the girdle side ; B, auxospore formation ; C, cell 

 divided into two sporangia preparatory to the formation of microspores ; D, spore formation 

 in the sporangia ; E, swimming microspore. (A-D x 228, E x 570. After P. BERGON.) 



reproduction by means of so-called microspores ( 21 ) ; the formation of these has been 

 accurately followed by BERGON in Biddulphia moUliensis. A cell first divides into 

 two daughter cells or sporangia, the contents of which round off and by repeated 

 division form many (32) microspores. These emerge as naked swarm-spores, each 



