328 



BOTANY. 



7J 



the most probable explanation is that it is due to protrusions 

 of the protoplasm through orifices in the rigid wall. 



306. Diatoms bear a close resemblance to the Desmids 

 in their modes of reproduction ; the differences that exist 

 are easily referable to the differences in the wall. The 

 asexual reproduction is a true fission, although at first sight 

 it might not be recognized as such. The protoplasmic con- 

 tents of the cells divide in a plane parallel to the valves , 



each portion then forms a 

 new valve in the plane of the 

 division. As during this pro- 

 cess the two original valves are 

 pushed apart, the new valves 

 are fitted, the one into the 

 larger and the other into the 

 smaller one (B, Fig. 154). By 

 a slight subsequent increase 

 of their contents, the two 

 daughter- cells are pushed out 

 so as to be free from each 

 other ; in many cases they sep- 

 arate, while in others they re- 

 main in contact, although 



really free. This process re- 

 Fig. 154. Namcula viridis. A, front nl1 ; T . PQ f rrml +l-,vnp +n fmir rlnvc 

 view of a frustule ; , front view of a quires IlOHl tnrCC tO IOUr days 

 frustule undergoing fission; C, sideview f nr if H pnTrmlpfinn Tf will 

 of a frustule, showing the central line, IOr 3n ' 

 called the raphe, the central and termi- rea dily be Seen that the CO11- 

 nal nodules, and the surface markings. J 



After (Ersted. tinued formation of individu- 



als in this way must result, in all species whose valves are of 

 a slightly unequal size, in producing smaller and smaller 

 cells. This reduction of size does not, however, take place 

 in those species whose valves are simply opposed, as in Dia- 

 toma. The reduction of size is corrected by the formation 

 of what are termed auxospores ; * these are large individu- 

 als, which form either by an asexual or a sexual process. 

 The asexual formation of auxospores takes place by the 



* From the Greek ai>dvu, to increase. 



