228 



BOTANY. 



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- 



A, front nll ;,, oa f vnm +lirno fr -frmv rlo^o 

 view of a frustule : B. front view of a <JU1I8B II Olll tlirCC tO IOU1 days 



for its completion. It will 

 S; readily be **? that the con- 

 After (Ereted. tmued 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 aiifdvu, to increase. 



