328 



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 difEerences that exist 

 are easily referable to the difEerences 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- 

 quires from throe to four days 

 for its completion. It will 

 readily be seen that the con- 

 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 



Fig. 154. — Navicula viridis. A, front 

 view of a frvisiule ; B, front view of a 

 f nietuie undergoing fission ; C. side view 

 of afrustule, eliowing tlie central line, 

 called theraphe, the cenlral and termi- 

 nal nodules, and the surface markings. 

 — After CErsted. 



* From the Greek ai^dva, to increase. 



