PELAGIC PLANT LIFE 313 



up' organic substance out of carbonic acid. They usually 

 lie in regular order along the cell-wall (Fig. 213, <?) ; but if the 

 light becomes too strong for them, they are able to huddle 

 more closely together, either in the middle of the cell or 



Fig. 212. — Cell-wall of a Diatom {Coscinodiscus subbuluens), ^i"*. 

 (7, External view ; b, vertical section ; c, section in cell-division. 



at some point where they can mutually protect each other from 

 the harmful effects of the rays (Fig. 213, ^ and c). This has been 

 demonstrated by Schimper. The assimilation of carbonic acid 

 produces a fat oil, which may form into comparatively large drops. 



Cells are produced by 

 r^^i division. The nucleus and 

 protoplasm divide into two 

 parts, the valves are pushed 

 a little apart, and two new 

 valves develop within the old 

 ones. Thus each of the 

 daughter-cells gets one of the 

 valves from the mother-cell 

 and a new valve that joins on 

 to it (see Fig. 212, c). When 

 once the valves have acquired 

 their shape they seem incapable 

 of expanding, so that the cell 

 generations will gradually be- 

 come contracted in the plane 

 in which division takes place. 

 It follows that the cavity of 

 the cell will also be dimin- 

 ished, though at the same time 

 the perpendicular axis of the plane of division is frequently 

 slightly prolonged. Algse can, however, regenerate their 

 original size, by throwing off their old valves, growing into a 

 larger bladder with a thin expansible skin, and forming within it 

 new valves that are two or three times as large as the old ones. 

 This is the so-called auxospore development (see Fig. 214). 

 Diatoms occur in quantities over the whole world in both 



Schimper. 

 Cell division. 



Fig. 213. 

 b, Lauderia annnlata. a, Cell with the pig- 

 ment granules (chromatophores) in normal 

 position, collected early in the morning ; b, 

 chain from the surface of the sea, 3 P. M. , 

 chromatophores congregated at the ends of 

 the cells ; c, Detonula schrxderi in the same 

 condition. All ^\^. 



Auxospore 

 development. 



