88 



BOTANY 



Less commonly, e.g. Isthmia, the chromatophores are numerous small 



oval ones, like those common in the higher plants. 



The shape of the Diatom-cell is to a great extent correlated with 



the habits of the different forms. 

 The commoner fresh-water types, 

 which live separately and form 

 coatings upon various objects at 

 the bottom of the water, are com- 

 monly oblong or spindle-shaped. 

 Those which are attached at one 

 end, e.g. Gomphonema, Isthmia, 

 etc., are usually shorter, and often 

 differently shaped at the free and 

 attached ends. The floating forms, 

 like the majority of those in the 

 plankton of the ocean (Fig. 62), 

 have special contrivances for in- 

 creasing their buoyancy. They 

 are either excessively slender, e.g. 

 Rhizoselenia, or have slender 

 extensions of the cell, e.g. Chaeto- 

 ceras, or they are thin discs, e.g. 

 Coscinodiscus, Planktoniella. 



Movements. The elongated 

 Diatoms, which live free, often 

 show active creeping movements, 



C 



that are probably caused by the 

 protrusion of protoplasmic pro- 

 cesses through the openings along 

 the raphe. By the contraction of 



these pseudopodia it is supposed that the cell is dragged along 



the surface to which it is applied. 



FIG. 62. Pelagic Diatoms. A, Chseto- 

 ceras boreale (X 175). B, C, Plank- 

 toniella Sol. B, from above. C, from 

 the side (x 125). (After SCHUTT.) 



Cell-division. The cells divide in a plane parallel with the surface of the 

 valves. The protoplast increases in size, forcing the two valves apart until only 

 their edges are in contact. The nucleus then divides, and this is followed by a 

 division of the protoplast into two, but without a cell-wall between them. The 

 division of the chromatophores may occur either before or after the division of 

 the protoplast. There are thus two new protoplasts enclosed within the original 

 pair of valves. Each of the protoplasts now forms a new valve on its inner side, 

 i.e. the side in contact with the other protoplast, so that the two new valves are 

 placed back to back, and fit into the old valves, and thus the two new Diatoms 

 are complete. As the original valves are of unequal size, and each becomes the 

 larger valve of one of the new Diatoms, it follows that the latter are of unequal 

 size, and that after each division one of the resulting cells is smaller than the 

 other. When the divisions are repeated rapidly, this soon results in a great 

 diminution in the size of part of the cells, and the same species may exhibit 



