324 Plant Physiology 



189. Cell division. - - Usually, in vegetative organs, 

 division results in such manner that any meristematic 

 cell, temporarily regarded as a primary (or often desig- 

 nated parent) cell, produces two more or less equal sec- 

 ondary (daughter) cells. Exceptionally, differentiation 

 may accompany division ; and, in any case, the subse- 

 quent life history of the secondary cells may or may not 

 be similar. It is obvious that cell division must carry 

 with it the division of most of the essential organs of the 

 cell. There is, in fact, division of nucleus, cytoplasm, 

 and plastids. The nuclear division is of peculiar interest. 



190. Nuclear division. - The nuclei of both plants and 

 animals seldom divide by a direct halving of the nuclear 

 substance, or direct division. Such a type of division 

 is, however, known. The usual process is complex, char- 

 acterized by several distinct phases, all of which are 

 apparently important in securing the equal division of 

 certain chromosomes, or nuclear segments, which appear 

 during division. This indirect process is termed mitosis, 

 or karyokinesis. The observation of nuclear division 

 usually requires material which has been carefully fixed 

 (with respect to protoplasmic structure), sectioned, and 

 stained. 



A meristematic cell from the root of Indian corn may 

 typify the usual phenomena (Fig. 82). During the 

 growth of the cell, the nucleus exhibits toward certain 

 stains definite reactions, and these are, for the most part, 

 greatly intensified during division. The nuclear retic- 

 ulum shows at first some small chromatic thickenings or 

 scattered areas taking the stain more deeply, whilst the 

 nucleolus is also deeply stained. When the reticulum- 



