100 



CHAPTER YIII. 



CELL-DIVISION. 



Having now examined some of the chief modifications of the 

 plant-cell, and gained an outline of the more important vital 

 processes to be demonstrated in it, attention may now be 

 directed to the manner in which fresh cells may arise from pre- 

 existing ones: and in this respect it is found that, in the 

 majority of instances, the nucleus is the structure in a cell which 

 undergoes the most marked changes. In the higher plants, in 

 fact, cell-division is always preceded by division of the nucleus. 



Of the types of cell-division met with there are two main 

 varieties — viz., the amitotic and the mitotic. In the former the 

 nucleus divides en masse, the cytoplasm becoming aggregated 

 round the resulting nuclei, after a process of redistribution ; 

 whilst in the latter, or mitotic type, certain changes take place 

 in the structure of the nucleus which lead to the development 

 of a well-marked karyokinetic or division-fignre, followed by the 

 formation of a partition-wall dividing the original cell into two. 



A. Amitotic Cell-division. 



This type is comparatively rare in the Higher Plants ; it 

 occurs, however, in cambial cells, and it is also seen in old 

 internodal cells of Tradescantia virginica. In the lower plants it 

 may occur at times, as in the case of the nuclei of Vamlieria,. 

 and in internodal cells of Chara fragilis. 



In amitosis, the nucleus becomes constricted in the middle,, 

 and this constricted part becomes narrower, until finally the 

 original nucleus has split into two daughter-nuclei. This type 

 of nuclear di^'ision is looked upon b)"^ many as an evidence of 

 degeneration (more especially in animal cells); but in a few cases 

 it is a sign of the need of rapid division, where time and space 

 will not allow of the more highly differentiated mitotic type. 



