140 



BOTANY: PRINCIPLES AND PROBLEMS 



cell multiplication is rapidly going on, the wall is very thin, and 

 the sap cavity, so conspicuous in mature cells, is absent. In the 

 formation of new cells in such growing regions three main stages 

 may be distinguished: Cell division, in which the number of 

 cells is increased by the division of each parent cell into two; 

 cell enlargement, in which these new cells expand rapidly to their 



E F & 



Fig. 70. — Diagram of mitosis in an ordinary body-cell. A, resting nucleus. 

 B and C, prophases. D, metaphase. E, anaphase. F, telophase. G, new cells. 

 The separate chromosomes, each of which has an individuality of its own, are 

 differently marked. It is evident that the chromatic material is divided exactly 

 evenly between the two daughter cells. (Modified from Sharp). 



final size, and cell maturation, in which they assume their mature 

 structure and characteristics (Fig. 71). 



Cell Division. — Cell division, technically known as rnitosis, 

 is not in most cases a simple splitting into two of the mother cells 

 but is accomplished in a rather complex manner (Figs. 69 and 70). 

 The most active part in the process is played by the nucleus. 

 Within this body is a characteristic granular material which stains 

 very deeply with certain dyes used in microscopical work and has 

 hence received the name of chromatin. In an ordinary mature 

 cell the chromatin is arranged in a fine network, but when the 

 cell is preparing to divide, the elements of the chromatin net- 

 work come together into thread-like or rod-like masses, the 

 chromosomes. .The number of chromosomes is the same in every 

 vegetative cell of the plant and is constant for any particular 

 species. Thus in Indian corn the chromosome number is 20, 

 in wheat 16, in peas 14, and in tobacco 48. Soon after the 



