72 GENERAL BOTANY 



with their great increase in length, b and c, they gradually form 

 the tissues of the bark, cortex, and woody cylinder of the fully 

 formed twig (c). While this extension and differentiation is pro- 

 ceeding, a new cylindrical cambium, formed within the wood 

 cylinder, adds new conducting tissue to the stem in a manner 

 to be explained later. 



From the above discussion it is apparent that roots and stems 

 grow in essentially the same manner, with slight modifications due 

 to the nature of the particular organ and its environment. 



Growth also plays a prominent part in the movements of plant 

 organs and in the ultimate form of the plant body, as shown in an 

 earlier chapter. 



THE CELL AND CELL DIVISION 

 MINUTE STRUCTURE OF THE CELL 



Before proceeding to discuss the complicated process of cell 

 division it will be necessary to review briefly the structure of 

 the cell and to point out certain details of cell 

 structure which we have not heretofore consid- 

 ered. In the brief discussion which follows, the 

 term resting cell will be used to indicate the con- 

 dition of a cell when it is neither dividing nor 

 in active preparation for cell division. Fig. 41 

 is a camera drawing of a resting cell of a root 



" . . tip, in which it may be seen that the protoplasm 



a root tip in the f> j r 



resting stage of such a cell presents the aspect of a mesh, or 

 Note the distributed network, inclosing light spaces. In the living 

 condition of the dark state fa Q meshwork would be made up of liv- 



chromatin net . 



ing cytoplasm, and the light spaces would 

 correspond to minute vacuoles. This appearance of the living 

 substance in artificial preparations has received various interpre- 

 tations, which cannot be discussed in an elementary textbook. 



The simplest conception for the beginning student is to regard 

 the living substance as spongy in structure, resembling the struc- 

 ture of a fine bath sponge, with lamellse, or plates, of living pro- 

 toplasm surrounding minute spaces filled with cell sap. Such a 

 spongework, when cut in thin sections, would appear like the 



