50 Introduction to Botany. 



tween the phloem and xylem are the cambium cells, which, 

 during the growing season, rapidly divide and form new 

 cells that finally become altered into the tubes, etc., of the 

 xylem and phloem. As the stems grow older, cambium 

 is formed between the bundles, and thus is produced a 

 complete ring of delicate cells which permits the bark to 

 be separated readily from the wood. 



The bark is composed of all the tissues outside the cam- 

 bium ring, so that when the bark is stripped off the 

 phloem portion of the vascular bundles goes with it (see 

 Fig. 21). Turning again to the section of Aristolochia, 

 one sees that a portion of the pericycle (//, known as the 

 sclerenchyma ring) is composed of cells which are thick- 

 walled and lignified, as indicated by their being stained 

 red by phloroglucin, or blue by the cyanin. A longitudinal 

 section would show that these cells are elongated in the 

 direction of the long axis of the stem and are closely 

 bound together, being thus well adapted to give strength 

 to the stem. If the section is carefully studied, it will be 

 seen that these different tissues are not separated from 

 each other by lines, such as the beginner might be tempted 

 to use in drawing them, but that they owe their demarca- 

 tion to the fact that the cells composing them differ in 

 form and size, in the character of their contents, and in 

 the thickness and kind of their walls. 



The tissues are adapted to perform various functions 

 by their different characters and relative positions. The 

 epidermis has an outer wall which is infiltrated with waxy 

 substances and is thus adapted to prevent the inner tissues. 

 from drying up ; the epidermis has, therefore, a protective 

 function. The thick-walled cells of the outer part of the 

 primary cortex (z, known as the collenchyma) are chiefly 

 for giving strength. The inner cells of the primary cortex 



