THE ORGANIZATION OF THE PLANT 



65 



Structure of the stem. The stem, like the root, has a central 

 cylinder and a cortex. When young it also has an epidermis, 

 but in perennial steins this soon gives place to the outer bark, 

 which serves the same purpose. The central cylinder is made 



a b 



FIG. 39. Structure of stems 



a, basswood, a dicotyledon; b, corn, a monocotyledon 



up of many thin-walled cells, called pith cells, through which 

 run strands of heavier fibers and two sets of tubes which form 

 the fibrovascular bundles. It is the woody tissue of these fibro- 

 vascular bundles, packed closely together, that gives the trunks 



FIG. 40. A single dicotyle- 

 don bundle. (Much enlarged) 



pa, parenchyma ; ph, phloem ; 



c, cambium; d, ducts; w, 



wood ; p, pith 



p 



FIG. 41. A monocotyledon 

 bundle. (Much enlarged) 



ph, phloem; d, ducts; 

 ic, wood ; p, pith 



of trees their great solidity and strength. Plants that live but 

 a single season and rise only a short distance above the earth 

 do not need to develop these bundles so extensively, though 

 some are always necessary. 



