194 



STEMS 



ened but elastic walls. Being fonued early, they are of much 

 importance in affording strength to the young regions of the stem 

 where bast fibers and woody tissues are not yet well formed. 

 Most of the cortex is made up of thin-walled parenchyma cells, 

 known as chlorenchyma, since they contain chloroplasts and func- 

 tion like the cells of leaves in the manufacture of food, being sup- 

 plied with air through the stomata of the epidermis. The starch 

 sheath, comparable to the endodermis in roots, is not distinct 

 from the other cells of the cortex in most stems. Its function is 



Fig. 173. — A portion of a cross section from near the base of an Alfalfa 

 stem. X, xylem, which has formed a compact cylinder; p, pith; c, cam- 

 bium; t, phloem; e, cortex; a, epidermis. Highly magnified. 



in dispute. Some think that its function is to conduct carbohy- 

 drates, while others think that it is the tissue which perceives 

 geotropic stimuli, and is thus responsible for the direction that 

 stems take in response to gravity. 



The vascular cylinder, consisting of vascular bundles so joined 

 as to form a compact cylinder in the older regions of the stem, as 

 shown in Figure 173, at first consists of separate vascular bundles 

 having a circular arrangement about the stem and widely sepa- 

 rated by bands of pith. At the outer border of each mass of 

 phloem are hast fibers, often called sclerenchyma fibers, — an im- 



