194 STEMS 
ened but elastic walls. Being formed 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 
Pe oo 
veee 
- oe 
Fic. 173. — A portion of a cross section from near the base of an Alfalfa 
stem. z, xylem, which has formed a compact cylinder; p, pith; c, cam- 
bium; ¢, 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 bast fibers, often called sclerenchyma fibers, —an im- 
