140 PHARMACEUTICAL BOTANY 
largement of the cells of the primary tissues. Such stems show no 
differentiation into cortical and central regions. 
Histology of Annual Dicotyl Stem.—(In both annual and perennial 
dicotyledonous stems endodermis and pericambium are rarely seen 
since each has become so similar to cortex through passage of food, 
etc.) 
Fig. 68 . —Photomicrograph of cross-section of stem of Aristolochia sipho, 
where cambial activity is just beginning, a, Epidermis; b, collenchyma; c, thin- 
walled parenchyma of the cortex, the innermost cell layer of which is the starch 
sheath or endodermis; d, sclerenchyma ring of the pericycle; e, thin-walled paren¬ 
chyma of the pericycle; /, primary medullary ray; g, phloem; h, xylem; i, inter¬ 
fascicular cambium; j, medulla or pith. X 20. (From Stevens.) 
1. Epidermis, cutinized, with hairs. 
2. Cortex composed of three zones: an outer or exocortex, whose 
cells are thin walled and contain chloroplasts; a middle ormedio- 
cortex, consisting of cells of indurated walls giving extreme pliability 
and strength,.an inner or endocortex, a very broad zone of thin- and 
thick-walled parenchyma cells. 
3. The innermost layer of cells of the cortex called endodermis. 
(Not generally distinguishable.) 
4. Pericambium. (Not generally distinguishable.) 
