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.) 



J 



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.) 



