PLANT ORGANS AND ORGANISMS 
IOI 
increase materially in thickness due to absence of cambium. The 
limited increase in diameter that does take place is due to the en- 
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 
Fig. 41. — 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,\x ylem; i, inter- 
fascicular cambium; j, medulla or pith. X 20. ( From Stevens.) 
since each has become so similar to cortex through passage of food, 
etc.) 
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 or medio- 
cortex, consisting of cells of indurated walls giving extreme pliability 
and strength, an inner or endocortex, a very broad zone of thin- and 
thicked-walled parenchyme cells. 
3. The innermost layer of cells of the cortex called endodermis. 
(Not generally distinguishable.) 
