THE STEM. 



85 



recurved spines, tendrils, sensitive leaf stalks, or even by 

 recurved normal branches. (See ITf 99, 131.) Others wrap 

 themselves about objects of suitable size, and are called 

 twining stems. (See \ 249.) 



107. Primary structure. — If a thin section be cut from 

 an internode which has just reached its full length, three 

 definite regions may be distinguished, viz. : (i) the epidermis; 

 (2) the cortex; (3) the stele (figs. 72, 73). 



I. The epidermis is a single layer of cells forming the 

 extreme edge of the section, being, therefore, the layer which 



Fig. 72. Fig. 73. 



Fig. 72 — Diagram of a transverse section of stem of Iheris nmnra, showing outline, 

 and paired vascular strands, Tlie black is the wood strand ; the gray is the bast 

 strand. The outer line represents the epidermis ; a circle including the iDundles would 

 mark the limits of the stele, with its central pifh ; the cortex lies between the epidermis 



- and stele.— After Nageli. 



Fig. 73. —Diagram of a transverse section of a palm stem. The epidermis is represented 

 by the outer line ; the narrow cortex lies between this and the inner circle ; the stele, 

 with numerous bundles scattered through the pith, is within the cortex. — After 

 Frank. 



covers the surface of the stem. Here and there are minute 

 openings which permit communication between the outside 

 air and spaces between the cells of the cortex. These open- 

 ings are usually bordered by two specialized cells, and are 

 called stomata. (See ^ 137.) Naturally they are wanting 

 in submerged stems of water plants and in most subterranean 

 stems. The epidermis is often furnished with hairs, scales, 

 and like outgrowths (figs. 74, 75> 200-203). 



