THE STEM. 



99 



air, not by their own rigidity, but by the development of 

 special members for climbing purposes, such as recurved 

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

 normal branches. (See ^[ 115, 158.) Others wrap them- 

 selves about objects of suitable size, and are called twining 

 stems. (See ^[291.) The direction of twining varies with 

 different plants, but most commonly corresponds to the 

 movement of the hands of a watch, the support being sup- 

 posed to be in the center. 



124. Primary structure. — The origin of the stem-tissues 

 has already been described. (See ^[ 100.) 



In following the stem from apex to base it is readily 

 observed that the structure changes as the parts grow older. 

 It is possible, however, to select a point at which the stem in 



Fig. hi. Fig. 112. 



Fig. hi.— Diagram of a transverse section of stem of Iberis amara, showing outline, 

 and paired vascular bundles. The black is the xylem bundle ; the gray is the phloem 

 bundle. The outer line represents the epidermis ; a circle including the bundles would 

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

 and stele. — After Nageli. 



Fig. 112. — Diagram of a transverse section of a palm stem. The epidermis is represented 

 by the outer line; the endodermis by the inner one, with the narrow cortex between 

 them ; the stele, with numerous bundles scattered through the pith, is within the 

 endodermis. — After Frank. 



all cases attains a definite development. This point is at the 

 internode which has just reached its full length. The struc- 

 ture of the stem at this point may be designated as its primary 

 structure. If a thin section be cut from such an internode, 



