THE STRUCTURE AND GROWTH OF STEMS 165 



meable to water. This and other substances also appear 

 in the cells of the epidermis of stems and they, too, are 

 thereby rendered impermeable to water. 



C. Arrangement of the Vascular Bundles. — In stems, 

 which are unlike roots in this respect, the tissues of the 

 stele are organized into separate strands or bundles called 

 vascular bundles. Each bundle is composed of xylem and 

 of phloem. The xylem furnishes the ascending or out- 

 ward path of movement, while the phloem furnishes the 

 descending or inward path of movement. Note, however, 

 that the movement through the phloem to flowers and 

 fruits is usually ascending and outward. 



There are two distinct ways in which the vascular 

 bundles are arranged in stems, and the two distinct kinds 

 of stem structure are based upon these two ways of arrange- 

 ment of the vascular bundles. In one of these ways the 

 bundles are arranged in a cylinder; in the other they are 

 scattered. We will call one the cylindrical arrangement 

 and the other the scattered arrangement. (Compare Figures 

 48 and 49, pages 145 and 146.) 



D. The Cylindrical Arrangement. — Nearly all perennial 

 stems and many annual stems have the cylindrical arrange- 

 ment of vascular bundles. We will consider it first in 

 perennial stems. A good way to understand the cylindri- 

 cal arrangement and the growth which occurs in connec- 

 tion .with this arrangement is to study the twigs of trees 

 and what happens to them. 



The cut end of a twig looks very different from the cut 

 end of the trunk that bears it. (Compare Figures 56, 57, 

 and 5#.) Yet the trunk once had exactly the same struc- 



