104 BOTANY: PRINCIPLES AND PROBLEMS 



the rays therefore tend to form broad constrictions in the ring. 

 In more dehcate herbaceous stems the constrictions finally 

 become complete, the broad rays disappear, and the cylinder is 

 thus broken up into a ring of separate segments or fibro-vascular 

 hmidles. Each of these consists of a group of wood cells on its 

 inner side and of bast cells on its outer, with a vestige of cambium 



Fig. 58. — Stem-bundle of a monocotyledonous plant. Transverse section 

 of a fibro-vascular bundle from the Corn stem (Fig. 57). A, vessels. B, sieve- 

 tubes, with companion-cells in their corners. The bundle is surrounded by a 

 bundle-sheath of thick-walled cells. 



between. Connecting the cambium layers of two adjacent 

 bundles there may be a weak interfascicular cambium, producing 

 a few layers of parenchyma cells. In many herbaceous stems, 

 however, the bundles, each surrounded by a bundle-sheath of 

 thick-walled cells, are quite distinct and widely separated from 

 one another, with no remnant whatever of a cambial zone between 

 them. 



In still more highly specialized stems, characteristic of mono- 

 cotyledonous plants, the bundles are no longer arranged in a ring 

 but are scattered irregularly throughout the whole area of the 



