462 ANGIOS PERMS i.Ess. 



are arranged in a circle, and there is a closed cambium 

 cylinder from which new xylem is added internally, and new- 

 phloem externally. Moreover, in trees and shrubs, i.e., 

 plants which survive from year to year instead of dying down 

 at the end of one or two seasons, a cork-cambium is formed 



fihl 



Fig. 123. — Diagrammatic transverse section of the stem of a Lily, 

 showing the epidermis (ep), cortical parenchyma containing chloro- 

 phyll (cor), and axial cylinder of parenchyma surrounded by the pericycle 

 (pre) and containing vascular bundles, each consisting of phloem (phi) 

 and xylem (xy). 



in the cortex from which an external layer of cork is pro- 

 duced, the epidermis disappearing. So that the phenomena 

 of growth in thickness can be studied as conveniently in 

 any dicotyledonous tree as in a pine or cypress. 



In Monocotyledons — in a lily, for instance — the arrange- 

 ment of tissues is different. The vascular bundles (Fig. 123) 

 are arranged in a number of irregular circles scattered 

 throughout the central parenchyma or ground tissue, which 



