y'V 



84 THE CAMBIUM CYLINDER 



appear as minute points upon the surface of young stems but 

 upon old trunks they often become greatly elongated, forming 

 the characteristic bands on the bark of the birch and cherry, etc. 

 (Fig. 52, B). In bottle cork, derived from the cork oak of the 

 Mediterranean, the lenticels appear as minute lines often errone- 

 ously referred to as worm holes. 



39. The Cambium Cylinder. — Let us now consider the changes 

 that are effected in the vascular bundles through the activity of 

 the cambium. The increase in the diameter of the stems of 

 many annual plants and especially of our trees and shrubs is 

 largely brought about by the formation of new cells derived 

 from the cambium. While the vascular bundles are very small 

 it will be noticed that the parenchyma cells separating the vascu- 

 lar bundles begin to divide so as to form a line of cells connecting 

 the cambium of each bundle (Fig. 53). In some cases these 



Fig. 53. Cross-section of a stem of castor bean showing the formation of 

 the cambium between two vascular bundles: x, xylem; ph, phloem; c, cam- 

 bium of the bundle. The faint lines, ic, are the first divisions of the parenchyma 

 cells between the bundles that result in the formation of the cambium 

 cylinder. 



divisions are at first somewhat irregular but soon the growth 

 results in the formation of cells with parallel walls and in this 

 way a line of regular cells is formed which in cross section appear 

 as a band or ring of cells but in longitudinal extent they constitute 

 a cylinder. These cells are therefore termed the cambium cylin- 

 der and they continue to divide as already noted in the cambium 

 of the bundles (Fig. 54). Consequently new cells are now added 

 not alone to the vascular bundles but also along the entire extent 



