NATURE OF PLANTS 75 



called a lenticel (Figs. 49; 51, A). Soon this growth becomes 

 localized in a layer of cells situated on the inside of the lenticel 

 (Fig. 50). This layer of cells is a part of the cork cambium, 

 but strangely enough only loose cells are added to the lenticel 

 during each year's growth and consequently a passage way is 

 kept open to the living cells within the stem. The lenticels 

 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. 51). 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. 



38. The Cambium Zone. — 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 



cam 



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

 of the cambium zone, cam, as a ring of regular cells, has been completed ; 

 p, pith ; v, vascular bundles ; st, stereome ; c, cortex. Compare Fig. 39. 

 — H. O. Hanson. 



