THE STEM AND THE LEAF 



45 



seen in the youngest twigs of most of our dicotyledonous 

 trees and shrubs, such as willow, poplar, sassafras, and elder. 

 The early structure of dicotyledonous stems is in some ways 

 best shown in the stems of woody climbers. Fig. 29 shows 



e b c p 



EIG. 29. Diagrammatic cross section of one-year-old Aristolochia stem 



e, region of epidermis ; 6, hard bast ; o, outer or bark part of a bundle (the cellular 

 portion under the letter) ; w, inner or woody part of bundle ; c, cambium layer ; 

 p, region of pith ; m, a medullary ray. The space between the hard bast and the 

 bundles is occupied by thin-walled, somewhat cubical cells of the bark. Consid- 

 erably magnified 



FIG. 30. Diagrammatic cross section of sunflower stem 



p, pith ; fv, woody or fibrovascular bundles ; e, epidermis ; b, bundles of hard bast 

 fibers of the bark. Somewhat magnified. After Frank 



the relative position of the structural components of the one- 

 year-old stem of Dutchman's-pipe, as seen in a cross section. 

 The outer cylinder (e-c) is bark ; the central portion (^?) is 

 pith. Between bark and pith, extending both inward and out- 

 ward from the cambium layer (<?), are fibrovascular bundles 

 (0-w), seven of which are shown in the figure. Each bundle 



