60 



STRUCTURE OF THE STEM 



The wood cylinder may be discontinuous, that is, broken up 

 into separate fibre-vascular bundles, as shown in Fig. 57 ; but 

 even then the position of the wood between an inclosed pith 



FIG. 56. Diagrammatic cross section of an annual dicotyledonous stem 



p, pith; fv, woody or fibro-vascular bundles; e, epidermis; b, bundles of hard- 

 bast fibers of the bark. Somewhat magnified. After Frank 



e b c p 



FIG. 57. Diagrammatic cross section of one-year-old Aristolochia stem 



c, region of epidermis; b, hard-bast fibers; o, outer or bark part of a bundle; w, 

 inner or woody part of bundle; c, cambium layer; p, region of pith; m, a 

 medullary ray. Considerably magnified 



The space between the hard bast and the bundles is occupied by thin-walled, 

 somewhat cubical cells of the bark 1 



and an inclosing bark is notably different from the way in 

 which the bundles are scattered in monocotyledonous stems. 



1 In this and the following figure the relative prominence of the cambium 

 layer is a good deal exaggerated. 



