60 



STRUCTURE OF THE STEM 



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

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

 even then the position of the wood between an inclosed pith 



FIG. .")(>. Diagrammatic cross section of an annual dicotyledonous stem 



p, pith : /r, woody or fibro-vascular bundles ; e, epidermis ; b, bundles of hard- 

 bast fibers of the bark. Somewhat magnified. After Frank 



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 l 



and an inclosing bark is notably different from the way in 

 which the bundles are scattered in monocotyledonous stems. 



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

 layer is a good deal exaggerated. 



