STRUCTURE OF STEM 



449 



or phloem (phi), a middle green layer of cortical parenchyma 

 (cor) containing resin-canals, and an outer brown layer, the 

 cork (ck). Lastly, in the trunk and larger branches the wood 

 forms by far the greater part of the whole section, the bark 

 being a comparatively thin layer, easily stripped off, with no 



ck 



FIG. 118. Diagrammatic transverse sections of three branches of 

 Pinus of different ages. 



A, very young axis, showing epidermis \ep), cortex (cor] with resin- 

 canals (r. c}, medulla (med), and ring of vascular bundles, separated by 

 medullary rays (med. r), and each consisting of xylem (xy), cambium 

 (cb\ and phloem (phi). 



B, older axis, in which the cambium forms a complete cylinder, owing 

 to the formation of interfascicular cambium (cb') between the bundles. 



c, Axis of the third year, showing xylem of first (xy 1 ), second 

 (xy z ), and third (xy 3 ) year's growth ; cork (ck) ; and cork -cambium 

 (ck. cb.) 



cortical parenchyma, and with its corky outer layer much 

 thickened, gnarled, and wrinkled. 



The wood has been stated to exhibit both concentric and 

 radial striations. The radial markings are called medullary 

 rays (Fig. 118, c, med. r) and follow the "grain" of the 



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