G TMNOSPERM^E. 



407 



surface. In many cases oil or resin receptacles occur in, or 

 immediately beneath, the epidermis. 



516. The fibro-vascular bundles are for the most part 

 of the collateral form, and in the young stem they are ar- 

 ranged so as to form an inner xylem cylinder ensheathed by 

 a phloem cylinder (Fig. 301). The xylem of these first- 

 formed bundles is composed of an inner mass of annular and 

 spiral vessels, which gradually pass outwardly into tracheides. 

 The phloem is mostly composed of an outer mass of bast 



Fig. Wl. Cross-section through the new wood (A-A), cambinm ( x - x ), and bark 

 (b-b) of the stem of Jiirdnerus communis, made at the end of September, m, m, me- 

 dullary rays. In the bark are shown the layers of bast fibres, o, ft, b. Magnified. 

 After De Bary. 



fibres, which is bordered internally by a mass of sieve tissue 

 (latticed or cambiform cells) and parenchyma. Between the 

 xylem and the phloem a layer of cells always remains as a 

 meristem tissue ; this constitutes the cambium layer of the 

 -bundles (c, Fig. 301, B., and X-X, Fig. 301a). 



517. The increase in the diameter of the stem takes 

 place by the multiplication of cells in the cambium layer ; 

 the cambium cells undergo longitudinal fission by the forma- 

 tion of partitions at right angles to the radii ; these new cells 



