142 



THE LIGNEOUS SYSTEM. 



104 THE CORTICAL LAYERS sometimes accumulate to a considerable thickness 

 (maple, hickory, oak), but are finally rent and furrowed by the expanding wood. 

 In the cork oak (Quercus suber) they attain an excessive growth, furnishing that 

 useful substance, cork. In birch (Betula papyracea) these layers resemble paper, 

 long abiding by their elasticity the expansion of the trunk. 



705. THE MEDULLARY RAYS (medulla, pith) are those fine lines which 

 appear in a cross-section passing like radii from the pith to the bark, 

 intersecting the wood and dividing it into wedge-shaped bundles or 

 sectors. They consist of firm plates of parenchyma (muriform tissue, 

 the cell resembling brick-work) belonging to the same system with the 

 pith. 



706. The medullary r.ays are no less frequent in 

 the outer layer of wood than in the inner. Hence 

 their number must increase .yearly, and a new set 

 commence with each successive layer, extending 

 with those already formed through the subsequent 

 layers to the bark, as shown in the diagram. (595.) 



707. THE SILVER GRAIN. In a radial section 

 (597, 598) the medullary rays are more conspicuous 

 as shining plates of a satin-like texture, called the 

 silver-grain, quite showy in oak, maple. A tangen- 

 tial section shows their ends in the form of thin 



597, Woo of Oak ; section lon- 

 gitudinal, s lowing, a, medullary 



rays ; &, wood-cells ; 

 ducts. 



porous 



708. THEY SERVE AS BONDS to combine into one 

 firm body the successive wood layers, and as chan- 

 nels of communication to and from the bark and 

 heart-wood. They also generate, at their outer ex- 

 tremities, the adventitious buds. 



709. THE CAMBIUM LAYER. Between the 

 liber and the wood there is formed in the 

 spring, at the time of the opening of the 



buds, a mucilaginous, half-organized 

 layer of matter. Its presence loosens 

 the bark and renders it easily peeled 

 from the wood. The cambium is a 

 sap solution of the starchy deposits 

 of the preceding year, now rapidly 

 being organized into cells. 



710. THIS IS THE GENERATIVE LAYER 



whence spring all the growths of the lig- 

 neous system. From this, during each 

 growing season, two layers are developed, 

 one of liber and one of wood, both at first 

 a cellular mass, but the cells with wonder- 

 ful precision transforming, some into the 

 slender bast-cells of the liber, some into 

 the dotted ducts and fusiform cells of the 



wood, some into the muriform tissue of the 598, Wood of Maple ; a medullary rays ; 



ducts ; c, wood- colls. 



