142 



THE LIGNEOUS SYSTEM. 



704. 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. 



t06. The medullary rays are no 1 less frequent in 

 the outer layer of wood than in the inner. Henco 

 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 tho 

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

 tial section shows their ends in the ^form of thin 

 ellipses. 



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 outeV 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. Prom 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 593, Wood of Maple ; a modullary rays ; 5, 



ducts c, wood-colls. 



597, Wood of Oak ; section lon- 

 gitudinal, showing,' , medullary 

 rays ; b, wood-cells ; c, porous 

 ducts. 



