142 



THE LIGNEOUS SYSTEM. 



ma 



704 TnE cortical layers sometimes accumulate to a considerable thickness 

 (maple, hickory, oak), hut are finally rent and farrowed hy 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 

 *ectors. They consist of firm plates of parenchyma (inuriform tissue, 

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

 pith. 



70G. The medullary rays are no less frequent in 

 tho outer layer of wood than in the inner. Henoa 

 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, 508) 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 show3 their ends in tho form of thin 

 ellipses. 



708. They serve as bonds to combine into ono 

 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. 



097, Wood of Oak ; section loa- 709. TlIE CAMBIUM LAYER. Between th© 



ZTt ISS,"' ?££> Hber and the wood there is formed in the 



duota. 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 tho growths of tho lig- 

 neous system. From this, during each 

 growing season, two layers are developed, 

 one of liber and one of wood, both at first 

 « cellular mass, but the cells with wonder- 

 ful precision transforming, some into tho 

 Blonder bast-cells of the liber, some into 

 tho dotted ducts and fusiform cells of the 



WOOd, some into the muriform tissue of the 89S, Wood of Msple ; a modullary r»ya ; 



ducts; c, wood- colls. 



