270 TIMBERS AND THEIR USES 



wood the greater the strength of the timber, 

 other things being equal. 



Even a casual examination of a cross-cut tree 

 stem will, in most cases, show that the wood 

 of the central portion is different from that 

 nearer to the tissue-forming cambium. Very 

 often this difference is rendered still more 

 marked by the fact that the central wood is 

 not of the same colour as that of the outside ; 

 it is usually, though not invariably, darker. The 

 central wood comprises tissue whose cells have 

 lost their contents, and whose sole function is 

 the mechanical one of supporting the tree ; this 

 is the heartwood. Surrounding the heartwood 

 is the sapwood, the cells of whose tissues still 

 perform their physiological function of affording 

 a channel for the raw food material. 



The medullary rays vary in size considerably 

 with the kind of tree. In the cross section of 

 Oak (Fig. 72) they are frequently many cells 

 in width ; in some trees they are so small that 

 they cannot be seen with the naked eye. In 

 tangential section (Fig. 71) the rays are approxi- 

 mately the shape of a double convex lens, with 

 a single cell at either end. In radial section, 

 the medullary rays form lustrous bands (Fig. 

 70) variously known as " felt," " silver grain " 

 or " mirrors." The medullary rays form channels 

 through which elaborated food material may 



