cn. IT. 



COUESE OF THE SAP. 



63 



pith. He tells us that each division between the 

 annual layers of wood is a pith for the layer outside it 

 (let us call these concentrical piths, to distinguish them 

 from the central pith) ; and that, in addition to the 

 original medullary rays, or silver grain, which run 

 from the central pith or medulla to the bark, and 

 which are annually prolonged through each successive 

 concentrical pith and layer of wood — in addition to 

 these, intermediate medullary rays are developed from 

 each new concentral pith, which run from that new 

 concentrical pith to the bark, and are annually pro- 

 longed. Indeed, as very few medullary rays could be 

 developed in a seedhng of perhaps half an inch in 

 girthing, it seems only natural that the number of rays 

 should increase with the growth of the tree. Other- 

 wise, when the girthing of the tree had increased from 

 half an inch to thirty feet, the medullary rays would 

 stand very far apart at their outward ends ; and in the 

 bark of thirty feet circumference there would be only 

 the same number of rays as in the bark of half an inch 

 circumference. The new rays, however, have no right 

 strictly to the name of medullary^ since they do not 

 originate in the central pith or medulla. The medul- MeduUary 



rays longi- 



lary rays, which appear like the spokes of a wheel pi^t^g^^^^i^ 

 when the stem is cut across, are, in fact, thin plates J^en'Jth^b/'^ 

 running the whole length of the stem, roots, and of ti^^t^and 

 branches. In width they increase every year by the branches 



and roots. 



width of the new layer of wood across which they 

 extend to the bark, and in length they increase every 



