70 



COUESE OF THE SAP. 



PT. 11. 



shoot will be seen to originate in an extra deposit of 

 cheesey pitli on the last so-called concentrical pith of 

 the stem-wood. In the ash, the cheesey point of the 

 central pith of the shoot is bright green, and the extra 

 mass of cheesey pith of the stem-wood is a yellowish 

 white ; and I think their junction is very visible. This 

 fact, if it is a fact, seems to confirm Dutrochet's bold 

 suggestion, that the outside of each concentrical layer 

 of wood really is a pith, T have always doubted this 

 idea, because the substance of these concentrical piths 

 appears so different from that of the central pith ; their 

 substance appears to be not only wood, but the hardest 

 and most durable part of the wood. If the ends of 

 fir-trees are left resting on the moist ground, these so- 

 called concentrical piths will remain after the layers of 

 wood have rotted away from between them. 



In favour of Dutrochet's idea, I have observed 

 that, if young Scotch firs are decapitated with a saw, 

 the resinous sap may be seen to stand in drops on their 

 concentric piths. The stems, also, of Scotch firs, when 

 cut down, appear to bleed from them alone ; and it is, 

 perhaps, possible that the longitudinal upward flow of 

 sap may be through these piths only, and the lateral 

 flow through the silver grain, or medullary raj^s. 



But whether the central pith of the shoot of a 

 pollard or coppice-wood stool originate in the wood of 

 the stem, or of the last crop of branches, or in the 

 supposed outside concentrical pith of one of these, 

 should either or neither of these observations be correct, 



