82 
COURSE OF THE SAP. 
[Part II. 
seen each to originate in a point surrounded by 
the old wood, and in conjunction with no other 
central pith. But I should rather say that the 
central pith of each shoot will be seen to origi¬ 
nate in an extra deposit of cheesey pith 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 Du tro¬ 
chees bold suggestion, that the outside of each 
concentrical layer of wood really is a pith. I 
have always doubted this idea, because the sub¬ 
stance 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 
