70 
COURSE OF THE SAP. 
[Part II. 
for ever unchanged, though engrafted on common 
stocks, as a single branch of a plant accidentally 
variegated will for ever retain its character. 
When peach-scions are grafted or budded on 
plum-stocks from four to five feet high, the plum- 
stocks taper in the usual way, from below up¬ 
wards ; but in the course of years the growth 
of the peach appears to overpower the stock, 
and it will be seen to taper from above down¬ 
wards. This over-growth says distinctly that it 
comes from above; but that this over-growth 
is plum, not peach, says as distinctly that it 
is not solely from above. I think it, then, pro¬ 
bable that the upward sap may communicate 
laterally throughout from the wood to the bark; 
and that, for the growth in girthing, it may be 
necessary to bring together, on the common 
ground on which the new external layer of 
wood and the new internal layer of bark are 
deposited, a sap which has been subjected to a 
triple elaboration, namely, juices of the upward 
sap — the product of chemical decomposition, as¬ 
similation, and elaboration in the stem, and 
those of the downward sap — which have been 
subjected to respiration, transpiration, and elabo¬ 
ration in the leaf, and to all these processes in 
their descent through the bark ; finally, that 
