46 
COURSE OF THE SAP. 
[Part II- 
The upward sap 
flows through 
the sap-wood ; 
proof by exam¬ 
ple. 
have pierced the sap-wood as well as the heart- 
wood, who shall say whether the stream which 
followed came from the sap-wood, or from the 
heart-wood, or from both, or how much from 
each ? If, according to MM. Coulon, Desfon- 
taines, and Thouin, it came from the heart-wood 
alone , the experiment would prove too much. 
It would prove that the sap-wood is not a con¬ 
duit for the sap. This is not only nonsense, but 
undoubted nonsense. 
Or if any one does doubt that sap-wood con¬ 
ducts the sap, let him look at the case of plashed 
hedgerows; where the entire heart-wood and 
pith are cut through, and a strip of sap-wood 
left no thicker than a lath : yet this thin conduit 
supplies sap to long thick branches sufficient to 
enable them to live and to grow permanently. 
I do not allude to layers laid sideways in the 
earth, but to plashers laid sideways in the air, 
as in hedges. 
People have indeed always doubted, and some 
still do doubt, whether the heart-wood is a con¬ 
duit to the sap; among others, Dr. Lindley, in 
1849 (nineteen years after Coulon’s discovery), 
sticks to this old error. -He says: “ When the 
tissue of the concentric layers is filled with 
secretions, it ceases to perform any vital func- 
