COUKSE OF THE SAP. 



PT. II. 



The tip- Or if any one does doubt tliat sap-wood conducts 



•u-ard sap 



flows tlie sap, let him look at the case of plashed hedojerows ; 



througn ^ i o ^ 



wood; proof where the entire heart-wood and pith are cut through, 

 ampiV 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 perma- 

 nently. 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 conduit 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 functions. 

 The dead and fully formed central layers are called 

 heart- wood.' 



The Doctor is not the first of physiologists, but he 

 is the first of them who has told us the reason of the 

 death of the heart-wood. It chokes itself with its 

 own secretions. Yet, tliough the heart-Avood has no 

 longer room for the upward sap, it is not so choke-full 

 but what, by-and-by, we shall find the Doctor forcing 

 it to swallow the downward sap. 



In fact, both the heart-wood and the sap-wood are 

 conduits for the upward sap. To convince us that this 

 is so, we want nothing further than these two proofs 

 adduced by myself : that is, the boring through an old 

 scar into the heart- wood of a birch proves, by the 



