Chap. II.] 
COURSE OF THE SAP. 
47 
tions. 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, 
though the heart-wood 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 con¬ 
vince us that this is so, we want nothing farther 
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 stream 
which follows, that the heart-wood is a conduit 
for the sap; and the existence of plashers 
proves that the sap-wood is a conduit for the sap. 
How I wish that Dr. Lindley would allow us 
this one bit of undoubtful ground in vegetable 
physiology to catch our breath on! 
Certainly, in respect to first getting the sap 
into the tree, we have had doubt and difficulty 
enough. Now for the next doubt and difficulty. 
Now for getting the sap up the tree. I was 
going to say, that no one has an idea of the 
By what me¬ 
chanical power 
is the upward 
sap raised? 
