Chap. I.] BY THE SURFACE OF THE ROOTS. 
21 
thread being comparatively tough, the outer 
coatings may be pulled off it for many inches. 
The internal thread is the wood, and is the con¬ 
duit for the ascending sap which approaches 
nearest to the extremity of the root. The ex¬ 
ternal coatings are the bark, and the inner bark 
is the conduit for the descending sap, and the 
means of the growth of the root, as well in elon¬ 
gation as in diameter. 
The silver ends of roots are, in fact, a mere 
prolongation of the bark, without wood. Down 
the bark all physiologists allow a descending 
current, but none allow an ascending current. I 
believe, indeed, that there is a power of absorp¬ 
tion and of lateral transmission of moisture across 
the bark into the wood, and that the layers of 
wood are the upward conduits for the sap. But if 
we suppose the possibility of the absorption of 
moisture by the immature and silver ends, we 
cannot suppose the possibility of its transmission 
upwards, where there is no wood, but only unripe 
bark in process of formation and deposition from 
above, which bark, even when ripe, is the de¬ 
scending, not the ascending, conduit: and though 
on extraordinary emergencies the sap may for a 
short distance flow up the downward conduits 
of the bark, it is rather too much to suppose that, 
Ends of roots 
only bark,which 
is the descend¬ 
ing, not the 
ascending, con¬ 
duit. 
