76 
COURSE OF THE SAP. 
[Part II. 
Lateral as well 
as longitudinal 
flow of sap 
through the 
whole wood. 
and I think that the medullary rays may be 
seen to prolong themselves into the latest layers 
of bark, if the stem of a living oak-tree is cut 
across ; at least, there is a white line across 
these layers of bark, opposite the end of each 
medullary ray. 
Some physiologists have supposed a lateral 
communication of the sap between each and all 
the annual layers of wood by means of these 
medullary rays, or silver grain. But to show 
what guess-work vegetable physiology consists 
of, others suppose that these rays are merely 
conductors of atmospheric air between the bark 
and the pith. That there is a lateral trans¬ 
mission of sap throughout the wood by some 
means or other, I think may be argued from the 
existence of ringed branches. Indeed, were it 
not for this lateral communication, whether 
branches were ringed or not, their buds, leaves, 
fruits, and shoots could only be supplied with 
upward sap from the last year’s growth of wood 
on which they are placed; and the upward sap 
of every annual growth of wood, except the last, 
would be confined within the limits of the cone 
formed by each annual growth. 
I have a bough of a pear-tree which I ringed 
for the space of an inch in June, 1832, and 
