238 
PHYSIOLOGY. 
BOOK II. 
shoot being conical is, that, as the wood originates from the 
base of the leaves, the lower end of the shoot, which has the 
greatest number of strata, because it has the greatest number 
of leaves above it, will be the thickest ; and the upper end, 
which has had the fewest leaves to distend it by their deposit, 
will have the least diameter. Thus that part of the stem 
which has two leaves above it will have wood formed by two 
successive deposits ; that which has nine leaves above it will 
have wood formed by nine successive deposits; and so on: 
while the growing point, as it can have no deposit of matter 
from above, will have no wood, the extremity being merely 
covered by the rudiments of leaves hereafter to be developed. 
If at this time a cross section be examined, it will be found 
that the interior is no longer imperfectly divided into two 
portions, namely, pith and skin, as it was when first ex- 
amined in the same way, but that it has distinctly two, internal, 
perfect, concentric lines, the outer indicating a separation of 
the bark from wood; and the inner, a separation of the wood 
from the pith : the latter, too, which in the first observation 
was fleshy, and saturated with humidity, is become distinctly 
cellular, and altogether or nearly dry. 
With the spring of the second year and the return of 
warm weather vegetation recommences. 
The uppermost, and perhaps some other, buds which were 
formed the previous year gradually unfold, and pump up sap 
from the stock remaining in store about them ; the place of the 
sap so removed is instantly supplied by that which is next it ; 
an impulse is thus given to the fluids from the summit to the 
roots ; fresh extension and fresh fibrils are given to the 
roots ; new sap is absorbed from the earth, and sent upwards 
through the wood of last year ; and the phenomenon called the 
flow of the sap is fully completed, to continue with greater or 
less velocity till the return of winter. The growing point 
lengthens upwards, forming leaves and buds in the same way 
as the parent shoot : in like manner also each bud sends down 
its roots, in the form of fibres within the bark and above the 
wood of the shoot from which it sprang ; thus forming on the 
one hand a new layer of wood, and on the other a fresh 
deposit of bark. In order to facilitate this last operation, the 
