32 
INTRODUCTION TO BOTANY. 
" The cause of the figure of the perfect shoot 
being conical, is, that as the wood originates 
in the base of the leaves, the lower end of the 
shoot, which has the greatest number of 
strata, because it lias 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 
lias nine leaves above it, will have wood formed 
by nine successive deposits ; and so on : 
while the growing point, as it can have no de- 
posit of matter from above, will have no wood, 
the extremity being merely covered bythe rudi- 
ments 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 examined 
in the same way, but that it has two internal 
perfect concentric lines, the outer indicating 
a separation of the hark from the wood, and 
the inner a separation of the wood from the 
pith ; the latter too, which in the first obser- 
vation was fleshy, and saturated with humidity, 
is become distinctly cellular, and altogether, 
or nearly, dry. 
" III. — 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 win- 
ter. The growing point lengthens upwards, 
forming leaves and buds in the same way as 
the parent shoot : a horizontal increase of the 
whole of the cellular system of the stem takes 
place, and each bud sends down organizable 
matter within the bark, and above the wood 
of the shoot from which it sprung ; thus form- 
ing on the one hand a new layer of wood, and 
on the other a fresh deposit of liber. 
" In order to facilitate this last operation, the 
old bark and wood are separated in the spring 
by the exudation from both of them of the 
glutinous slimy substance called cambium, 
which appears to be expressly intended in the 
first instance to facilitate the development of 
the subcortical tubular tissue ; and in the se- 
cond place, to assist in genera' ing the cellular 
tissue, by which the horizontal dilatation of 
the axis is caused, and which maintains a com- 
munication between the bark and the centre of 
the stem. This communication has by the 
second year become sufficiently developed'to 
be readily discovered, and is effected by the 
medullary rays spoken of in the last book. It 
will be remembered that there was a time 
when that which is now bark constituted a 
homogeneous body with the pith ; and that it 
was after the leaves began to come into ac- 
tion, that the separation which now exists 
between the bark and pith took place. 
"At the time when the latter were indisso- 
lubly united, they both consisted of cellular 
tissue, with a few spiral vessels upon the line 
indicative of future separation. When a de- 
posit of wood was formed from above, between 
them, they were not wholly divided the one 
from the other, but the deposit was effected in 
such a way as to leave a communication by 
means of cellular tissue between the bark and 
the pith ; and as this formation or medullary 
ray is at all times coetaneous with that of the 
wood, the communication so effected between 
the pith and bark is quite as perfect at the 
end of any number of years as it was at the 
beginning of the first ; and so it continues to 
the end of the growth of the plant. 
" The sap which is drawn from the earth into 
circulation by the unfolding leaves is exposed, 
as in the previous year, to the effect of air and 
light j is then returned through the petiole to 
the stem, and sent downwards through the 
bark, to be from it either conveyed to the 
root, or distributed horizontally by the medul- 
lary rays to the centre of the stem. 
"At the end of the year the same phenomena 
occur as took place the first season : wood is 
gradually deposited by slower degrees, whence 
the last portion is denser than the first, and 
gives rise to the appearance called the annual 
zones : the new shoot or shoots are prepared 
for winter, and are again elongated cones, and 
the original stem has acquired an increase in 
diameter proportioned to the quantity of new 
shoots which it produced ; new shoots being 
to it now, what young leaves were to it before. 
" IV. — The third year all that took place the 
year before is repeated ; more roots appear, 
sap is again absorbed by the unfolding leaves, 
and its loss is made good by new fluids intro- 
duced by the roots and transmitted through 
the alburnum or wood of the year before; new 
wood and liber are formed from matter sent 
downwards by the buds; cambium is exuded: 
the horizontal development of cellular tissue is 
repeated, but more extensively ; wood towards 
the end of the year is formed more slowly, 
and has a more compact character ; and another 
ring appears, indicative of this year's increase, 
" In precisely the same manner as in the 
