BOOK II. 
GENERAL CONSIDERATIONS. 
281 
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 ligneous matter 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 
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 descent of the subcortical tissue from the 
growing buds; and, in the second place, to assist in gene- 
rating the cellular tissue by which the horizontal dilatation of 
the axis is caused, and which maintains a communication be- 
tween the bark and the centre of the stem. This communi- 
cation 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 action that the separation which 
now exists between the bark and pith took place. At the 
time when the latter were indissolubly united they both con- 
sisted of cellular tissue, with a few spiral vessels upon the line 
indicative of future separation. When a deposit of wood was 
formed from above between them they were not wholly di- 
vided 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 ; is then returned through the 
petiole to the stem, and sent downwards through the bark. 
