BOOK II. 
GENERAL CONSIDERATIONS. 
239 
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 fibres 
of the growing buds ; and, in the second place, to assist in 
generating the cellular tissue by which the horizontal dilata- 
tion of the axis is caused, and which maintains a communica- 
tion between the bark and the centre of the stem. These 
lines of communication have, by the second year, become suf- 
ficiently developed to be readily discovered, and are in fact 
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 they were indissolubly united they 
both consisted 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 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 is at all times coaetaneous .with that of the wood, 
the communication so effected between the pith and bark is 
quite as perfect at the end of tlie third year as it is at the 
beginning of the first ; and so it will continue to be to the 
end of the growth of the plant. The sap which has been 
sucked into circulation by the unfolding leaves is exposed, as 
in the previous year, to the effect of air and light ; is then re- 
turned 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 medullary 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 de- 
posited 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, as was the first ; and this latter 
has acquired an increase in diameter proportioned to the 
