68 
ORGANOGRAPHY. 
BOOK I. 
the centre and the circumference, which are connected by thin 
vertical plates of the same nature 
as themselves. The central part 
(a, fig. 32.) is the pith, that of the 
circumference (b) is the bark, and 
the connecting vertical plates (c) are 
medullary rays. 
The pith is a cylindrical or an- 
gular column of cellular tissue, 
arising at the neck of the stem 
and terminating at the leaf-buds, 
with all of which, whether they are lateral or terminal, it is in 
direct communication. Its tissue, when cut through, almost 
always exhibits an hexagonal character, and is frequently 
larger than in any other part. When newly formed, it is 
green, and filled with fluid ; but its colour gradually disap- 
pears as it dries- up, and it finally becomes colourless. After 
this it undergoes no further change, unless by the deposition 
in it, in course of time, of some of the peculiar secretions of 
the species to which it belongs. It has been contended, in- 
deed, by some physiologists, that it is gradually pressed upon 
by the surrounding part of the vascular system, until it is 
either much reduced in diameter or wholly disappears ; and in 
proof of this assertion, the Elder has been referred to, in 
which the pith is very large in the young shoots, and very 
small in the old trunks. Those, however, who entertain this 
opinion, seem not to consider that the diameter of the pith of 
all trees is different in different shoots, according to the age of 
those shoots ; — that in the first that arises after germination, 
the pith is a mere thread, or at least of very small dimensions 
— that in the shoots of the succeeding year it becomes larger 
— and that its dimensions increase in proportion to the gene- 
ral rapidity of developement of the vegetable system : the pith, 
therefore, in the first-formed shoots, in which it is so small 
compared with that in the branches of subsequent years, is 
not small because of the pressure of surrounding parts; it 
never was any larger. 
The pith is always, when first forming, a uniform compact 
mass, connected without interruption in any part ; but the 
