in between the rays of the wood, meet in the center, and confti-
tute the pith.
O r -t h e S a p V e s s e l s . '
The moft numerous and the largeft apertures are generally to
be found in the wood, which are perceived very difiinctly in a
tranfverfe feftion, in which the ends o f the veffels are feen as cut
through by the knife. The fcarlet oak o f America is recommended
as a proper objeft for exhibiting them. I f a fhort cylinder o f
a three years branch o f this oak, a little macerated, be hollowed
away with a chiffel, we lhall fee what a large portion of the wood
is occupied by thefe veffels ; they are thick and ffrong, and it is
ealy, with fome care and attention, to loofen feveral o f them.
I f a number o f thefe, thus feparated, be put into a vial o f
rain water, and frequently lhook for feveral days, fome will at
length be found perfeftly clean, thefe are then to be put into
fpirit o f wine, and when that has been two or three times changed,
they will be in a condition to be viewed for underftanding their
ftrufture ; another method o f preparadon has been already fhewn
in page 162.
Thefe are the veffels which have been called by fome writers
air, by others tracheal veffels. It is, however, to be remarked,
that moft o f thofe who have conlidered them as air veffels, refer
us to the tree while in a more herbaceous ftate; in this cafe they
fay, that we lhall find thefe parts filled with a fine fpiral filament.
As thefe veffels are often to be found empty, they have been fup-
pofed to anfwer the purpofes of lungs to the plant. Malphigt
4 afferts,
affertis, that if they are examined in winter, they often exhibit a
vermicular motion, which aftOnilhes the fpeciator.
Thofe who fuppofe the corona to contain the whole ftruflure
o f the tree in miniature, and that it is the embryo o f future fhoots,
fuppofe it to contain the veffels proper for each part, a fubject
that muft be left to the decilion of future obfervers.
O f t h e V a s a P r o p r i a I n t i m a .
Thefe are the only veffels which remain to be fpoken of.
They are large, confpicuous, and important; their natural place
is- in the blea, though they are fometimes repeated in the wood
and the corona. Their coats are thicker than thofe of any other
Veffels.* It ismot difficult, after a fuCcefsful maceration, to fepa-
rate fome of thefe veffels from the' blea ; in this ftate they appear
pe'rfefl Cylinders, with thick white coats, o f a firm, folid, and
uniform texture.
It has generally been fuppofed, that each o f thofe concentric
circles, which are to be obferved in the tranfverfe feffion of
almoft every tree, was the produft o f one year, or the quantity
o f wood added, to the tree in that fpace; here, however, Dr.
Hill differs again from the general opinion.
From what has been faid we may deduce the following general
ideas relative to the organization of trees. The moft obvious
and remarkable parts of a plant, or tree, are the root, the Item,
4 N 2 the