P L A T E
X i .
F i g .
T H E CON: S T RUC T IO N OF TIMBER
T o follow the order of their arrangement in the Tree, their kinds are thefe j
I. T h e Vafa exteriora, lodged between the Rind and the Bark. 2. T h e Vafa interiora,
fituated in the Bark. 3. The Vafa intima, lodged in the Blea. 4. The Vafa peculiaria,
in the Cororia. Befide thefe, there are the Sap Veflels in the Woodj but they are
common to all Trees.
T h e firft of thefe, the Vafa propria exteriora, have been ihewn in their place and
proportion at Fig. 3. where they appear as round dark-coloured bodies, lodged between
the Rind a, and the Bark b, and entrenching upon the fubftance of both, making
their own beds half within the Bark, and hal f within the Rind.
- T o know their ftrucfture we muft carefully feparate the outer Rind from the inner
Bark ; and this may be done, with fome attention, in a living Shoot, jiift at the time
o f its fwelling for the Spring, or for the Midfummer Shoot 5 but much eafier by the
means of maceration.
When the Rind is perfeitly feparated that way, it leaves the Vafa propria of this
clafs behind it:- they fcarce adhere to the inner Bark j not at all to the Rind j and
therefore lie undifturbed upon the piece thus ilripped. We fee them as rcprefented in
. Plate XI. at Fig. i. they are difpofcd in little packets, like cords, and do not run
ilrait down the Branch; but interweaving with one another, form a very pretty kind
o f net.
W h e n we raife any one or more of thefe packets of Veflels, we perceive that it here
and there fticks a little to the fubftance of the Bark, but no where to the other veffels
: they part very freely where they pafs over one another, and will indeed fall afunder
in thofe places, if they be clipped ihort, and ihook about in a paper.
When we examine a thin tranfverfe piece of one of thefe packets, we perceive that
i t is compofed of twelve or fifteen diftin<a_ veflels, whofe Rinds feem hard, and muft
indeed be fo; for they prefervc their roundnefs notwithftanding their mutual preffure
of one another, and the preilure alfo of the Rind and Bark. A view of fuch a
F i g . 2. piece is ihewn at Fig. 2.
Laying feveral longer pieces of thefe packets before the Microfcope, we iliall, with a
great power, iind fome happy fragment in which w e may fee the caufe and nature of
Fig. 3. the adhefion of thefe parts to the Bark. Such a view is given at Fig. 3. We fee at
a a a one,end the veflels o f the packet a little feparated, and in the parts a a perceive
upon the fides oval depreflions, dotted as it were with pin holes; thefe are moft probably
a kind of glands, which feparate, from that general ftore of fap with which
the Bark is filled, the peculiar, juices which are found in thefe veflels.
A great deal of patience, a vail number of objeits, a good Microfcope, and a fair
day, are requifite for viewing the Vafa exteriora, for this purpofe} but he who takes all
thefe precautions will not be difappointed.
I f