T H E C O N S T R U C T I O N OF TIMBER
P L A T E
V.
Fig i .aa
T h i s o b j e i t muil always be viewed dry, and is beftkept in one of thofe Sliders which
the Noble Perfon, to whom, in a manner, all that are called my Improvements are
originally owing, has direfted to be made with giaffes inllead of Tales.
I f a piece of the Rind, thus impregnated, be gently rubbed between the fingers till the
pnrts are feparated, we fliall be able in one placc or other to get a view of the veiTels all
round, and of the films which form the Blebs between them. Thefe laft confill: of
mere membrane : no power of the Microfcope ihews any thing vafcular in their ftructure
: they are a kind of bladders, clofed at bottom and open at the top, with a fpacc,
greater or lefs, between the top of one and the bottom of another. Fig. 2. a a.
Fig. 3. As to the Veflels, their compofition is much more to be regarded; they are every
where pierced with openings; but of thefe the outer ones, firft feen, are by far the
largeft: there are two other feries of them i the largeft of which, tho' ftill much lefs
than the former, are placed againft the interilitial fpaces, between Bleb and Blebj and
the iinalleft open into the Blebs t (t b hemfelves. Fi g . 3. a b.
I ihould think it is not eafy to err as to the ufes of thefe openings j when we fee their
conftrudtion fo exadl y : and, thofe ufes being underftood, we Îhall have made no fmall
advance in the knowledge of vegetation. Let us, if you pleafe, philofophic Reader,
confider them firft in tliat part of the Tree which is under ground, the Root : here
they are always furrounded with fome degree of moifture : and let us, together with
thefe objefls, confider thofe everlafting agents heat and cold ; not to fay heat in its various
degrees, for that were fpeaking too abftrufely. Heat can be no where prefent
but it expands fubflances : cold no where but it contrafts them.
We fee a Root equalling more than a third part of the Tree above ground, in the extent
of its furface. This furface is covered with the Rind, thus pierced ^ which is
connefled alfo wi t h the parts underneath it. The cold of winter contrat s the whole ;
the parts are drawn clofer together; and the mouths of thefe innumerable veflels are
ihut, or nearly ihut, by this contraâion : a little, and but a very little, of the half-congealed
moifture of the ground gets into them. This faffices for the fervice of the
Tree, when there is little heat alfo to caufe perfpiration; and when in the deciduous
Trees, (the far greater part of thofe of our country) the very organs of the greateft
perfpiration, the Leaves, do not exift.
The warmt h of fpring arrives : the fluids of the earth grow, thinner , every part of the
Root expands j this opens the mouths of the veflels, and the torrent of nutrition ruihes
in. A great deal of it afcends, but more diffufes itfelf among the circumjacent parts :
the mouths of the fécond order deliver out a great deal to all the interftitial fpace j and
thofe of the third into the Blebs themfelves j and thefe being naturally open, foon run
over J thus every part of the Rind, and every coat of it, and even the interftitial fpace
between its innermoft coat and the Bark, are filled with a fine fluid j and the whole is
fupple, and it then eafily feparates from the under coverings.
7 Th e