B O T 
confifts of two parts, the cuticle and the true bark. The 
cuticle of plants affords an external covering to all their 
parts. It confifts-of numerous layers, eatily f'eparable 
from each other, and of which the fibres are circular. The 
true bark may be confidered as a congeries of cellular fub- 
ftance, in which ate placed two kinds of organs, tite vef- 
fels peculi&r to the plants, and the longitudinal fibres. Of 
the life of thefe, nothing can be faid at prefent. On re¬ 
moving'the bark, the wood appears. Its fubftance is denfer 
than the bark, and its flrufUtVe more difficult to be de- 
monffrated. But it has been difeovered likewife to con¬ 
tain vafa propria, and longitudinal fibres, and, befides 
thefe, large veffels with fpiral coats, which run from one 
end of the tree to the other, and are denominated vafa 
atria, or air-veflels. Between the wood and the pith lies 
a green-coloured fubftance, firff accurately deferibed by 
Dr. John Hill, and by him affirmed to contain all the parts 
of the plant in embryo : he gave it the name of corona. 
In the centre of the tree refides the pith, which, in young 
plants, is very abundant. As they approach to maturity 
it grows drier, and appears in a fniaiter quantity ; and, in 
very aged trees, it is entirely obliterated. Its fubftance is 
cellular, and, according to tlac author juft mentioned, it 
is of a fimilar ftrudture in all plants. Thefe are the folid 
parts of vegetables. But there are likewife fluids or juices 
in vegetables; and thefe are of two kinds. The one is of 
the lame nature in ail the variety of vegetables : the other 
varies according to the different plants in which it exifts. 
The former, which is called the fuccus communis, when col¬ 
lected early in the 1'pring, from an incilion made in the 
birch or vine, differs little from common water. The lat¬ 
ter, which is named the fuccus proprius, poffefles various 
properties in various plants, and gives to each its fenfible 
qualities. 1 hefe two juices never mingle with each other 
in the tree, and the latter is found in the unfa propria only, 
It is hot yet afeertained, whether the juices of plants are 
tranfmitted through velfels, ora cellular fubftance. Each 
fide of the queftion has had its advocates, who have fup- 
ported their refpective opinions with probable arguments; 
but it is to he regretted, that, on fo interefting a fubject, 
no conclufion can be formed from the afiual difleftion of 
vegetables. It, however, feems mod probable, that all 
the fluids of plants are tranfmitted through veffels. 
Bofattifts have made many experiments to ascertain the 
eourfe of the fap. Early in the fpring, when the fap be¬ 
gins to flow, incifions have been made in the trunk and 
branches of the trees, as far as the pith ; and, in fitch 
cales, it has been conftantly found, that a larger quantity 
of lap flowed from the fuperior, than from the inferior, 
margin of the incilion. This circumftance led to the opi¬ 
nion 1 , that in the beginning of the fpring great quantities 
of ino.fture are abforbed by trees from the atmof'phere, 
and hence the fource of the abundance of fap. But this 
conclulionis found to difagree with the phenomena of na¬ 
ture, from the two following experiments, i. Incifions 
of various heights being made in the fieni of feveral plants, 
their roots were immerfed in a decodtion of log-wood. The 
roots abforbed the coloured liquor, which at length began 
to flow front tire fuperior, and not from the inferior, mar¬ 
gins ot the incifions; nor had the liquor extended itfclf 
much upwards, beyond the margin of the incifion from 
which it was difeharged. i. In the feafon when the fap 
flows mod abundantly, called the bleeding feafon , a deep 
Cut was made into the branch of a growing vine, and the 
grenteft quantity of lap was difeharged from the upper 
margin of the incilion : but a branch of the fame tree, cut 
in the fame manner, being inverted, the lap flowed molt 
copioufly from the other margin of the incifion, which of 
eourfe was now that next the root. On the other hand, 
the following experiments may be brought to prove, that, 
in the bleeding feafon, the fap afeends from the roots to¬ 
wards the branches, i Early in the fpring; when little 
or no fap had as yet entered the plant, Dr. Hope made a 
number of incifions, of different altitud’es, into the root 
and ffem'of a birch. As the fap role, it firfl: flqwed from 
A NY. 23* 
the fuperior margin of the loweft incifion, and then, in re¬ 
gular lucceffion, from the upper margins of the other in¬ 
cifions, till, at laff, it reached the higheft. 2. If, in the 
beginning of the bleeding feafon, before the fap is found 
in the Item or branches, an incifion be made in the root of 
a vine, a confiderable flow of fap will follow the wound. 
3. The quantity of fap is very generally proportioned to 
the humidity of the foil. 
When a portion of the bark and wood of the pine is cut 
from the ftem, the fuccus proprius flows in confiderable 
qua'ntity, both from the upper and under margin of the 
incifion. Hence it occurred to botanifts, that this juice 
might have little or no motion, and that its efflux from 
Inch an orifice might depend entirely on its being freed 
front the preffure of the bark and wood. But vvq cannot 
accede to this opinion : for although in the beginning, the 
fuccus proprius flows from both margins of the incilion, 
in a little while it is difeharged from the fuperior margin 
only. Hence it appears clearly, that the eourfe of this 
juice in its Veffels is never from the roots towards the 
branches, but always in the contrary direiffion. But befides 
the velfels of the fuccus proprius, and thofe conveying the 
lap, we mult take into our view the air-veffels, or vafa atria, 
Thefe are chiefly (iruated in the wood, leaves, and petals; 
bur are wanting in the bark of trees, and in the herbaceous 
plants. They are formed by a number of fmall filaments, 
fpirally rolled up, fo as to form a cavity in the middle. 
The name of vafa aeria has been given them, becaufe they 
are empty of liquor, and becaufe a great quantity of air 
is certainly found in the wood of plants, where thefe vef¬ 
fels are chiefly placed, and where there is no peculiar or¬ 
ganization. They are fuppofed to be the inftruments of 
refpiration in vegetables ; but in what manner this function 
is performed, is not yet clearly underftood. 
Dr. Hill litis demonftrated, that the cuticle of plants is 
an organized fubftance, containing veffels. In trees and 
ffirubs thele veffels have an external opening : but in th{S 
herbaceous plants this is wanting. Trees and ffirubs only 
are poffeffed of vafa aeria, and, when a plant is placed 
under the exhauffed receiver of an air-pump, the air en¬ 
ters through the cuticle, and only ilfues fron the wood, in 
which the vafa aeria are fituated. From thefe circurji- 
ftances taken together, and conlidered attentively, we have 
reafon to conclude, that the air’s proper entrance to the 
vafa aeria is through thefe cuticular velfels. Thus, in 
the early part of the fpring, the gentle heat expands the 
mouths ot thefe veffels, before contracted by the winter's 
cold. Into thefe orifices the external air nifties and preffes 
down to the roots. To thefe it gives energy, as it does to 
the moving fibres of animals ; and, by its preffure, it may 
ailiff: in propelling the juices upwards. An additional quan¬ 
tity of air is evolved by the internal motions of the plant, 
and the whole paff’es off" with the perfpirable matter. In 
this way, there feems to be a circulation of air through 
plants, affifting and alliffed by the powers which move the 
juices. On this account, trees overgrown with mofs have 
few leaves, weak ihoots, and no fruit. The practice of 
gardeners is, therefore, to be commended, who, in tile 
fpring, ffrip the mofs from the bark of aged trees, and, 
tints facilitating the acceffion of the air, reftore them to 
verdure and fi uitfulnefs. 
We are next to confider capillary attraction, which has 
generally been accounted the caufe of the motion of the 
juices of plants; and the permanence of the action of this 
power lias been fuppofed to depend on the evaporation 
from the leaves. Of late years, indeed, botanifts hav$ 
aferibed to plants a viral power, which they believe affiffs 
the flow ot the juices; and this opinion is fupported-by 
the following reafons. 1. The defeent of the juices, that 
is, their return from the branches -o the roots, cannot be 
explained without the fuppofition of a vital power regu¬ 
lating the motion. A flow of fluids, through capillary 
tubes, will only take place when the refiftance at the one 
end is diminilhed. This might account for the riling of 
the fap, when warmth is applied to the leaves, but cannot 
2 account 
