2 36 B O T 
account for its defcending in the fame circumftances, that 
is, when the atmofphere is warmer than the earth. But, 
this takes place conftantly, with refpeft to the fuccus pro- 
prius, and it is probable that part of the fap has the fame 
courfe both in the day and night. 2. The exertions of 
many plants, on the application of ftimuli, afford another 
argument in fupport of their mufcular power, and the 
fpontaneous motions of other plants confirm the fame opi¬ 
nion. 3. Light, admitted to plants, increafes their per¬ 
fpiration, and caufes a leaf, before inverted, to reftime its 
natural pofition. The influence of darknefs counterafts 
thefe effedls, and it produces what is called fleep in plants, 
although the heat of the atmofphere be not diminiflied, 
Thefe fadts feem to prove the irritability, or mufcular 
pow'er, of vegetables. 4. If the fluids of plants are con¬ 
veyed through velfels, can we fuppofe thefe tubes to be of 
fo final! a diameter, as, by capillary attraction alone, to 
raile the juices from the roots to the fummits of the loftieff 
trees ? 5. On the fuppofition of the fluids being moved 
entirely by capillary attraction, how happens it, that the 
fap of rhe vine flows from an inc.ilion made in the fpring, 
and not from ene made in the fummer ? In this cafe, as 
the veffeis remain the fame, and the heat is at lead not di- 
miniflied, the efflux of fap ought to be equally copious in 
fummer as in fpring. 6. Capillary tubes, filled with li¬ 
quor, do not difcharge their contents when broken acrofs. 
But from the (tern of a vine, Cut tranfverfely, a large quan¬ 
tity of fluid is difcharged, as has been demonftrated by 
Dr. Hales. 
Plants, as well as animals, perfpire, and, in both cafes, 
this function is effential to health. By the experimenr of 
Dr. Hales, and M. Guettard, it appears, that the perfpi- 
rable matter of vegetables differs in no refpedl from pure 
water, excepting that it becomes fooner putrid. The quan¬ 
tity perfpired varies, according to the extent of the fur- 
face from which it is emitted, the temperature of the air, 
the time of the day, and the humidity of the atmofphere. 
As the leaves form the greateft part of the furface, it is 
natural to fuppofe, that the quantity of thefe will very 
materially affeCt the quantity of the perfpiration. Ac¬ 
cordingly, the experiments of Dr. Hales have afcertained, 
that the perfpiration of vegetables is increafed or diminifti- 
ed, chiefly, in proportion to the increafe or diminution of 
their foliage. The degree of heat in which the plant was 
kept, according to the lame author, varied the quantity 
of matter perfpired ; this being greater, in proportion to 
the greater heat of the furrounding atmofphere. The de¬ 
gree of light has likewife confiderable influence in this re- 
fpect; for Mr. Philip Miller’s experiments prove, that 
plants uniformly perfpire mod in the forenoon, though the 
temperature ot the air, in which they are placed, fliould 
he unvaried. M. Guettard likewife informs us, that a 
plant, expofed to the rays of the fun, has its perfpiration 
increafed to a much greater degree than if it had been ex¬ 
pofed to the fame heat under the (hade. Finally, the 
perfpiration of vegetables is increafed in proportion as the 
atmofphere is dry, or, in other words, diminiflied in pro¬ 
portion as the atnfbfphere is humid. The more vigorous 
and healthy, the plant, the more copious the perfpiration; 
this function, like the reft, depending much on the vital 
energy. Exceflive perfpiration feems to hurt, and even 
fometimes to deftroy, vegetables ; defective perfpiration is 
equally injurious. It is alfo found, that this funCtion is 
performed, chiefly, if not altogether, by the leaves and 
young (hoots. That it may be proprrly carried on, all 
leaves are deciduous; inthofe trees called ever-grecns, there 
being a conftant fucceflion of leaves, to prevent the organ 
of perfpiration from becoming rigid. 
Dr. Hales firft obferved, that a quantity of moilture is 
abforbed by plants, when expofed to a humid atmofphere. 
This abforption, as well as the perfpiration, is performed 
by the leaves ; but in what manner has not yet been afcer- 
tajned. Experiments made by M. Guettard fliew, that 
perfpiration is more confiderable from the upper, that} 
from the under, furface of the leaves) and thofe of the 
AN Y. 
fame author, of Duham.el, and Bonnet, demonftratc, that 
abforption, on the contrary, is much greater at the infe, ■ 
rior furface than at the fuperior. To prove this, the fu- 
perior furface of one leaf, and the inferior furface of ano¬ 
ther, were covered with varnifli, and the confequence was, 
that the former, in a given time, differed little diminution 
of weight, but the latter became much lighter. Again, 
(imilar leaves were laid upon a furface of water, and it fol¬ 
lowed, that thofe which had their fuperior furface inverted, 
gained little weight, and for the mod part died in a few 
days; while fuch- as had their inferior furface applied to 
the water, became much heavier, and flouriflied many 
months. Thefe faCls make it evident, that perfpiration 
and abforption are not performed by the fame veffeis, but 
that each has its peculiar organs-. 
It has been commonly fuppofed, that perfpiration takes 
place, chiefly, when the air is warm ; and abforption, on 
the other hand, when it is cold and moifl. But, unlefs 
the velfels peculiar to abforption, which are placed in the 
under furface of the leaves, were kept conftantly in action, 
they would neceffarily collapfe or decay. All ablorbing, 
organs have a peculiar ftrudture, and an action depending 
on life : that fuch an organization is prefeiit in the leaves 
of plants, it is reafonable to conclude, becaufe dried leaves 
do not ablorb. The fame reafoning is applicable to the 
abforption performed by the roots : for when a (mail .por¬ 
tion of the l oot of a hyacinth, growing in water, is cut 
oft, the whole root dies, and new roots are (hot out, hav¬ 
ing their extremities peculiarly adapted to the abforption of 
nourifhment. 
The noxious matter, carried off by perfpiration, requires - 
large dilution to prevent its hurting the delicate ftructure 
of the leaves, and in this (late accordingly it is thrown out 
on their furface. Here the noxious part is excreted, but 
part of the diluting fluid is reabforbed, to ferve the pur- 
pofe of fecretion, 'which could not be performed, unlefs 
the common juice, or fap, were previoufly prepared. In 
the fame manner, in the animal body, the filling and pu¬ 
trid matter, carried oil'by the urine, mull be liberally di¬ 
luted, to prevent it from injuring the tender ftrudture of 
the kidneys; yet, when it is (afely lodged in the bladder, 
a part is reabforbed, and the groffer excrementious matter 
is alone thrown out. Something of the fame kind happens 
in the perfpiration of animals. They certainly take in 
fomething ufeful from the furface of their bodies ; and 
this is probably performed by veffeis opening outwards, 
different from the common exhalants. The great quantity 
of water abforbed during the ufe of the pediluvium, and 
that Angular fymptom in diabetes, of the patient’s void>- 
ing a much greater quantity of urine than there is liquor 
taken in by the month, feem t@ confirm this affertion. 
Plants are poffeffed of a pow'er of forming their differ¬ 
ent parts, and this is done by fecretion. We may con- 
jedture what the agents are which produce this effect, but 
in refpedt to the manner of the operation we are entirely 
in the dark. In animals, where the vital power is (trong, 
this is the principal agent in producing the new arrange¬ 
ment of parts, which is made in every fecretion ; but in 
plants, where this power is weaker, it would be unequal 
to perform the fundtion if it were not afftfted by abforp¬ 
tion and fermentation. Wherever any firm matter is to be 
fecreted, the veflelshave a convoluted courfe, to allow the 
juice to be fermented, and the thinner parts to be abforb¬ 
ed. In this manner, the. (tones and kernels of fruits are 
fupplied with nounfliment by fibres, which are much con¬ 
voluted. The proper juice feems to be formed only when 
the fap has afeended towards the leaves, and is defcending 
to the roots. The pabula, from which vegetables receive 
the matter of fecretion, are contained in the furrounding 
elements. 
Some botanifts have conceived, that plants, as well as 
animals, have a regular circulation of their fluids. Others 
think this very improbable. On both fides, recotirfe has 
been had to experiments; and, from tliefp, conchilions 
perfectly oppofite have been deduced. When a ligature 
