390 I R R 
the following pertinent and judicious obfervation on the 
irritability of plants in general. “The whole of my ex¬ 
periments (he obferves) on the lettuce, as well as on dif¬ 
ferent fpecies of plants, inconteftably prove that vegeta¬ 
bles poffefs a principle fimilar to that which in animals is 
termed irritability. All vegetables appear to pofi'efs this 
power, though in different degrees. On this principle 
depends not only the abforbing power, and the afcent of 
fap in vegetables, but alfo the motion, or the circulation, 
of the juices in their veffels. It is beyond a doubt, that 
fluids are abforbed and propelled by vegetables, fo long 
as their vitality remains; and that, as foon as this princi¬ 
ple is deftroyed, the circulation immediately ceafes. 
“ Senebier did not think that irritability was compati¬ 
ble with the rigid Hate of the lymphatic or fucciferous 
veffels in vegetables; and that, from this condition, even 
Were they endowed with that principle, it would be una¬ 
vailable, as they could neither be fufceptible of contraction 
nor dilatation. Hence he was led to maintain that the 
abforbing power of vegetables depends on an hygrofcopic 
or tkermometric aCtion, fimilar to that of a fponge. Ac¬ 
cording to this hypothefis, however, it can hoof no con- 
fequence whether plants be furniflied with an abforbent 
fyltem of veffels, or be compofed merely of a congeries of 
inorganic and impervious fibres. In faff, Senebier reduces 
vegetable life to a iimple effeCt of phyfical caufes. 
“But how, upon his principle, can we admit any dif¬ 
ference between a recent and a dry vegetable, or a living 
and a dead plant ? How, befides, can we explain the 
force with which the fap of plants is fometimes propelled 
to a confiderable height, as obferved by Hales and other 
naturalifts ? The ligneous fibres or fucciferous veffels of 
plants are hard, fay the fupporters of this opinion ; but 
this liardnefs never approaches to rigidity, they always 
retain a greater or lefs degree of foftnels, and are perfectly 
fufceptible of impreflions. Even the hardeft ligneous 
fibres are not wholly inflexible, efpecially thofe through 
•which the lap flows in the greatelt abundance. It affords 
bo folid objection to this doCtrine that thefe veffels are 
connected together at every point, provided that the con¬ 
necting medium be of fuch a yielding nature as to af¬ 
ford free play to the motion of the veffels; and the cellu¬ 
lar tifl'ue of vegetables, it is well known, poffeffes fuch a 
property. Neither is it available to allege, that, if the 
lymphatic or fucciferous veffels of plants were endowed 
with a contraCtile power, it mult be manifeft to our fenfes 
by fome change in their appearance, as is the cafe in the 
animal fyllein ; for it is evident, that, as this contraction 
mull neceflarily be in proportion to the hardnefs and 
fmall calibre of the ligneous fibres, it may exift without 
producing any fenfible alteration in the afpect of the plant. 
It appears to me then, unlefs we contradict known fads, 
that vegetables mult be admitted to poffefs a principle of 
life, or irritability, dn which their primary functions; as 
the circulation of the fap, &c. depend. Whether they 
be likewife endowed with fenfibility, or a fpecies of in- 
ftinCt, or volition, is a jnetaphyfical queltion wholly fo¬ 
reign to the prefent inquiry. It has, befides, been already 
difcuffed by men of the greatelt celebrity, who have 
maintained it with all the power of their imagination, and 
all the aids derivable from fcience. I fhall here therefore 
only obferve, that, if the apparently-fpontaneous motions 
of plants be luppofed to proceed from a principle of in¬ 
telligence, it mult at the lame time be admitted, that this 
volition is dependent on the laws of chemical affinities. 
“Setting alkie, however, all theory, this principle of 
life or vegetative power, on which the circulation of the 
fap in plants depends, has been demonftrated by direCt 
experiments. Coulomb has evinced by fome decifive tri¬ 
als, that the fap-velfels of certain plants contraft on the 
application of ftyptics; and Van-Marum has fatisfaClorily 
demonltrated the lame faCt, by Ihowing that vegetables 
may be deprived of this contractile power by the appli¬ 
cation of electricity, in the lame manner as it deltroys the 
irritability of the mufcular fibre in animals. Girunner 
i r n 
and Humboldt have likewife experimentally proved the 
irritability of plants, as well as Delametherie, who, in a 
work entitled Confderations fur les Etres organife's , has 
proved, by numerous and convincing experiments, that 
vegetables poffefs functions analogous to thofe of animals, 
“The following Iimple procefs, in my opinion, fully 
demonllrates that the circulation of the juices in plants is 
produced by vafcular aCtion. Let any one feleCt a young 
plant of fpurge. Euphorbia cypariffias, the Item of which 
is not furnilhed with branches; and, on detaching or 
cutting off the leaves, a milky liquor will immediately 
iffue from the wounds ; but, on afterwards cutting the ex¬ 
tremity of the Item, the fluid will ceafe to drop from the 
firft wounds, and begin to flow from the one laft inflicted. 
If this operation be performed acrofs, that is, if we di¬ 
vide, in the firft inftance, the extremity of a fimilar plant, 
and afterwards punCture the Hem, or detach feveral of 
the leaves underneath the firft incifion, very little of the 
milky juice will flow from thefe laft wounds, while, on 
the contrary, it will iffue copioufly from the one at the 
upper extremity of the ftem. Hence, it fhouldfeem that, 
there is a revulfion or derivation of fluids in plants as well 
as in animals, fince the fap may be recalled from one ex¬ 
tremity to another of the ftem of a plant, fo as to flow 
out at the part to which it is recalled. As it there finds 
a more prompt and eafy out-let, it leaves empty thofe 
veffels which it formerly occupied ; a circumftance that 
could not happen unlefs it was propelled forward by the 
aCtion of the veffels. 
“ I mud not, however, be underftood as maintaining 
that this principle, or motive force, with which I fuppoie 
vegetables to be endowed, and which I denominate irri¬ 
tability, is in every refpeCl fimilar to animal irritability. 
It is well known that, in animals, this principle refides irr- 
the mufcular fibres, and manifefts itfelf by their contrac-? 
tion ; but, in refpeCl to vegetable irritability, nothing is 
known except that it may be brought into aCtion by fti- 
muli of various kinds, and be renewed by the reiterated 
application of the exciting body ; in this refpeCl, it per¬ 
fectly agrees with animal irritability. We know not, 
however, in what part of the vegetable fubftance this prin¬ 
ciple refides; nor have we been able precifely to afeer- 
tain its mode of aCtion in the internal ftruCture of the 
organs endowed with it. 
“ If the laws and properties of vegetable irritability, on 
which we have fuppofed the circulation of their juices 
to depend, be carefully examined, it will be found in 
many refpeCls to differ from animal irritability. It is a 
fact, i’ufficiently well known, that many plants revive af¬ 
ter having been dried for a confiderable time, and that 
the drying procefs may be repeated without wholly de- 
ftroying the life of the plant; but mufcular irritability, 
on the contrary, when Jong fuffered to remain inert, is 
wholly deftroyed ; becaufe it is well known, from the laws 
of irritability, that the mufcular fibre lofes that principle, 
either by a too-prolonged contraction, or by being fuf- 
fered to remain, for a great length of time, in a ftate of 
inaCtion, or relaxation. Thus then there exifts in plants 
a fpecies of irritability, on which depends the circulation 
of the fap, and eonfequently their nutrition ; though k 
mult, at the fame time, be admitted, that the laws, by 
which it is governed, are in many refpeCls different from 
thofe of animal irritability. 
“ Are there then other vital functions, different from 
irritability, and fenfibility, with which we are yet unac¬ 
quainted ? It is not impoffible that nature may have gra¬ 
dations or varieties in the animating powers as well_as 
in the external forms of organized exiitences, which re¬ 
gulate their economy, and preferve the different claffes of 
being in the fcale of creation.” 
HERITABLE, adj. Eafily irritated.—The wife will 
determine from the gravity of the cafe, the irritable from 
their fenfibility to oppreffion. Burke. 
To IR'RITATE, v. a. [ irrito , Lat. irriter, Fi\] To pro¬ 
voke ; to teafe; to exafoerate.-^Laud’s power at court 
a could 
