118 M O T 
when afleep, bend towards the earth; by which means the 
noxious effefts of rain or dew are prevented. All thefe 
motions have been commonly afcribed to the fun’s rays; 
and Mr. Smellie informs us, that in fome of the examples 
above-mentioned the effefts were evidently to be afcribed 
to heat ; but plants kept in a hot-houfe, where the tem¬ 
perature of the day and night are alike, contract their 
leaves, and deep in the fame manner as if they were ex- 
pofed to the open air; “ whence it appears (fays he), that 
the deep of plants, is owing rather to a peculiar law, than 
to a quicker or dower motion of the juices.” He fuf'pefts, 
therefore, that, as the deep of plants is not owing to the 
mere abfence of heat, it may be occadoned by the want of 
light; and, to afcertain this, he propofes an experiment of 
throwing upon them a ftrong artificial light. If, notwith- 
ftanding this light (fays he), the plants are not roufed, 
but continue to deep as ufual, then it may be prefumed 
that their organs, like thofe of animals, are not only ir¬ 
ritable, but require the reparation of fome invigorating 
induence which they have loft while awake, by the agita¬ 
tions of the air and of the fun’s rays, by the aft of grow¬ 
ing, or by fome other latent caufe.” On this, however, 
we muft remark, that the throwing of artificial light upon 
plants cannot be attended with the fame confequences as 
that of the light of the fun, unlefs the former were as 
ftrong as the latter, which is impodible; and, even grant¬ 
ing that we could procure an artificial light as ftrong as 
that of the fun, a difference might be occadoned by the 
different directions of the rays, thofe of the fun being 
very nearly parallel, while the rays of all artificial light 
diverge very greatly. If, therefore, we are to make an ex¬ 
periment of this kind, the rays ftiould be rendered parallel 
by means of a burning mirror. Here again we fhould be 
involved in a difficulty : for the rays of the fun proceed 
all in one direftion ; but, as of necefiity we muft employ 
different mirrors in our experiment, the light muft fall 
upon the plant in different directions, fo that we could 
not reafonably expeft the fame refult as when the plants 
are direftly expofed to the rays of the fun. 
The motion of plants, not being deducible from fenfa- 
tion, as in animals, muft be afcribed to that property 
called irritability; and this property is poffeffed injenfibly 
by the parts of animals in a greater degree than even by 
the molt irritable vegetable. The mufcular fibres will 
contraft on the application of any ftimulating fubftance, 
even after they are detached from the body to which they 
belonged. The heart of a frog will continue to beat when 
pricked with a pin for feveral hours after it is taken out 
of the body. The heart of a viper, or of a turtle, beats 
diftinftly from twenty or thirty hours after the death of 
thefe animals. When the inteftines of a dog, or any 
other quadruped, are fuddenly cut into different portions, 
all of them crawl about like worms, and contraft upon 
the flighted touch. The heart, inteftines, and diaphragm, 
are the moll irritable parts of animal bodies; and, to dis¬ 
cover whether this quality reiides in all plants, experi¬ 
ments ftiould be made chiefly on leaves, flowers, buds, and 
the tender fibres of the roots. 
In a paper read before the Academy of Sciences at Paris, 
by M. Broufl’onet, the author inclines to confound irrita¬ 
bility and fenlibility together. “ The different parts of 
plants (fays he) enjoy the faculty of motion ; but the mo¬ 
tions of a vegetable are very different in their nature from 
thofe of an animal: the molt fenfible, thofe that are pro¬ 
duced with moll; rapidity in plants, are always influenced 
by fome ftimulating caufe. Irritability, which is nothing 
but J'erifibility made manifeft by motion, is a general law 
to which nature has fubjefted all living beings ; and it is 
this that continually watches over their prefervation. 
Being more powerful in animals than in plants, it may be 
often confounded in thefe laft with phenomena that de¬ 
pend on a quite different caufe. In the vegetable it is 
only the organ which is expofed to the aftion of the fti- 
mulating power that moves. Irritation in particular 
places never produces that prompt combination of fenfa- 
i 
I ON* 
tions which we obferve in animals; in confequence of 
which certain parts are put in motion without being di¬ 
reftly aft'efted, and which otherwife might have been paf- 
ftve. The more perfedf the organization in the different 
parts of an animal is, the more apparent are the figns of 
irritability. The parts that come neareft to thofe of vege¬ 
tables, and in which of confequence the organization is 
moft imperfeft,are the leaft irritable. The fame law holds 
with regard to plants; but the refult is oppofite : the 
figns of irritability are moft fenfible in proportion to the 
analogy of the parts with thofe of animals; and they are 
imperceptible in thofe that are dillimilar. This affertion 
is proved by what we obferve in the organs deftined in 
vegetables to perpetuate the fpecies. Thofe parts alone 
l'eem fenfible to ftimuli; the bark, leaves, ftalks, and roots, 
fhowing no figns of irritability. 
“ The motions efientially vital, which have in plants 
the greateft affinity with thofe of animals, are the courfe 
of the lap, the pafl'age of the air in the trachea, the diffe¬ 
rent pofitions which the flowers of certain plants take at 
certain hours of the day, &c. But, if we attend to the 
manner in which all thefe motions in plants are performed, 
we {hall find that they prefent a greater number of mo¬ 
difications than the analogous motions that take place in 
animals. The temperature of the atmofphere, its agita¬ 
tion, light, See. have great influence on the motions of 
plants, by accelerating or retarding the courfe of their 
fluids ; and, as they cannot change their place, thefe va¬ 
riations produce in them changes more obvious and more 
uniform than in animals.” 
Thofe motions which are particularly obferved in the 
organs deftined to the reproduftion of the individual, not 
appearing except in circumftances that render them abfo- 
lutely neceffary, feem in fome meafure to be tbe eft'eft of a 
particular combination : they are, however, merely me¬ 
chanical ; for they are always produced in the fame way 
and in the fame circumftances. Thus the rofe of Jericho, 
and the dry fruit of feveral fpecies of Mefembryanthemum, 
do not open but when their vefl'eis are full of water. 
The fudden dil'engagement of fluids produces a kind of 
motion. To this caufe we muft attribute a great number 
of phenomena obfervable in the leaves of feveral plants, 
and which do not depend on irritability. The fmall 
glands in each leaf of the Dionsea are nofooner punftured 
by an infeft, than it inftantly folds up and feizes the ani¬ 
mal : the punfture feems to operate a difengagement of 
the fluid which kept the leaf expanded by filling its vef- 
f'els. This explanation is the more probable, that in the 
early ftate of the vegetation of this plant, when the fmall 
glands are hardly evolved, and when probably the juices 
do not run in fufficient abundance, the leaves are folded 
up exa6Hy as they appear when punftured by an infeft at 
a more advanced period. We obferve a phenomenon fimi- 
lar to this in both fpecies of the Drofera (fun-dew), men¬ 
tioned above. The mechanifm here is very eafily obferva¬ 
ble: the leaves are at firft folded up ; the juices are not yet 
propelled into the fine hairs with which they are covered ; 
but, after they are expanded, the prefence of the fluid is 
manifeft by a drop feen at the extremity of each hair: it 
is by abforbing this fluid that an infeft empties the veffeb 
of the leaf, which then folds up, and relumes its firft ftate ; 
the promptitude of the aftion is proportioned to the num¬ 
ber of hairs touched by the infeft. This motion in fome 
degree refembles that which takes place in the limb of an 
animal kept in a ftate of flexion by a tumor in the joint; 
when the matter which obftrufted the motion is dif- 
cliarged, the limb inftantly refnmes its former politioh. 
The phenomena that depend on the abundance of fluids 
are particularly evident in plants which grow in wet 
foils ; the Drofera and Dionsea are of this kind ; and it is 
known by the experiments of Du Fay and Du Hamel, 
that fenfitive plants are particularly lenlible when the fun 
is oblcured by clouds, and the air warm and moift. 
The influence of external caufes fometimes fo modifies 
the vital motions in plants, that we might be tempted to 
aferibe 
