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phar Be. a eee 
234 
siderable height, as observed by Hales, 
and other naturalists? 
“ The ligneous fibres or succiferous 
vessels of plants are hard, say the sup- 
porters of this opinion ; but this hardness 
never approaches to rigidity, they always 
retain a greater or less degree of softness, 
and are perfectly susceptible of impres- 
sions. Even the hardest ligneous fibres 
are not wholly inflexible, especially those 
through which the sap flows in the great- 
est abundance. It affords no solid ob- 
jection to this doctrine that these vessels 
are connected together at every point 
provided that the connecting medium be 
of such a yielding nature, as to afford 
free play to the motion of the vessels; 
and the cellular tissue of vegetables, it is 
well known, possesses such a property. 
Neither is it available to alledge that if 
the lymphatic or succiferous vessels of 
plants were endowed with a contractile 
power, it must be manifest to our senses 
by some change in their appearance, as 
is the case in the animal system, for it is 
evident, that as this contraction must ne- 
cessarily be in proportion to the hardness 
and small calibre of the ligneous fibres, 
that it may exist without producing any 
sensible alteration in the aspect of the 
plant. It appears to me then, unless we 
contradict known facts, that vegetables 
must be admitted to possess a principle 
of life or irritability, on which their pri- 
mary functions, as the circulation of the 
sap, &c. depend. Whether they be like- 
wise endowed with sensibility, or a spe- 
cies of instinct, or volition, is a metaphy- 
sical question wholly foreign to the pre- 
sentinquiry. Ithas, besides, been alrea- 
dy discussed by men of the greatest cele- 
brity, who have maintained it with al] the 
power of their imagination, and all the 
aids derivable from science, I shall here 
therefore only observe, that, if the appa- 
rently spontaneons motions of plants be 
supposed to proceed from a principle of 
intelligence, it must at the same time be 
admitted, that this volition is dependent 
on the iaws of chemical affinities. 
“‘ Setting aside, however, all theory, 
this principle of life or vegetative power, 
on which the circulation of the’ sap in 
plants depends, has been demonstrated 
by direct experiments. Coulomb = has 
evinced by some decisive trials, that the 
sap-vessels of certain plants contract on 
the application of styptics; and Van- 
Marum has satisfactorily demonstrated 
the same fact, by showing that vegetables 
may be deprived of this contractile pow- 
er by the application of electricity in the 
Experiments on the Irritability of Plants. 
[Oct. 1, . 
i PX 
~_ a 
J 
. 
same manner, as it destroys the irritabi+ — 
bility of the muscular fibre in animals, — 
Girtanner and Humboldt have likewise 
experimentally proved the irritability of 
plants, as well as Delametherie, who, in 
a work entitled Considerations sur les 
Etres Organisés, has proved, by nume- 
rous and convincing experiments, that 
vegetables possess functions analogous to 
those of animals, Na 
. . . ‘ 
‘“‘ The following simple process, in my 
opinion, fully demonstrates that the cir-_ 
culation of the juices in plants 1s pro- 
duced by vascular action. 
select a young plant of spurge, Euphor- 
bia cyparissias, the stem of which is not 
furnished with branches, and on detach- 
ing or cutting off the leaves, a milky li-’ 
quor will immediately issue from the 
wounds; but on afterwards cutting the 
extremity of the stem, the fluid will cease 
to drop from the first wounds, and begin 
to flow from the one last inflicted. If 
this operation be performed across, that 
is, if we divide, in the first instance, the 
extremity of a similar plant, and after- 
wards puncture the stem, or detach seve- 
ral of the leaves underneath the first in-_ 
cision, very little of the milky juice will 
flow from these last wounds, while, on 
Let any one | 
the contrary, it will issue copiously from ~ 
the one at the upper extremity of the 
stem. 
Hence, it should seem that, there is a 
revulsion or derivation of fluids im plants 
as well as in animals, since the sap may 
be recalled from one extremity to another — 
of the stem of a plant, so as to flow out at 
the part to which it is recalled. As it 
there finds a more prompt and easy out- 
let, it leaves empty those vessels, whichit — 
formerly occupied; a circumstance that 
couid not happen unless it was propelled — 
forward by the action of the vessels. 
“« T must not, however, be understood as 
maintaining that this principle, or motive 
force, with which I suppose vegetables to 
be endowed, and which I denominate ir- 
ritability, is in every respect similar to 
animal irritability. It is well known, 
that in animals, this principle resides in 
the muscular fibre, and manifests itself by 
their contraction ; but in respect to vege- 
table irritability, nothing is known except 
that it may be brought into action by sti- 
muli of various kinds, and be renewed by 
the reiterated application of the exciting 
body; in this respect, it perfectly agrees 
with animal irritability. We know not, 
however, in what part of the vegetable 
substance this principle resides; nor have 
we been able precisely to ascertain its 
. ; mode 
