SENSITIVE PLANT. See MIMOSA and DIONEA Muscipula.
The sensitive and humble plants are arranged by Linnaeus under the same genus with the acacias. These are well known to possess a kind of muscular motion, by which the leaves and stalks are contracted and fall down upon being slightly touched, or shaken with some degree of violence. The sensibility of these plants is lodged in the young branches, in the common foot-stalk of the winged leaves, and in the nerve or middle rib to which the lobes or lesser leaves are attached. These different motions, which seem to be totally independent of each other, may be aptly enough compared, by analogy, with the irritability of certain parts in animals.
The sensitive plant has two kinds of motion; the one natural, occasioned by the action of warm nourishing vapours; the other artificial, in consequence of being touched or shaken.
Mr Duhamel having observed, about the 15th of September, in moderate weather, the natural motion of a branch of sensitive plant, remarked, that at nine in the morning, it formed with the stem an angle of 100 degrees; at noon, 112 degrees; at three afternoon, it returned to 100; and after touching the branch, the angle was reduced to 90. Three quarters of an hour after, it had mounted to 112; and, at eight at night, it descended again, without being touched, to 90. The day after, in finer weather, the same branch, at eight in the morning, made an angle of 135 degrees with the stem; after being touched, the angle was diminished to 80; an hour after, it rose again to 135; being touched a second time, it descended again to 80; an hour and a half after, it had risen to 145; and upon being touched a third time, descended to 135; and remained in that position till five o'clock in the afternoon; when being touched a fourth time, it fell to 110.
With whatever body the sensitive plant is touched or irritated, it is remarkable that the sensibility resides particularly in the articulation or joining either of the branches of the common foot-stalk, or of the particular foot-stalk of each wing.
The time which a branch requires to recover itself after being touched, varies according to the vigour of the plant, the hour of the day, the season of the year, or the heat and other circumstances of the atmosphere.
The order in which the parts recover themselves varies in like manner: sometimes it is the common foot-stalk; sometimes the rib to which the lobes are attached; and sometimes the lobes themselves are expanded, before the other parts have made any attempt to be reinstated in their former position.
If, without shaking the other smaller leaves, we cut
Sensitive. off the half of a lobe belonging to the last pair, at the extremity or summit of a wing, the leaf cut, and its antagonist, that is to say, the first pair, begin to approach each other; then the second; and so on successively, till all the lesser leaves, or lobes of that wing, have collapsed in like manner. Frequently, after 12 or 15 seconds, the lobes of the other wings which were not immediately affected by the stroke, shut; whilst the stalk and its wing, beginning at the bottom, and proceeding in order to the top, gradually recover themselves. If, instead of one of the lesser extreme leaves, we cut off one belonging to the pair that is next the foot-stalk, its antagonist shuts, as do the other pairs successively, from the bottom to the top. If all the lobes of one side of a wing be cut off, the opposite lobes are not affected, but remain expanded. With some address, it is possible even to cut off a branch without hurting the leaves or making them fall. The common foot-stalk of the winged leaves being cut as far as three-fourths of its diameter, all the parts which hang down collapse, but quickly recover without appearing to have suffered any considerable violence by the shock. An incision being made into one of the principal branches, to the depth of one half the diameter, the branches situated betwixt the section and the root will fall down; those above the incision remain as before, and the lesser leaves continue open; but this direction is soon destroyed, by cutting off one of the lobes at the extremity, as was observed above. Lastly, a whole wing being cut off with precaution near its insertion into the common foot-stalk, the other wings are not affected by it, and its own lobes do not shut. No motion, likewise, ensues from piercing the branch with a needle or other sharp instrument.
From the preceding experiments, to omit many others which might be mentioned, these inferences are clearly to be deduced. 1. That when the plant is in its greatest vegetative force, its motions are greater and more sensible. 2. That when the sky is serene, and the sun bright during the whole day, the plant is more sensible in the morning than at noon. 3. That in the circumstances in which they are less sensible, the leaves continue to fold and collapse, although the foot-stalks, which through age become stiff and woody, have lost their motion. 4. That a stroke, or an irritation, produces a more forcible effect than an incision or even an entire section. 5. That a slight irritation only acts upon the neighbouring parts, and extends its influence in proportion to its force. 6. That any given irritation acts more strongly upon some parts than on others. 7. That whatever can produce any effect upon the organs of animals, acts upon the sensitive plant; as a stroke, excess of heat or cold, the steam of boiling water, that of sulphur and volatile spirits, &c. 8. That plunging it in water, or lodging it in the exhausted receiver of an air-pump, seem to have no other effect than that of diminishing its vigour. 9. That there appears to be no more intimate a communication betwixt the opposite lobes of a winged or pinnated leaf, than betwixt the other parts of the plant. 10. That the muscular motion of the sensitive plant is owing to a strong contraction: each foot-stalk seems to be terminated with a kind of joint, on which the leaves turn in all directions with surprising facility.
Different from all the kinds of sensitive plants hi-
ther to know, is the dionea muscipula, or Venus's mouse-trap, a plant lately discovered in the swamps of North America; for a description of which, see DIONEA Muscipula.—The negroes in Senegal call a large species of sensitive plant which grows in that country, guractias, that is, "good-morrow;" because, say they, when you touch it, or draw near to speak to it, the plant immediately inclines its leaves, to wish you, as it were, a good-morrow, and to show you that it is sensible of the politeness done it. In the same country is produced a small sensitive plant, that is rampant, not spinous, and which Mr Adanson affirms to be infinitely more delicate and sensible than all the other species.
To conclude, the cause of this and the other motions of plants is merely external. The motions themselves, therefore, are not spontaneous as in perfect animals, which have that cause dependent on their choice and will. How many imperfect animals, however, are there, such as those in animal and vegetable infusions, the polypes and animalcules in seed, whose certain motions, like those of the plants in question, are perhaps to be attributed to heat, light, and other external causes? and again, how many, as the gall-insects, the oyster, and other shell-fish, have not a motion so perceptible nor rapid as that of Venus's mouse-trap and the sensitive plant!