The sensitive plant was the earliest Up-gathered into the bosom of rest; A sweet child weary of its delight, The feeblest and yet the favourite, Cradled within the embrace of night.
The leaves of the common wood-sorrel not only close in the evening, but if gathered roughly seem to shrink from the touch like the sensitive plant. Another species of wood-sorrel (Oxalis sensitiva), which is a native of Amboyna, is said to be so delicately sensitive that it will not bear the blowing of the wind upon it, without contracting its leaves. Dr. Roxburgh said of it, that "it was like a maiden, though common on every wayside, it may be looked at but is not to be touched."
Thus much is sufficient to explain what are the most manifest phenomena of those which have received the name of "sensitive plants": in other words, plants which exhibit irritability in a manifest degree. In these instances the leaves possess the power of moving themselves in response to a touch, either by elevation or depression, such movement being independent of another motion, termed the "sleep of plants," which is exhibited as daylight declines and night comes on, by the gradual folding or closing of leaves. There is undoubtedly an intimate relationship between the phenomena of motion in leaves when touched, and in those which close spontaneously on the decline of light, and also in such plants as turn themselves towards or away from the sun. Nevertheless, for convenience, we have preferred to write separately of "sensitive plants," of the "sleep of plants," and "heliotropes," or "sun-turners."
It is needless to explain that sensibility, as implied in the term "sensitive plants," does not exist in the vegetable kingdom in the same manner as in the animal. Without brain, and without nervous system, that which we characterize as sensibility does not exist. Yet there is an apparent sensibility to external impressions, and there is also the power of transmitting impressions from one part of the plant to the other. Who will attempt, and how is the limit to be defined to sensibility, or what other term have we which will adequately supply its place, when applied to plants? A curious coincidence of the effects produced on animals and sensitive plants, under the same conditions, was demonstrated by M. Blondeau. Induction currents of electricity have little or no effect on animals when under the influence of anaesthetic agents. In order to see what would be the effect, in the case of the Mimosa pudica under like circumstances, he exposed a specimen to the anaesthetic effect of a few drops of ether sprinkled in the glass enclosing the plant. In a short time the plant experienced the effect of the anaesthetic, its leaves refused to move when shaken, and manifested no sensibility even when the induction current was passed through them. The same experimenter found that when a current from a galvanic battery was passed through the leaves of this "sensitive plant," no result was produced, and the plant did not respond to the stimulus. On the other hand, when in place of the direct an indirect current was employed, by the use of a small Ruhmkorff's coil, the results were entirely different, the leaflets folded up, and the leafstalks drooped along the whole course of the stem. If the current were continued for a short time the plant after a period of repose raised its leaves, and resumed its ordinary state; but if the experiment was prolonged for twenty-five minutes, the organism seemed to become entirely exhausted, and the following day was found withered and blackened, as though struck by lightning.
Some interesting observations have been made by Dr. Maxwell Masters on the effects of ether upon plants, which have a relationship to this phase of the subject. He writes: "On allowing a drop of ether to fall on one of the leaflets of Mimosa pudica from a height of five or six inches, contraction of the leaflet instantly took place, and was immediately followed by the motion, in successive order, of the adjacent folioles, proceeding from the apex towards the stalk of the leaf. When, on the other hand, the drop of ether was placed as gently as possible on the surface, the leaflet did not move, but seemed paralysed by the anaesthetic agent, while the adjacent ones, not touched by the ether, moved as in the preceding case. Ether spray applied with the jet had precisely similar effect. When the spray fell directly on the leaflets, that is, with some force, the impact of the falling drops counteracted any paralysing power that the ether might have; but when the spray was so directed as not to fall directly or with force on the leaflets, then such of them as came within its influence were rendered motionless, the adjacent folioles contracting from the apex towards the base as before. A spray of water directed on to the leaflets caused them to fall, but if not allowed to impinge directly on them no motion ensued, though, of course, the water did not, as the ether did, stop their mobility, as a touch was sufficient to make them collapse after the water spray, while after the ether spray contact produced no effect."
"The effect of the ether spray on certain other plants was, in two instances, remarkable, though the results now to be mentioned were only obtained in two instances out of many trials on various plants in hot-houses in November, 1867. On applying the spray to the extremity of one leaf of Iresine Herbstii, which from having been grown in heat, was what gardeners call 'drawn,' that is, had comparatively long intervals between the leaves, and a flaccid texture, a thin film of ice was speedily produced on the distal end of the leaf. In less than two minutes the whole shoot, four or five inches long, was observed to bend quickly downwards. Next morning the whole shoot was dead. To what precise circumstances the rapid transmission of the effect from one end of the shoot to the other, and its ultimate death, are due, it would be premature to assert, as it is difficult in such a case to eliminate the irritant effect of the ether (clearly it did not here act as an anaesthetic) from the effect of the cold, and ice produced by its rapid evaporation. It may be stated, that two or three drops placed on the leaf in the ordinary way had no effect at all. A few days after, similar trials were made in the Botanic Garden, Chelsea, on some plants of the same species, grown in a colder house, and which were 'short-jointed,' and altogether firmer in texture. In these instances no other effect was produced than the death of the leaf. The other case was a Maranta, also growing in a stove, and in which the application of ether spray to the tip of a leaf caused it to roll up on to the underside, like a roll of paper. In the young state the leaves of this plant are rolled lengthwise, but the effect of the ether was to cause the leaf to roll up along the under surface, from the tip towards the stalk."
Returning from this digression to the special subject of this chapter, it may be intimated that temperature and a healthy condition are important factors in the manifestations of irritability. The tropical wood-sorrel (Oxalis sensitiva), which is remarkably sensitive in countries where it grows naturally, scarcely exhibits this peculiarity, even when grown in hot-houses, in this country. Both Desmodium gyrans and Mimosa pudica show an evident increase of susceptibility with an elevation of temperature. A plant of the Mimosa, carried about in his carriage by Desfontaines, although at first exhibiting its usual susceptibility, by and by ceased to respond to the stimulus, and its leaves became motionless as a result of the continued vibration. Dr. Masters experienced similar results in a plant carried by railway. It is stated that the Mimosa, which is an annual, is more feeble towards the close of the year than when in full vigour. In its native country, where the plant grows wild, it is said that the touch of children affects the plant's movements more than that of adults will do; and Dr. Sigerson has stated that it is more active in its movements when excited by a person in a tonic condition than when he is weary or exhausted. These are curious facts, although they contribute but little to the solution of the problem. "Why a touch," writes Mr. Darwin, "slight pressure, or any other irritant, such as electricity, heat, or the absorption of animal matter should modify the turgescence of the affected cells in such a manner as to cause movement, we do not know. But a touch acts in this manner so often, and on such widely distinct plants, that the tendency seems to be a very general one; and, if beneficial, it might be increased to any extent." We may, therefore, be excused from any attempt to explain that which such an experienced authority confesses that "we do not know." The facts themselves are placed on record, and hereafter the key to the mystery may perhaps be found.
Spontaneous movements are common in the organs of reproduction, being more or less associated with the process of fertilization. "In Stylidium, an Australian genus, the style and filaments are adherent into a column, which hangs over on side of the flower. When touched it rises up and springs over to the opposite side, at the same time opening its anthers and scattering the pollen. The stamens of the various species of berberry (Berberis and Mahonia) exhibit this irritability to a remarkable degree. If touched with a pin or other object at the base of the inside face of the filament, the stamen will spring violently forward from its place within the petal, so as to bring the anther into contact with the stigma, and will, after a time, slowly resume its original position. At first sight it may seem as if this contrivance were intended to ensure the fertilization of the pistil from the pollen of its own flower. In reality, however, the reverse is the case; the excitation takes place in nature when an insect entering the flower for the sake of the honey in the glands at the base of the pistil touches the inside of one of the stamens. The pollen is thus thrown on to the head or body of the insect, which carries it away to the next flower it visits, and leaves some of it on the stigma, and thus cross-fertilization instead of self-fertilization is secured. Similar motion of the stamens towards the pistil, but spontaneous, takes place in the London Pride and other species of Saxifraga." This may be studied also in that pretty marsh-flower, the grass of Parnassus (Parnassia palustris). In this flower each of the white petals has a glandular appendage at the base which forms a portion of the disc. It consists of a crescent-shaped scale, bearing on its margin from sixteen to twenty slender hair-like pedicels, each of which supports a globose vesicle filled with fluid. The base of the flower is a rather large ovary, with a stigma on the top, and surrounding this five stamens on short filaments, which are indeed so short that the anthers can scarce reach the top of the ovary. When the pollen is ripe the filaments of the stamens lengthen themselves one by one, and apply the face of the anther to the stigma. After the pollen is discharged the filaments bend back to the level of the petals, and the empty pollen-cases soon fall off and leave the five filaments spread out in a star-like manner alternate with the petals of the flower. The same phenomena have been observed in other species of the same genus.