Here terminates our observations on insectivorous plants. It will have been remarked during our progress through the preceding chapters, that there are grades of perfection in the modes by which animal food is obtained. In the more highly developed forms, as in the Sundews, the Venus's Fly-trap, the Butterworts, and the Pitcher-plants, the structure of certain parts is adapted for the capture of insects, which are afterwards covered with an acid secretion and digested in a manner analogous to digestion in animals, the products being absorbed for the benefit of the plant. In the second class there are facilities for the capture of insects in the pitchers of the Side-saddle flowers, and the bladders of the Bladderworts, but the power of digestion is not present, or has not been demonstrated, although there may be the power of absorbing from the decay of imprisoned animals their nitrogenous products. In the first class there are also, in some cases, spontaneous movements designed to aid in the capture of insects which are absent in the second. In the Sundews, the captive, rendered helpless by a viscid secretion, is slowly embraced by sensitive tentacles. In the Fly-trap it is caught by a quick movement, as in a trap, and squeezed to death. In the Butterworts, the capture is accomplished by the profuse tenacious slime, little, or not at all, aided by the curvature of the margins of the leaves; and in the Pitcher-plants there is no spontaneous movement, but an adaptation of parts so as to lure insects to their own destruction.
In order that there may be no doubt that the capture of insects is not "accidental," but a vital process resulting in benefit to the plant, investigations by the aid of the microscope have demonstrated that the absorption of animal matter, or fluids of a like composition, act in a peculiar manner by causing aggregation of protoplasm in the adjacent cells. Further, that this aggregation has not been observed under any other conditions, and yet has been discovered in all our carnivorous plants, down to the lowest and most uncertain of all, the "Christmas Rose." In the face of these facts, however reluctant we may at first find ourselves in accepting a theory so contrary to our previous experiences, we must even be "convinced against our will" that there are plants, such as we have described, which are truly carnivorous.
What's this I hear About the new carnivora? Can little plants Eat bugs and ants And gnats and flies?
And, if so, we feel disposed to consider it
A sort of retrograding; Surely the fare Of flowers is air, Or sunshine sweet; They shouldn't eat, Or do aught so degrading.
Whatever our feelings may be at first, these must give way under reflection that many yet stranger things than these may yet be revealed. Our knowledge of the operations is as yet very elementary.
CHAPTER VII
GYRATION OF PLANTS.
IT is only very recently that any regular and systematic investigation has been pursued to ascertain the causes and mode of action in certain phenomena of motion in plants. By whatever names these motions may have been designated, they appear to resolve themselves into modifications of one simple type of movement to which the name of "circumnutation" has been applied. This movement in its ordinary form consists in the revolution of the growing point, which has been described in the following terms:—"If we observe a circumnutating (or revolving) stem, which happens at the time to be bent, we will say towards the north, it will be found gradually to bend more and more easterly until it faces the east, and so onwards to the south, then to the west and back again to the north. If the movement had been quite regular the apex would have described a circle, or rather, as the stem is always growing upwards, a circular spiral. But it generally describes elliptical or oval figures; for the apex after pointing in any one direction commonly moves back to the opposite side, not, however, returning along the same line." That plants did exhibit motion during growth had been observed, but it was left to more recent times to demonstrate how general this kind of growing movement was, and the direction it assumes. Heliotropism, geotropism, sleep of plants, and circumnutation are expressions for forms of the same phenomena of revolving motion in growing plants.
In the volume which Mr. Darwin has written on the movements of plants, he has distinctly expressed his opinion that apparently every growing part of every plant is continually rotating, though often on a small scale. This movement may be traced in seedlings before they have broken through the ground, and in the extremities of their young roots; in the stems of some climbing plants, and in the tendrils of others; in leaves and leaflets, and in all the growing points.
When seeds commence to germinate they thrust out a small radicle, or rudiment of a root, which at once bends downwards and endeavours to enter the ground. The young rootlet is clad with delicate hairs, which, by the softening of the outer surface and its subsequent hardening helps to attach the plant to the soil. As soon as it emerges from the coat of the seed the young rootlet begins to rotate, and thus early in its career active spontaneous motion commences. It is true that the oscillations may seem small to the unaided eye, being only about one-twentieth of an inch, or less, in the garden bean, on each side of a central line, but nevertheless distinct and decided. Rootlets were induced to grow down sloping plates of smoked glass, on which their own vibrations were traced, and thus they recorded the extent of their own lateral motion. This rotation of the young radicle seems to be almost universal, whenever not prevented, or until prevented, by the close pressure of the soil. In a loose soil, or when entering worm-holes, this oscillation would doubtless be useful in enabling the root to enter the ground. The bean having been selected for experiment it was shown that the following is an explanation of how the root enters the earth.
"The apex (of the radicle) is pointed, and is protected by the root-cap; the terminal growing part is rigid, and increases in length with a force equal, as far as our observations can be trusted, to the pressure of at least a quarter of a pound, probably with a much greater force when prevented from bending to any side by the surrounding earth. Whilst thus increasing in length it increases in thickness, pushing away the damp earth on all sides with a force of above eight pounds in one case, of three pounds in another case. It was impossible to decide whether the actual apex exerts relatively to its diameter the same transverse strain as the parts a little higher up; but there seems no reason to doubt that this would be the case. The growing part, therefore, does not act like a nail when hammered into a board, but more like a wedge of wood, which whilst slowly driven into a crevice continually expands at the same time by the absorption of water, and a wedge thus acting will split even a mass of rock."
From the seed upwards rises the short rudimentary stem which supports the cotyledons, or seed-leaves. This short stem (if present) raises the cotyledons above the surface of the soil through which it breaks in the form of an arch. When the cotyledons, or seed-leaves, appear, they are at first vertical, with their faces applied to each other; but they soon separate and exhibit the phenomena of rotation. In all the seedlings of dicotyledonous plants examined, the seed-leaves were in constant movement, chiefly in a vertical direction, and most often once up and down in twenty-four hours. In one plant they moved upwards and downwards thirteen times in the course of sixteen hours, but this is unusual. In different species and in different individuals of the same species, there will be variation and gradation in the oscillations. In one species of wood-sorrel, whilst one of the seed-leaves moved upwards the opposite one moved downwards. Although the up and down movement was the most common, this was sometimes accompanied by lateral and zigzag oscillations. In a great many cases the seed-leaves sink a little in the forenoon and rise a little in the afternoon, so that they stand rather more highly inclined during the night than at mid-day, when they would be nearly horizontal. The position of the seed-leaves by night and by day was observed by Mr. Darwin in the seedlings of plants in 153 genera. Of these there were twenty-six in which the cotyledons stood vertically at night, or at least sixty degrees above or below the horizon. There were thirty-eight in which the cotyledons (seed-leaves) which at noon were horizontal were at night more than twenty and less than sixty degrees above the horizon. In the remaining eighty-nine the cotyledons did not change their position at night so much as twenty degrees.
Proceeding with its growth, the plumule, or miniature bud of the future stem and true leaves, rises between the cotyledons, or seed-leaves, and as it grows it nutates or rotates as the radicle and the seed-leaves had done.
Before quitting the seedling and its radicle some mention must be made of a subject which occupies an entire chapter of the work to which we have alluded, and that is the sensitiveness of the tip of the radicle.
The radicle of the bean was selected for the majority of the experiments in this connexion, the results of which appear to prove that "when one side of the apex (of a radicle) is pressed by any object the growing part bends away from the object; and this seems a beautiful adaptation for avoiding obstacles in the soil and for following the lines of least resistance. Many organs when touched bend in one fixed direction, such as the stamens of Berberis, the lobes of Dionæa, &c.; and many organs, such as tendrils, whether modified leaves or flower peduncles, and some few stems, bend towards a touching object; but no case, we believe, is known of an organ bending away from a touching object."
That the radicle of many plants, indeed, of most, if not all, are sensitive to pressure continuously exerted upon them, or to injury, and are capable of bending away from it, was shown by many experiments, such as attaching small objects to the side of the tip, touching it with caustic, or cutting off a slice from it. All these interferences seemed to act in a similar manner, causing the tip to diverge from its direct downward course and turn in the direction opposite to the obstruction or injury. This sensitive power appeared to be confined to the tip of the radicle for about one-twentieth of an inch. When the part was irritated by contact or slicing, its influence extended upwards for one-third or half an inch, causing the radicle to bend away from the point of irritation in a symmetrical curvature. This occurred sometimes within six or eight hours, and almost always within twenty-four hours of the commencement of the irritation. The curvature often amounted to a right angle, occasionally the tip bent upwards like a hook with its point to the zenith, or it curved and formed a loop or a spire.
The method by which these observations were made was by soaking the beans in water, for about twenty-four hours, then suspending them so that in germinating the radicle might grow downwards freely, and without obstruction. When the radicles were sufficiently protruded, small objects, such as fragments of card or paper, were attached by gum-water, or other cement, on one side of the tip of the radicle, these were then watched for deflection. In other instances slices were cut with a razor from one side of the tip, so as to cause continued irritation. In all these instances the radicles responded, and turned away in the opposite direction to the source of irritation. In cases where the tips were only temporarily irritated, by striking or pricking, no deflection occurred. Other tips were touched lightly on one side with dry nitrate of silver. The injury caused by the caustic was permanent, and the tips turned away in the opposite direction usually with more certainty than when objects were attached.