For the proper observation of these movements the radicle should be developed at the normal rate. If subjected to high temperature, so that it grows rapidly, or checked so as to germinate slowly, as in winter, the irritability is much less pronounced. The common garden pea, treated in the same manner, was found to be even more sensitive than the bean.
A condition of irritability, or sensibility, had been previously demonstrated in radicles, but this always took place higher up, and not at the tip. In such cases the radicle always turns towards the touching object, and not away from it. This movement is of a kindred nature to that exhibited by tendrils, which turn towards and embrace the touching object. But the sensibility of the tip seems to be of quite a distinct character, and one which would prove of considerable service to the plant. As Mr. Darwin says: "The direction which the apex takes at each successive period of the growth of a root ultimately determines its whole course; it is, therefore, highly important that the apex should pursue from the first the most advantageous direction; and we can thus understand why sensitiveness to geotropism, to contact, and to moisture, all reside in the tip, and why the tip determines the upper growing part to bend either from or to the exciting cause. A radicle may be compared with a burrowing animal, such as a mole, which wishes to penetrate perpendicularly down into the ground. By continually moving his head from side to side, or circumnutating, he will feel any stone or other obstacle, as well as any difference in the hardness of the soil, and he will turn from that side; if the earth is damper on one than on the other side he will turn thitherward as a better hunting-ground. Nevertheless, after each interruption, guided by the sense of gravity, he will be able to recover his downward course, and to burrow to a greater depth."
From seedlings we are led to mature plants, and here again we encounter systematic rotatory movement so universal in its character that it is doubtful if it does not exist more or less in all plants.
A hybrid raspberry, about a foot high, that was growing vigorously, was watched in the month of March. During the morning the growing point of the stem almost completed a circle, and then deflected to the right. In the afternoon it reversed its course, and continued to move in that direction for forty hours.
Some young ivy-plants were seen to rotate their growing points at a slow rate, and over a small space during six successive days.
The stems of some plants only describe one large ellipse during twenty-four hours, others describe several smaller ones. A plant which describes but one ellipse during one day, may on the next describe two smaller ones. Sometimes the course is almost circular, but generally elliptical, and the ellipse may be broad or very narrow. Whatever the figure may be it is not a regular one, but varied by loops or zigzag vibrations in other directions.
In coniferous trees it was observed by Dr. Masters in 1878 that the leading shoot has a rotatory movement, and this has subsequently been confirmed by him, in Abies nordmanniana. The same authority has also directed attention to other movements which affect the leaves, and relate to modified forms of heliotropism. "Some of the silver firs," he says, "are endowed with a power of motion by means of which the leaves are raised or lowered. M. Chatin asserts that in the middle of the day the plant (Abies nordmanniana) has a predominant green hue, but when the light is more diffused, as in the evening or early morning, then the plant assumes a milky-white appearance. This appearance is due to the elevation of the glaucous under-surface of the leaves. The movements in question are so obvious during the period of active vegetation that no doubt can exist on the subject. Unlike M. Chatin, however, I have observed that the white hue of Abies nordmanniana is more conspicuous when the branches are exposed to the full rays of the sun. The same remark holds good of other species."
The tips of runners, or stolons, of such plants as the garden strawberry, exhibit just the same rotating movements as stems. The same may be said also of the flowering stems of various plants, such as that of the rape, as observed by Sachs; wood-sorrel, as observed by Darwin and others, to which the last author has alluded. A larger number of observations has been made on the rotation, or circumnutation of leaves; and, as the phenomena are similar in all, illustrations of this latter kind will suffice.
The leaves of the cabbage rise at night and fall by day, an irregular ellipse being formed every twenty-four hours. The leaves of the Swedish turnip draw together so much in the evening, that, according to Mr. Stephen Wilson, "the horizontal breadth diminishes about 30 per cent. of the daylight breadth."
In the common bean (Vicia faba) the whole leaf and the terminal leaflets pass through regular well-marked diurnal movements, rising in the evening and falling during the latter part of the night, or early morning, whilst during the middle of the day they rotate around the same small space.
More complex movements take place in Venus's Fly-trap (Dionæa). In a young state the two lobes of the leaf are pressed closely together. In the evening one of these young leaves formed an ellipse in the course of two hours. An older leaf did not rotate plainly. A young and unexpanded leaf was carefully watched with a micrometer. It moved onwards generally by rapid jerks. After each jerk the apex drew itself backwards slowly for part of the distance that it had advanced, and then, slowly afterwards, made another jerk forwards. Four conspicuous jerks forwards, with slower retreats, were on one occasion seen to occur in exactly one minute. Sometimes the apex remained quite motionless for a short period. An older leaf was tested in a similar manner at a lower temperature, the apex oscillated forwards and backwards in the same way, but the jerks forward were less, and the motionless periods longer. During one of these motionless periods a wax taper was held close to the leaf. After ten minutes violent oscillations commenced, perhaps owing to the stimulus of the warmth of the taper. The light was then removed, and in a short time the oscillations ceased. Looked at again after a period of an hour and a half, and it was again oscillating. The plant was taken back into the hothouse, and in the following morning was seen to be still oscillating, but not vigorously.
A cyclamen was also watched under the usual conditions for three days. On the first day the leaf fell more than afterwards. On all three days it fell from early morning until about seven in the evening, and from that time it rose during the night, with a slightly zigzag movement. Although the whole distance travelled was considerable, yet the motion would hardly attract attention, or be observed, unless some method of tracing or measuring the movements were adopted.
Plants of seakale (Crambe maritima) were also under observation. In the first instance, a leaf nine inches long was selected. Its apex was in constant movement, but this could hardly be traced, from being so small in extent. A more vigorous young plant with four leaves was then selected. One of the leaves was specially watched and found to be continually rotating, and its movements were distinctly traced. One of the leaves changed its course at least six times in fourteen hours.
The leaves of a camellia were also observed. These leaves are firm and leathery, with short footstalks, so that but little apparent rotation was expected. Nevertheless the apex of a leaf changed its course completely seven times in eleven hours, but moved to only a very small distance. On both days the leaf rotated in the forenoon, fell in the afternoon, and then rose, falling again during the night or early morning.
Peculiar movement in the frond of a fern has been recorded by Professor Asa Gray. "A tuft of Asplenium trichomanes, gathered last autumn in the mountains of Virginia, is growing in the house of Mr. Loomis, in a glass dish. About two months ago he noticed that one of the fronds,—a rather short and erect one, which is now showing fructification,—made quick movements alternately back and forth, at right angles to the frond, through from 20° to 40°, whenever the vessel was brought from its shaded situation into sunlight or bright daylight. The movement was more extensive and rapid when the frond was younger. When I first saw it (on 23rd January), its compass was within 15°, and was about as rapid as that of the leaflets of Desmodium gyrans. It was more rapid than the second hand of a watch, but with occasional stops in the course of each half-vibration. This was in full daylight, next a window, but not in sunshine. No movement had been observed in the other fronds, which were all sterile and reclining, with the exception of a single one which was just unfolding, in which Mr. Loomis thinks he has detected incipient motion of some kind." Subsequently to the publication of this notice, Mr. Loomis furnished the following additional particulars. "Four other fronds starting from two different roots exhibit motion, but in less degree than the one first noticed. These are not new fronds, but old ones which were fully developed as to size when taken up, but have fruited since transplanting. It seems to me that the motion is confined not only to the fruitful fronds, but to the period of fructification, since these four fronds have been subjected to the same conditions as the first, but have exhibited motion only since the fruiting began. The stimulus of artificial light is sufficient to excite motion in the fronds for a few minutes, but after the lapse of five or six minutes the motion ceases, and is not resumed. I have noticed that the end of the frond does not describe a straight line, but it moves in a long and very narrow ellipse with the hands of a watch. The motion is more vigorous and through a larger arc in the middle of the day." Interest attaches to this narrative on account of the very few instances of spontaneous movement as yet recorded in the higher cryptogams; and it would be well if it stimulated closer observation of the ferns and their allies.
There is one other point which comes within the limits of this chapter, and may be referred to here with advantage. There are some plants which as the flowers fade point their ovaries downwards, and then by the curvature or lengthening of the peduncle, these ovaries are made to enter the ground and mature their seeds in the earth. The explanation which is offered to account for such movements is that these parts are more than ordinarily sensitive to gravitation, and that it is "geotropism" by which such phenomena should be called, because of their turning to the earth, as "heliotropism" is applied to those which turn towards the sun. Whatever the explanation may be, the phenomena are interesting as exhibiting a curious type of movement of plants.
We will commence with a species of clover (Trifolium subterraneum) which is indigenous to the south of England, and therefore of more interest than an exotic would be. For the details of its burying propensities we must again follow the lucid narrative of Mr. Darwin, but somewhat condensed. "The flower-heads of this plant produce only three or four perfect flowers. All the other flowers are abortive and modified into rigid points. After a time five long elastic claws which represent the divisions of the calyx are developed on their summits. As soon as the perfect flowers wither they then bend downwards, as the peduncle stands erect, and closely surround its upper part. The imperfect flowers, which are the central ones in the flower-heads, ultimately follow, one after another, the same course. Whilst the perfect flowers are bending, the whole peduncle (flower-stalk) curves downwards, increasing in length until the flower-head reaches the ground. Nineteen upright flower-heads, arising from branches in all sorts of positions were marked, and after twenty-four hours six of them were vertically depressed, having travelled through 180°. Ten were extended horizontally, and these had passed through about 90°. Three very young peduncles had as yet only moved a little downwards.