Fig. 31.—Tendrils of Virginia creeper, with discs attached.
<!-- image -->Fig. 32.—Tendrils of Virginia creeper, discs not attached.
When a tendril does not attach itself it ultimately winds up into a close spiral (fig. 33, a), but if it attaches its extremity to any object it winds itself into a more open spiral for some distance, then reverses, and winds in the opposite direction (fig. 33, b, c). The reason for this will be obvious if we attempt to twist a piece of twine with its extremity fixed; the torsion will soon become so great that we must cease or reverse the spiral. The latter movement relieves the torsion, and the twist in the second direction soon compensates the first. If any tendril with its extremity attached be examined, this reversal of the twist will be found of universal occurrence. Indeed, it must be so, as a physical necessity, to which the tendril is compelled to submit. The above figures in illustration are from a cultivated passion-flower (Passiflora edulis).
Fig. 33.—Tendrils of Passiflora edulis.
<!-- image -->There remain only the two sections, of scramblers, or plants which ascend merely by hooks, and root climbers, which ascend by means of rootlets, to be described. As these do not exhibit many remarkable phenomena a few observations will suffice. The scramblers are represented by that very common weed the "cleavers" or "goosegrass" (Galium aparine), which scrambles up hedges and amongst thickets by means of the recurved hooks with which the stems are liberally provided. The young shoots appear to possess no spontaneous rotation, and the climbing habit is literally reduced to a scrambling, the lowest and most imperfect climbing with which we are acquainted. Some kinds of roses would also find a place in this section, for they will scramble up the walls of a house if there is a trellis-work to assist them. Professor Asa Gray, explaining this phenomenon, in reference especially to the Michigan rose (Rosa setigera), remarks that the summer shoots are strongly disposed to push into dark crevices and away from the light, so that in pursuance of this habit they would be sure to thrust themselves under a trellis, whilst the lateral shoots, developed in the following spring, will emerge from the trellis in search of the light. This alternate mode of growing inwards and outwards is just the process which would be mechanically adopted to secure a rose to a trellis-work.
Fig. 34.—Cleavers (Galium aparine).
<!-- image -->Of root-climbers our most familiar indigenous illustration is the ivy (Hedera helix), which ascends by means of rootlets, which adhere to the wall or old trunks, and thus enable the plant to reach the summit of its ambition. Dr. Spruce, alluding to a South American plant (Marcgravia umbellata) which grows against the trunks of trees by means of claspers or roots, remarks, that when it has reached the light and the branches become free, the stems which before were flattened become rounded, and the leaves are altered in character and general appearance. To a certain extent this is also true of the ivy, for when it has reached the top of an old trunk and the free branches are produced, they are destitute of rootlets, and the leaves are smaller, more narrowed towards the footstalk and otherwise modified.
There is also a species of fig (Ficus repens) which climbs a wall in the same manner as the ivy. The rootlets of this plant, when pressed lightly on slips of glass, were found to emit minute drops of a clear fluid, which is slightly viscid. This fluid exhibited the remarkable faculty of remaining fluid during 128 days. Other rootlets, left in contact with glass for a longer period, secreted larger drops of fluid, which were more tenacious, and could be drawn out in threads. Other rootlets, left for a still longer period in contact with glass, became firmly cemented to it, and when torn away atoms of yellowish matter were left behind. The inference from these observations, strengthened by chemical tests applied to the secretion, is that the fig has the power of transuding from the rootlets a kind of cement, similar to caoutchouc, by means of which the rootlets become attached to the supporting object.
As we have intimated, the tendril-bearers seem to be the most highly organised of climbing plants. The most interesting point in their history is, as Mr. Darwin has pointed out, the varied movements they display according to their wants. The first action of a tendril is to place itself in a proper position. Secondly, if a twining plant, or tendril, gets into an inclined position accidentally it soon bends upwards again. Thirdly, climbing plants bend towards the light by a movement analogous to that which causes them to revolve so that their revolution is accelerated or retarded in travelling to or from the light. A few tendrils bend towards the dark. Fourthly, there is the spontaneous rotation which is independent of external stimulus. Fifthly, tendrils all have the power of movement when touched, and bend towards the point of irritation. If the pressure be not permanent, the part soon straightens itself again. Lastly, the tendrils soon after clasping their support, effectually contract themselves in a spiral manner.
Reflecting upon these movements we are prepared to assent to the concluding paragraph of the work in which most of the observations in this chapter have been founded. "It has often been vaguely asserted that plants are distinguished from animals by not having the power of movement. It should rather be said that plants acquire and display this power only when it is of advantage to them; this being of comparatively rare occurrence, as they are affixed to the ground, and food is brought to them by the air and rain. We see how high in the scale of organization a plant may rise, when we look at one of the more perfect tendril-bearers. It first places its tendrils ready for action, as a polypus places its tentacula. If the tendrils be displaced it is acted on by the force of gravity and rights itself. It is acted on by the light and bends towards or from it, or disregards it, whichever may be most advantageous. During several days the tendrils or internodes, or both, spontaneously revolve with a steady motion. The tendril strikes some object, and quickly curls round and firmly grasps it. In the course of some hours it contracts into a spire, dragging up the stem and forming an excellent spring. All movements now cease. By growth the tissues soon become wonderfully strong and durable. The tendril has done its work, and has done it in an admirable manner."
CHAPTER X
SENSITIVE PLANTS.
CULTIVATED in green-houses as curiosities several species of exotic plants have received the name of "sensitive plants." These are, perhaps, the most decided in their exhibition of irritability, or movement, when touched; but the same phenomenon in a less degree is to be found in a vast number of plants. Poets have taken advantage of this extraordinary faculty, and invested those which possessed it with mystery and romance.
A sensitive plant in a garden grew, And the young winds fed it with silver dew, And it opened its fan-like leaves to the light And closed them beneath the kisses of night.
Travellers in foreign climes have delighted to descant on the wonderful sensitive plants. "Looked upon with such interest in our green-houses, but which here abound (Brazil) as common as wayside weeds. Most of them have purple or white globular heads of flowers. Some are very sensitive, a gentle touch causing many leaves to drop and fold up; others require a ruder hand to make them exhibit their peculiar properties, while others, again, will scarcely show any signs of feeling, though ever so roughly treated. They are all more or less armed with sharp prickles, which may partly answer the purpose of guarding their delicate frames from some of the numerous shocks they would otherwise receive."
One of the best known "sensitive plants" is the one usually called the "sensitive plant"—that is, the Mimosa pudica of botanists, a plant of which stands on the table before us as we write. In this the leaves are bipinnate, then quadripinnate. There is a pair of pinnae at the end of a long peduncle; with maturity two others are developed. These pinnae consist each of about eight to twelve pairs of opposite leaflets, the two pinnae standing almost at right angles to each other. At a slight touch all the leaflets rise and close the upper surfaces together, at the same time the two pinnae approach each other so as to be nearly parallel, instead of at right angles as before. In this manner the leaves which have been touched respond, and remain closed for some time; but at length they recover gradually from the shock, and return again to their previously expanded position. The experiment may be repeated with similar results; but if repeated again and again the movements become more tardy, as if debility ensued from over-exertion. Too strong sunlight has a similar effect in causing the leaflets to close. A strong puff of the breath, or a shake of the pot, is enough to cause the movement. It is by no means a slow and gradual change, but an almost instantaneous one, sometimes of the leaflets on both sides simultaneously, and sometimes first of one side and then the other. The return movement is much more deliberate, so that it can scarcely be detected.
Fig. 35.—Leaves of sensitive plant, Mimosa pudica, awake and asleep.
<!-- image -->Yet more remarkable movement takes place in another celebrated plant, without a touch being required to stimulate it. This is the "telegraph plant" (or Desmodium gyrans), a native of Bengal. The lateral leaflets keep constantly moving all day long without any external impulse being given to them. They move up and down and circularly, this last motion being performed by the twisting of the footstalks, and while one leaflet is rising its corresponding one opposite is generally being depressed. The motion downwards is generally quicker, or more irregular than the motion upwards, which is steady and uniform. These motions are observable for twenty-four hours in the leaves of a shoot which is lopped off from the plant, if kept in water. If from any obstacle the motion is retarded, upon its removal it is renewed with greater velocity. The motion is most evident when the sun's rays are upon the plant.
This shrub belongs to the same natural order as the acacia, the furze, and the broom. Another, which belongs to the same order as our little wood-sorrel, but, on the contrary, grows to a big tree, is also remarkably sensitive. It is the camrunga-tree of India (Averrhoa carambola). The leaves are pinnated, or feathered, with alternate leaflets, and an odd one at the end. Their common position in the daytime is horizontal. On being touched they move downwards, frequently in so great a degree that the two opposite leaves almost touch one another by their undersides, and the leaflets sometimes either come into contact, or even pass each other. The whole of the leaflets of one leaf move by striking the branch with the finger-nail, or each leaflet can be moved singly by making an impression which shall not extend beyond it. Thus the leaflets of one side of the leaf may be made to move one after another, whilst the opposite ones continue as they were, or they may be made to move alternately in any order by merely touching the leaflet intended to be put in motion. After sunset the leaves go to sleep, first moving down so as to touch one another by their undersides; they, therefore, perform a greater motion at night of themselves than they can be made to do during the day by external impressions. The rays of the sun may be concentrated by a lens upon the leaflets without producing any motion; but when directed upon the leafstalk the response is almost instantaneous. The leaves move rapidly under the influence of an electric shock.