THE MOVEMENTS OF PLANTS
Not so very many years ago we were told that the chief difference between plants and animals was that animals had the power of movement and plants had not. But as soon as people began to study the life and the ways of plants more thoroughly, they found out that this was not at all correct. Plants most certainly can move. No one who has watched a sundew curl its tentacles over a captured fly, or the leaves of Venus's flytrap snap together the instant an insect touches its sensitive hairs, can have any doubt about it.
But, of course, plants do not move to the same extent that animals do. Plants only move when there is something to be gained by doing so. As a rule they are fast-rooted in the soil, and they find all the food they need in the sunlit air around them and the soft rain that falls upon them, so there is no occasion for them to move. But when on account of the poorness of the soil they cannot get enough nourishment, certain plants have learned to move, just as many lower animals do, in order to obtain for themselves a sufficient supply of nitrogen.
But it is not only to obtain food that plants move. Many plants alter their position when they go to sleep at night. Notice how the pansy in the garden bed bends down its head when evening comes, how the daisy draws its ray flowers over the delicate disk flowers in the center of its head. If you look about in the garden and in the meadows when the sun is going down in the west, you will see that not only the flowers, but the leaves of plants, often change their position when they go to sleep for the night. The clover folds the three leaflets in each of its leaves one over the other, each pair of leaves of the creeping stem of the chickweed lean towards each other, and the leaves of many other plants draw closer together or droop downward on their stems.
Many plants, too, move their flowers and their leaves so that they face the sun the greater part of the day. If you walk through a wood carpeted with wood-anemones on a sunny day, with your back to the sun, the ground seems covered with the little white flowers; but if you turn round and face the sun there are hardly any to be seen; for all the anemones are turning in the same direction, with their faces towards the sun, so they now have their backs towards you. The common hawkweed that grows in the meadows turns its yellow flower-heads in the direction of the sun; the sunflower in the garden moves its big round face so that as much sunlight as possible shall fall. upon it; and I am sure you must have noticed how plants growing in pots indoors will lean over towards the window so that they face the light.
There is, of course, a reason why plants bend towards the light in this way, for their very life depends upon the light of the sun. Only by the sun's power can the leaves do their work, which, as you know, is to catch the particles of carbonic acid gas floating in the air and work them up into starches, sugars, and living green stuff to feed and quicken the whole plant on which they grow. Just as an engine worked by electricity will stop if all the current is cut off, so the leaves of a plant will cease to work if every ray of sunshine is shut away from them.
Yet although most plants seek the light, and none can live where the sun's rays never penetrate, all plants do not need exactly the same amount of light ; so, if it is too strong for them, certain plants will turn away from the light. The eucalyptus trees in the forests of Australia turn their leaves edgeways to the sun in order that they may not be scorched up by its fierce rays; and growing on the prairies of North America, is a curious plant called the "compass plant" that twists its leaves round to face the rising and the setting sun, and so prevents the burning noon-day rays from falling upon them. The behavior of the compass plant shows very plainly that plants do not move unless they gain some advantage by doing so; for when it grows in damp, shady places, it does not twist its leaves in this way, for there the sun's rays are not too strong for it.
The shoots of the ivy turn away from the light, and so do the tendrils of the Virginian creeper and the vine, for, of course, they could not cling to the tree, or to the wall, if they grew outwards towards the sun. A creeper called the "ivy-leaved toad-flax," which often covers old garden walls with its ivy-shaped leaves, is still more interesting, for it stretches its small lilac flowers out towards the light, so that flying insects may see and visit them. But when the seeds are set, the flower-stalks curve backwards, in order that the seeds as soon as they are ripe, may drop into a crevice in the wall and there take root.
Many climbing plants move in a wonderful way in their efforts to raise themselves above their neighbors, in order to get as much air and sunshine as possible. Have you noticed how the nasturtium in the garden pulls itself up by coiling its leaf-stalks round anything they can clasp? and how the scarlet-runners struggle to reach the top of the pole by turning and twisting their stems round and round it ?
They have their own way of going to work too, and are very obstinate about it. Some climbing plants twist in one direction and some in another, and you cannot persuade them to wind the other way about. The hop and the honey-suckle always follow the sun and twine in' a right-handed spiral, while the bean plants twist round to the left in a left-handed spiral. The woody night-shade, or bittersweet, is not so particular, and winds sometimes in one direction and sometimes in another; and the Chili-nettle may start winding its stem one way, then change its mind and go the other way round.
The passion flower, the pea, and the vine put forth tendrils, like delicate feelers, which coil round everything they can lay hold of to support the climbing plants. They are so very sensitive that the slightest touch will induce them to begin to curl; but the tendrils of some plants move much more quickly than others. The tendril of the vine takes rather more than an hour to make one complete turn; the tendril of the passion-flower twists once round its support in about half an hour or a little longer; and the tendril of the pretty cup-and-saucer plant can take one turn in twenty-five minutes.
So, you see, plants move for many different reasons--some because they are hungry, others because they are sleepy, some turn towards the sun, others turn away from it; and some twist and twine because they want to climb up to the light. To enable them to move, plants need something to stimulate or excite them. Something must cause them to move. It may be an increase or decrease of light, a rise or fall in temperature, or the lightest touch on a sensitive part.
Plants most certainly have feelings; many shrink from rough handling, and their leaflets droop directly when they are touched.
The beautiful mimosa plants go regularly to sleep at night, and their pretty feathery leaves then hang down like little pieces of twisted string; but one member of the mimosa family, called the "sensitive plant," not only behaves like this when it goes to sleep, but does exactly the same thing when it is touched.
Should a hungry animal attempt to nibble a leaf of the sensitive plant, not only the leaf that is touched, but every other leaf on the plant as well, at once collapses, and this so startles the animal that it walks off and leaves the strange plant well alone. Gusts of wind, burning heat, drifting grains of sand, or the pattering of rain drops will also cause the sensitive plant to move in the same way.
Even more curious is the behavior of a plant that grows in tropical India, for it actually moves before it is touched. As soon as anyone approaches it, down drops every leaf, and all the tiny leaflets shrink closely together. So wonderfully sensitive is this plant that it even feels the disturbance of the air caused by anyone moving near it.