The Study of Plant Life by M. C. Stopes (best ebook reader for laptop TXT) 📕
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Think over the results of these three experiments, and you will see that it is only when the tip of the root is not cut off that the plant seems to “realize” that it is not in the right position. When the tip is removed it does not bend down even when the whole plant is lying horizontally, and in the other case (fig. 30, C1, D) it will keep on bending even after it has been put in its right position.
We noticed that it is not the very tip itself which bends, so that we see that the very tip is the part which “feels” what is happening, while the part just behind it grows and bends according to the need of the plant.
This is a somewhat similar case to what happens when you realize with your brain that you are in danger on the road, and your feet hurry you across.
When we come to consider why the root should grow downwards in this persistent way, we find that there is an outside influence at work on the plant. You know when a stone is left without any support that it always falls to the ground, and we say that it is attracted toward the centre of the earth by the force of gravitation. It has been proved that the strong tendency of roots to grow down into the soil is largely the result of the same attraction, while the stem is not attracted by it but driven away, and therefore grows away from the centre of the earth. To prove this, however, requires more complicated apparatus than you are likely to be able to use at present.
From the experiments which we have done already we see that plants, as well as animals, are affected by their circumstances, and can in some measure realize them, and move to alter themselves in accordance with them. Later on we shall find that plants have a similar power in relation to light, supply of water, and other things. Have we not already observed in plants nearly all the signs of life we set out to look for? (see p. 4).
There is one very important point about the growth of plants which is strikingly different from the growth of animals. A young kitten has four legs, a head, and a tail, and as it grows to be a cat these only alter a little in shape and get larger and stronger; the number of its legs remains the same. A baby plant, on the other hand, has its little root and shoot with a few tiny leaves, but as it gets older these increase very much in number, till it may have many branches and thousands of leaves. In fact, the number of its parts is much more indefinite than those of an animal; its body is built on quite a different plan. Yet both plants and animals show the same important thing in their growth, that is the increase of their living body, which they build up out of their non-living food.
MOVEMENT
While we have been examining plants to find out some of the facts about their other life properties, we have at the same time seen many cases of movement in their different parts.
For example, we found (Chapter IX.) how the tips of roots move round to get back their vertical position if they are placed horizontally, and how the shoots of young plants bend over towards the light when they are grown in a dark box where it can enter only from one side (Chapter VIII.). Then, too, as the root tip grows into the soil or between the crevices of rocks it bends round the stones or other things in its way, and it is also attracted towards water, thus showing a continual, slow movement in its growth. The shoot shows a parallel kind of movement in following the light and placing itself as advantageously as possible with regard to it.
Fig. 31. Tendrils of the Pea; A young tendrils which have not yet been touched; B beginning to curl fifteen minutes after being rubbed with a twig.
You may see a still faster movement if you carefully examine a twining tendril. Notice how the young tendrils of a sweet-pea are at first almost straight, growing out into the air (see fig. 31). Now choose such a one for the experiment, and another like it which you do not touch, but keep to compare with the one on which you have experimented.
Gently rub one side of the tendril with a small rough twig, and then leave it alone. You will see that in about five or ten minutes it has begun to curve, and in a quarter of an hour may have bent round completely. Such movement is more rapid than that in the ordinary growth, and this power of bending so quickly is one of the special characters of tendrils, and one that is very important in helping them to do their work for the plant and to seize on any support within reach as quickly as possible.
Fig. 32. Leaves of Wood-sorrel; A in the day position, B “asleep” at night.
Then there are other movements, one of which you must have often observed in the “sleep” of plants. Many flowers and leaves close up and bend down at night, taking up their usual position again next day. This is not the same thing as the opening of buds, for it may occur again and again in the fully grown parts of plants. For example, you may mark certain leaves of wood-sorrel or common clover, and watch them close up at night and re-open in the morning many times. These movements are not very fast, and you cannot see the plant moving as you can see a kitten waving its tail, but the difference is only one of degree.
Fig. 33. Leaf of Sensitive Plant in its usual position.
Fig. 34. Leaf of Sensitive Plant, leaflets at a beginning to close after being gently touched.
There are plants, however, which move so quickly that you can see them close up their leaves at once at the slightest touch. This is the case in the Sensitive Plant (fig. 33), and if you only tap one of its tiny leaflets with a straw, that pair of leaflets will immediately fold up, then the next pair, and the next, till the whole leaf has closed, when it drops quickly down (see fig. 35), this movement only taking a moment. If the shock is great, all the leaves on the plant will close up instantly, and they move so quickly that you can hardly see them doing it.
Fig. 35. Leaf of Sensitive Plant quite closed, and the leaf-stalk fallen, after being touched.
Some foreign plants swing their leaflets round slowly like the arms of a windmill, but we have not yet found out why they do this. Also in many flowers we find movement, and in flowers it is generally in relation to the insects which visit them. For example, some orchids shut up their big front petal with a sudden snap when an insect alights on it and shoot the astonished fly towards the middle of the flower.
Parts such as these, which have more power of movement than the rest of the plant, are called sensitive parts, but though in them we see it more clearly than in most plants, they only illustrate what is common to all, and that is some power of movement.
The movements which you have seen so far in plants are different from those of most animals in one way, and that is in the fact that the whole plant remains rooted in one place, and only parts of it can move as the circumstances require, while, though an animal moves its parts separately, the result of some of those movements is to carry its whole body about. This may appear to you a great difference between plants and animals, but it is not quite so great as it seems; nor must we forget that there are some simple slimy-looking plants which slowly crawl along the ground, as well as many minute, green plants, which you could only see with a microscope, which move their whole bodies and swim about just in the way that tiny animals swim.
We have now done a number of experiments with plants, and found out many facts about their way of life, and I think you will agree that we have collected enough evidence to prove the statement made at the beginning of Chapter II.—that on the whole plants show the same “signs of life” as do animals.
We have seen that like animals they breathe in a part of the air, and that they breathe out with the air the added carbonic acid gas, which is the characteristic “waste product” of the out-breathing of animals.
They practically “eat” when they take in substances as food into their bodies, even though they have no gaping mouths which can open and close. We noticed, too, the interesting parallel between young plants and young animals, where both (the plants in the food in the seed, the animals in their mothers’ milk) are supplied with ready-made food at first, and as they get older have to find what food they require for themselves. As regards their feeding, the plants do more work than the animals, for they manufacture the starchy food for themselves out of simpler elements, while the animals require their starch to be ready made.
Then the fact that plants grow, increasing in size and forming new structures, has been known to you ever since you were a baby yourself. Although we noticed here an important difference between the kind of growth in plants and animals, yet the growth itself is alike in the two cases, for both plants and animals build up their living bodies out of simpler substances which they take in as food and change till the not-living food becomes part of themselves and is living.
Movement is not nearly so great in plants as it is in animals, and most plants are firmly fastened in the ground. Yet there are some plants in which we can see very rapid movements of some of the parts, while many simple little plants living in water can swim actively about like animals. All plants show some form of movement, though it is generally slow.
As a result, we find that all the signs of life we noted in animals, viz. breathing, eating, growing, and moving, are to be found in plants, and we must look on them as being just as much alive as animals. We can see that their mode of life and the work they do are distinctly different from those of the animals, but they are no less vital, and important for the world as a whole.
PLATE II.
A WHOLE PLANT, TO SHOW ALL THE PARTS
A POPPY
THE PARTS OF A PLANT’S BODY AND THEIR USES CHAPTER XI.
ROOTS
If you have a garden of your own, or have even watched another person gardening, you must have found out that it is not always an easy thing to get rid of the weeds, and that when one tries to “pull them up by the roots,” they often resist it very strongly indeed. If you have never done this, try to pull up a large grass tuft or a hedge mustard, or any fairly big common plant, and you will find that often when it does not look very strong it may be extremely difficult to
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