The Study of Plant Life - M. C. Stopes (korean ebook reader TXT) 📗
- Author: M. C. Stopes
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Now the sunflower we have just thought of is probably growing in a garden well looked after by a gardener, who sees that it gets all the light and water and just the kind of soil it needs. It is therefore protected and cared for to a certain extent, but who looks after the wild plants which manage to grow everywhere? These have not only their own lives to live, but by their own efforts must overcome difficulties which are not even felt by the cultivated ones.
They succeed in a wonderful way, and some plants manage to grow under very difficult conditions, even in places where they get no water for months under a burning sun, or in forests where the overshadowing trees cut off the light and rain, or under the water where they get no direct air. They have to do all the usual work of plants, and at the same time struggle against the hardships of their surroundings. They are like men fighting for their lives with one hand and doing a piece of work with the other.
The result of this is that they sometimes make themselves strange-looking objects, and in some plants which have had a very hard struggle it is difficult to know which part of the plant is which. Look, for example, at a Cactus (see fig. 48), which grows in the desert; it appears to have neither stem nor leaves like an ordinary plant, and to consist merely of a roundish green mass covered with needle-like prickles. Yet when you come to study the Cactus you will find out that the thick, fleshy mass is really its stem, and the prickles its leaves which have taken on these strange shapes. By means of its unusual form the Cactus can live where our common plants would die of the dry heat in a day or two. The power plants have of changing their bodies so as to fit themselves to live under all kinds of conditions is one of the strongest proofs that they are alive.
All the parts of plants have some special life-work, just as we have legs and arms for different purposes, and every part is formed in some way to suit the needs of the plant and help it to get on well in its home.
The main thing to realise at the beginning of the study of plants is that they are living things, and therefore to try to discover the importance of the shape and arrangement of all their parts and their relation to the life of each plant as a whole.
We will begin by looking carefully for all the signs of life in them, and noting how often these are the same as those of the animals, even though the whole plant-body is so different from that of an animal.
SIGNS OF LIFE
Fig. 1. Jar (A) with well-fitting cork, in which young bean plants are growing. The tube leading from the jar dips into dish of water (s) which has risen to levels marked in the course of three days. (b) Small tube of caustic potash.
If you were asked to give the signs of life in an animal, it is likely that you would think at once of its power of breathing, eating, growing, and moving. Now when we ask the same question about plants the answer does not appear to be quite so easy to find, because at first sight plants do not seem to do any of these things except the growing. However, the same answer would be quite correct for plants, as well as animals, for they are really able to breathe, eat, grow, and move; all you have to do is to watch them in the right way to see that this is the case.
We are not in the habit of treating dry seeds as though they were alive; beans are stored away in sacks all the winter and may be left for months in dry cellars, and the precious seeds which will give us our beautiful flowers in the summer are put away in boxes through the winter. Yet you know that if you place seeds in the earth and keep them warm and moist, little plants will come up and will grow. What gives them the power of growth which is not possessed by the stones and earth around them? Warmth and moisture alone could not put this power into the seeds when we planted them. This power, which only belongs to living things, was there all the time, but was lying asleep, shut in and protected so that it was not easily disturbed till suitable conditions made it time for it to wake.
You know when you are asleep that you do not eat or run about, but simply lie still and breathe. This is what the seed was doing before the baby plant began to break through its protecting coat and show itself to the world as a living thing.
Let us watch some of these young plants just waking up to activity, and see if we can find in them the four signs we take as being the tests of animal life.
First let us see if we can show that they breathe.
You know that when you breathe you take air into your lungs, use some of it, and give the rest out. You can show that plants also use up some part of the air. If you would actually prove this to yourself or anyone else, take some peas or beans, soak them in water, and leave them in damp sawdust for a day or two till the tiny plant has just begun to show. Then put them on wet blotting-paper in a jar which has a very well-fitting cork with no leakage, and through which a fine bent glass tube is fitted. Place a small tube of caustic potash in the jar. Then place the end of the bent tube in a dish of water, which acts better if you have dissolved some caustic potash in it (see fig. 1). Once it has begun to rise in the tube, mark the level of the water with a small label. If then you mark it daily the labels will show how much water has risen each day, and the amount of water rising in the tube shows us the amount of air which has been absorbed by the growing beans.
This tells us, therefore, that air is absorbed by plants in the course of their growth. But there is another thing we must notice about breathing which is equally important.
You will find that you yourself, as well as all animals, not only use up a part of the air, but also give out a waste product which we call carbonic acid gas. You can see one of the characters of this gas from your lungs if you take a jar of lime water and breathe into it for some time. Compare this with a similar jar of lime-water through which ordinary air has been pumped at about the same rate for the same time, and you will see that the one you have breathed into has gone very much more cloudy-white than the other (see fig. 2). The cloudiness in jar A is caused by the waste gas (carbonic acid gas) which you breathe out, and which combines with the lime in the lime-water to make solid grains of chalk. Fine white chalk grains always form in lime-water when this gas is present, so that a jar of clear lime-water is a very good test for the presence of the gas.
Fig. 2. Jar A contains lime-water through which human breath has passed. Jar B, lime-water through which ordinary air has been pumped for the same time. Note how much greater is the milky deposit in A than in B.
The giving out of carbonic acid gas is one of the most characteristic things about animal breathing, and we can show that plants in breathing give out this gas too.
To prove this, take another jar with a well-fitting cork, and put some beans and peas, which are just beginning to grow, into it, with a little damp blotting-paper to keep them sufficiently moist. Leave the jar closed for a day or two and then open it and quickly and gently pour in some lime-water. Put the lid on again at once and shake it up. You will find that the lime-water turns quite milky, showing that the same waste gas is given out by the plants as was given out in your own breath.
These experiments show us that plants breathe in a part of the air, and also breathe out some of the same waste gas which is given off by animals in breathing. So that we have found that plants do breathe.
Now to go to the other signs of life. I think you will hardly need to do any special experiment to show that seedlings grow into big plants, you must have seen it so often for yourself in the woods and fields and gardens.
We have still to show that plants eat and move, but before we can do this properly, we must learn a little more about the parts of the bodies of the plants themselves, for they have quite a different set of organs to those we are accustomed to in animals, and their way of eating is so different from that of animals that we cannot understand it immediately.
SEEDS AND SEEDLINGS
If we wish to follow the whole life of a plant, we cannot do better than begin by watching the baby plant “hatching” out from its seed at the beginning of its active life.
There are many seeds which would be good to begin work on, any kind would be interesting, but it is best to use some nice big ones which allow us to see the parts easily. Good ones to choose would be broad beans or peas. Notice first the size and shape of the dry seed of the bean, make a drawing of it, and then place it in water. After a few hours you will see that the outside skin wrinkles up; this is because the skin absorbs water and increases in size, and so becomes too big for the rest of the seed (see fig. 3, A, B). After the water has soaked right into the substance of the seed you will find that the outer skin fits again and is once more smooth, and that the whole seed is larger than it was before it was soaked (see fig. 3, C).
Fig. 3. A single Bean seed, A dry; B half soaked, when the skin wrinkles; C fully soaked and swollen.
Take one of these soaked beans and examine its structure. Notice the black mark where it was attached to the parent pod, and the little triangular ridge pointing towards it (see fig. 4, A). Now carefully peel off the skin, noticing that there are two skins, an outer thick one and an inner thin one, which protect the parts within. When you have removed the skin, you will find that the inner portions of the seed split very readily into two thick fleshy parts, and
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