The Study of Plant Life by M. C. Stopes (best ebook reader for laptop TXT) 📕
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Fig. 24. Nasturtium covered over, so that the light only enters from below. The leaf surfaces bend over to face it.
Fig. 25. Spray of Maple showing stalks of leaves of the same pair of very different lengths, so as to place the leaves well as regards light.
In a small plant, or one with only a few big leaves, this desire for the light is easily arranged for, as there is room for each of them. But if all the leaves of a great tree were turned in the same direction, you will see that many of the under ones must be shaded by the others. This is not so bad as one might expect, however, owing to the wonderful way in which the leaves arrange themselves so as to use every bit of space they can, and yet to overlap and screen each other as little as possible. Particularly in plants which grow flat on the ground or against walls, and which therefore get all their light from one side, this is very well shown. In plants with the leaves in opposite pairs you will often find one leaf of the pair big, and the other one small, or that the leaf-stalks are of different lengths, and if you examine this pair in relation to the rest of the branch, you will see how it is developed in this way so as to use every bit of space it can and get as much light as possible without overlapping its neighbours (see fig. 25). Although it is true in one way that each leaf works as a separate individual, yet each separate leaf is only a small part of the plant, and they all work together for the good of the whole. Branches which have their leaves arranged in this way so that they seem to fit into a pattern, form what is called “Leaf Mosaic.” You may see this kind of arrangement among the leaves of very many plants (see figs. 25 and 26).
Fig. 26. Leaves of Ivy growing out from the stem so as not to overlap each other.
If, as we have already seen, light is so very important for the plant, what is the result of growing it in the dark? As you know, it will not be able to build itself food, and so would finally starve and die. If, however, we choose a plant which has already much food stored up and can therefore grow for a time without making a new supply, then we can study the effect of darkness on its growth.
Take some beans which are just beginning to sprout, plant them in a pot, and place the pot in some quite dark place such as a cellar or a dark room, or cover them with a well-made blackened box which shuts out all the light. Also take a potato which is just beginning to sprout at its “eyes,” and keep it in the dark. Both these plants have food in reserve; the beans have much in their nurse-leaves, and the potato is packed with starch, as you saw before. At the same time grow a potful of beans and a potato plant in the light, so that you can compare the growth of the plants under the two different conditions of light and darkness.
You will find that those grown in the dark are very straggling and of a sickly yellowish colour, and are a great contrast to the shorter sturdier young green plants grown in the open air. The stems of those grown in the dark are long and limp, and not able to support themselves upright, while the distance between the leaves is very great, and the leaves themselves are small and useless (see fig. 27).
Fig. 27. Seedlings of Bean of the same age, A grown in the light, B in the dark.
Why should these plants have such a great length of stem? It shows us, that when the plant is already supplied with food, darkness does not prevent mere growth in length. In fact it grows faster in length in the dark, which is an effort on the part of the plant to grow away from the darkness into the light. It economises in material and does not form stiff, thick stems and big leaves which would be useless until it reaches the light.
If you now make a small chink in the black box with which you cover the plants, you will find that they grow towards it and through it into the light. Once the tip of the stem is outside in the light, it will form the usual leaves at the proper intervals from one another.
The power of rapid growth in length of a plant growing in darkness, which economises the material generally used for strengthening the plant, and its power of growing towards the light, combine to be of practical use to a bulb or seed which is planted too deep in the earth. You will find that the part underground has much the same character as a plant grown in artificial darkness, until it reaches the surface. These weak underground stems bring the growing part into the light, and the plant does not waste material in forming large leaves and strong stems underground where they would be useless.
Although light is so important, it does not follow that the stronger the light, the better it is for the plant; just as it does not follow that because we like to be warm, we like to be as hot as possible. It has been found that plants bend away from the light when it is too strong for them, as you may see in some plants near one of those very brilliant electric lamps. The sun even is sometimes too brilliant (English plants, however, do not suffer from that very much), and many plants living in the tropics and regions of strong sunlight, protect themselves from its direct rays by a number of different devices.
GROWTH IN SEEDLINGS
When once the young plants start growing under suitable conditions they steadily get bigger. At first sight they appear to grow equally all over, stretching out in each direction as indiarubber does when it is pulled. Let us try to find out whether this is actually the case.
Fig. 28. A Bean seedling: A, with divisions marked on root and stem; B, after further growth, showing where most of the stretching has taken place.
Take a well-grown straight seedling and measure off along its stem and along its root, beginning from the tip, distances 1 or 2 mm. apart, marking them with a fine brush and waterproof ink. Take care not to injure the plant, and also not to make the mark blurred or too big. Draw the plant showing the marks on it as accurately as you can, and make the drawing exactly life-size. Grow it in damp, but very loose sawdust, so as not to rub off the marks, and after one or two days take it out and compare it with your original drawing.
You will find that the whole plant is bigger than when you first drew it. Look carefully at the marks on root and stem, and you will find that they are not all the same distance apart, as they should be if the plant had grown equally all over. The marks which are widest apart are those just behind the tip of the root and below the top of the stem, thus showing that there has been much more growth in these two regions than in the rest of the stem or root (see fig. 28). If you repeat this often with many plants you will find that these are the actively growing parts of the stems and roots; the individual leaves, of course, are also growing. Thus we see that growth is not a simple stretching of the whole, but that there are two definite regions where it is specially active. That of the stem and first root carry on the growth in opposite directions, as we noticed before (see p. 11), the normal stem growing up into the air and the root down into the soil.
Fig. 29. A, Bean seedling planted upside down. The root has bent right over and is growing vertically down. B, later stage of the same. The shoot has bent up.
You can see how very determined the directions of growth are by planting upside down a bean which is just beginning to sprout, so that its root points up into the air. As it grows you will see the root bending over till it points vertically downwards, while the stem bends up and grows straight into the air (see fig. 29). The same thing happens if you plant a seedling on its side, and even if you take quite a big seedling, which has grown in the usual way, and then place it upside down in moist air, you will see the root and shoot bending in order to get into their right positions. This very determined growth on the part of roots and stems seems to show us that they must have some means of “perceiving” and regulating their position. It is not an accident that they always grow in these very definite directions. Let us find out what we can about this question.
Take a seedling and mark its root as you marked the roots for the experiment on the region of growth (see fig. 28), lay this seedling on its side on soft, damp sawdust, so that the root can easily bend into it. Next day you should find that the end of the root has bent, and that the bend is in just about the same region as that which showed the most active growth.
Is this actively growing and bending region therefore the part of the root which “realizes” that the whole is in a wrong position, and which therefore bends to put it right?
To answer this question quite fully would require a great deal of work, but there are three simple experiments which you can do, and which will tell you the most important facts about it.
(1)[5] Take a seedling with a fairly long root which has been growing straight down, then very quickly and with a sharp knife or razor, cut off the last 2 mm. of the tip of the root. Lay the seedling on its side on damp sawdust and examine it next day. It will not have bent, even though it has grown in length (see fig. 30, A).
Fig. 30. Experiments on the bending of the root tips in Beans. (See description in text.)
(2) Take another like it and leave it lying on its side for an hour, and then cut off the tip in the same way as in number one, placing it on its side once more. Next day you will find that it has bent in the same way as one which had not been cut (see fig. 30, B).
(3) Take a third, as like the other two as possible, and lay it on its side all night; do not cut it till next day, when it has definitely begun to bend (see fig. 30, C), then quickly cut off the tip, and place it in the upright position (C1). You will find that it continues to grow in the bent form, the root tip going on to one side. It does not seem to know that it is growing along instead of down. If you keep it in this position for a few days it will then get a new tip and begin to grow downwards in
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