The Study of Plant Life - M. C. Stopes (korean ebook reader TXT) 📗
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Another simple experiment shows us that the leaves play an important part in giving off water. Take three flasks with long, thin necks, and of as nearly equal sizes as possible. In one place a branch to which a number of fresh, green leaves are attached, in another a branch of the same size with only small buds (cut off the leaves if necessary), and leave the third as a check to show how much water has simply evaporated away. Fill all the flasks up to the same level with water, and mark this in all three when you start. Leave them for a day or two and then mark the level of the water, some of which will now have evaporated (see fig. 18). This will show clearly that more water has gone from the one with the branch than from the empty flask, and that a great deal more water has gone from the one in which was the branch with big leaves attached.
Fig. 18. Experiment to show that leaves give off water. The flasks were all filled to the same level I., and left for the same time. The one with the leaves in it lose far more than the others.
You can see roughly the rate at which the water goes off from the leaves by completely filling with water an apparatus like that in fig. 19. As the leafy branch (which is firmly fastened in the cork with no air leakage) uses up the water, it must be drawn along the narrow tube, which is graduated so as to show the quantity lost.
Fig. 19. Experiment to measure the amount of water given off by leaves in a given time. At first the tube is full of water, which is drawn back to points 1, 2, etc., as the leaves use it.
From these experiments we find that even although we do not actually see it coming off, yet the leaves of the plant give off a great deal of water in the form of vapour. By this process large quantities of water are drawn through the plant, and the salts in weak solution in it are kept and used by the plant as they are needed for building up its structure.
Now you may think that the loss is simply the result of evaporation from the leaves, because the surface of the leaves is great, and they would therefore naturally lose a considerable amount of water by evaporation. But this view is only partly correct, because the giving off of water by leaves or “transpiration,” as it is called, is regulated by a number of little pores in the skin of the leaf, which can open and close. You can see the importance of these pores as water regulators in plants which have them only on one side of the leaf, because practically all the water escapes from the side on which they are situated.
Fig. 20. Leaf A greased on the lower side, leaf B on the upper side, and C not at all. B withers as fast as C.
To see this, take three leaves of the indiarubber tree, which is grown so often in rooms. Choose three which are as nearly as possible just alike in size and shape. Of one of them carefully cover the whole of the lower side, and the cut-end of the stalk, with vaseline or coco butter; do the same to the upper side and the cut-stalk of the second leaf, and leave the third untouched. Fasten all three separately on to a string so that they all hang with both sides exposed to the air, and leave them for some days. The leaf which was not greased will shrivel up; as it gives up its water and can get no more, it “withers” and dies completely. The leaf which was greased on upper side also withers at about the same rate as the ungreased one, but the one which was greased on the lower side remains fresh and green (see fig. 20). This is because all the pores are on the lower sides of these leaves, and in the one greased on the lower side the vaseline had completely closed them, and so prevented the water from passing away through them. The upper surface is well protected against ordinary evaporation by a thick skin which does not allow the water to pass through it. The leaf greased on the upper side had all its pores left open, and so in this way was withered as quickly as one not greased at all. Not all leaves have their pores only on one side, but in nearly all plants the pores can open and shut. These facts show that transpiration is more than mere evaporation; it is a “life process,” that is, a physical process which is regulated by the structure of the living plant.
Transpiration is very important for plants, for it helps to keep the continual stream of water going through them, which brings with it the necessary food salts. Some plants cannot afford to let much water pass away, for they find it very hard indeed to get enough to keep them fresh; such plants as live in deserts or on bare, sandy places, for example, protect themselves from much transpiration by various devices and special arrangements, which we will study in Chapter XVIII.
We have already observed the fact that water enters the plant at its roots, and have just seen that it passes off as water-vapour from its leaves. Let us now consider for a moment the manner of its entrance. How can water enter the roots of plants?
Let us first look at a somewhat similar case in non-living things which will, perhaps, help us to understand the process in living plants.
Take a small “thistle-funnel” and tie tightly over the wide opening a piece of bladder; then pour some very strong solution of sugar into the funnel and place it in a glass of pure water. Mark the level of the sugar with a label (see fig. 21, S). Leave this for a short time, and you will find that the water has entered the funnel tube and run up it for quite a long way.
Fig. 21. “Thistle funnel” covered with bladder B, filled with sugar solution up to level S, and placed in a jar of water. After a time the water is seen to have risen to W.
You should take another similar tube and do everything in the same way, except that you leave out the sugar solution. Then you will find that the water remains inside the funnel at just the same level as in the outer jar. This is the usual behaviour of water, and in the first case, where the water rose inside the funnel, the rise was due to the influence of the sugar, which has the power of drawing in water. Now we can compare the skin of the root hairs (see fig. 9) to the bladder membrane covering the funnel, and it has been found that inside the cells are substances which have the same power of attracting water as we found was possessed by the sugar. So that the entrance of water into the roots depends chiefly on the attraction of the substances within its cells.
That a large amount of water enters the root in this way you can see if you cut off a quickly growing plant (a vine is very good if you can get it) just near its base, and attach to the cut-end a long glass tube in place of the shoot you have cut away. You must fasten this tube by a very well-fitting indiarubber tube, which you bind tightly so that it will allow no leakage, and support the glass. Pour a few drops of water down the tube to keep the cut-end of the plant from drying up at the beginning of the experiment. Then mark the level reached by the water, and do this every day as it rises in the tube. You should find that for some time it steadily rises day by day (see fig. 22).
Fig. 22. Plant P, which has been cut off near the root, is attached by the indiarubber tube I to a tall glass pipe, which is supported by stand S. On the glass are marked the levels reached by the water rising from the root.
We see in this way that the roots take in a large and continual supply of water, and this must get pressed up the stem even without the influence of the transpiring leaves. This is called the “root pressure,” and is a very important factor in supplying the plant with water. In a plant which is growing under usual conditions, both the transpiration of the leaves and the root pressure are at work, and are both necessary to keep a good stream of water passing through the plant. This stream of water provides it with its mineral food materials, and also keeps it stiff and fresh, and is, as we have seen, absolutely necessary for the growth of the plant.
LIGHT AND ITS INFLUENCES
When we were experimenting on the building of starchy food in leaves (Chapter VI.) we saw how very important and even essential light is for the activity of the plant, and it is therefore natural to expect that light should influence its growth very considerably.
You may see the effect of light which comes only from one side on plants grown in the windows of rooms. If they are left in one position they grow in a one-sided manner with only the bare stalks toward the darker side of the room and all their leaves turned towards the window through which the light comes. If you want them to look pretty towards the room side also, they must be turned round frequently, so that the leaves are drawn in many directions instead of one only. The usual effect of light is to make the leaves grow towards it. You may see this still more clearly by placing a pot of seedlings in a blackened box with a small hole on one side. Very soon they will bend over towards the light entering by it (see fig. 23).
Fig. 23. Grass seedlings growing in an earthenware dish enclosed in a strong box blacked inside so that the light only enters at a. (Note how the seedlings bend towards it.)
Leaves can absorb most light when their upper surfaces are at right angles to it, and you will find some leaf-stems will bend right round in order to allow their leaves to get into this position. For example, if you take a pot of nasturtiums growing in the usual way, and support the pot on a stand, and cover it over with a bell jar which has been blackened, or with a black box, so that all the light reaches the plant from below, you will find that in a day or two the leaves will have turned completely round on their stalks and are now facing the light, so that they are upside down in their relation to the position of the whole plant (see
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