CHAPTER VII.

THE CIRCULATION OF WATER

As we have already found out, water is one of the things which are necessary for the well-being of plants. Seedlings can begin to sprout only when they are well supplied with it, and in the growing plant it is the water in the cells which keeps it firm and fresh. Directly the plant is deprived of some of its water it becomes limp and flabby, and “withers.” We noticed in Chapter IV. that the rootlets absorb the water (with its salts contained in solution) from the soil, and from them it travels all over the plant. The salts dissolved in water, however, are in very weak solution, and to provide the plant with sufficient of them for its growth ~it is necessary that a continuous stream of water should enter the plant~. How is this stream kept up?

The leaves play a very important part in the water circulation, their thin expanded surfaces giving a large area from which the evaporation of water can take place. The water which comes off from them is not generally visible to us, because it comes off as vapour. However, you can easily make experiments which will show you that it actually does come off from the leaves.

Take a large test tube or a small glass flask, and place it over a good-sized fresh green leaf, which you leave attached to a healthy plant or a branch in water. Round the leaf-stalk wrap cotton wool till it fits like a cork in the neck of the flask, so that it shuts the leaf into the vessel, leaving no communication with the outer air, and at the same time does not injure it in any way (~see~ fig. 17). Very soon, even after an hour or two, you will find a misty appearance inside the glass, and this will settle gradually in the form of drops of water which collect together and run down the sides of the flask. You do not see all this water coming off from the leaf under ordinary conditions because it goes into the air as invisible vapour, but when it is given off continually into a closed space the air soon gets saturated with all it can hold, and the rest must form liquid drops which we can see. If you keep a record of the time of your experiment, and also measure the amount of water collected in the flask, and then measure the size of the leaf, it only needs a little simple arithmetic to give you a rough idea of the quantities of water which must be given off every day by a single leaf. From that you can imagine the amount passing away from a whole plant or a great tree; and I think you will be surprised to find how much it is.

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.

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.

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.

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.

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).

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.