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.