[Illustration: Fig. 17. Experiment to show that leaves give off water. Notice the drops collecting in the tube, which is closed with cotton-wool.] 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. [Illustration: 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.]