Evaporation of Water.--During the day an enormous amount of water is taken up by the roots and passed out through the leaves. So great is this excess at times that a small grass plant on a summer's day evaporates more than its own weight in water. This would make nearly half a ton of water delivered to the air during twenty-four hours by a grass plot twenty-five by one hundred feet, the size of the average city lot. According to Ward, an oak tree may pass off two hundred and twenty-six times its own weight in water during the season from June to October. From which Surface of the Leaf is Water Lost?--In order to find out whether water is passed out from any particular part of the leaf, we may remove two leaves of the same size and weight from some large-leaved plant[14]--a mullein was used for the illustrations given below--and cover the upper surface of one leaf and the lower surface of the other with vaseline. The leaf stalks of each should be covered with wax or vaseline, and the two leaves exactly balanced on the pans of a balance which has previously been placed in a warm and sunny place. Within an hour the leaf which has the upper surface covered with vaseline will show a loss of weight. Examination of the surface of a mullein leaf shows us that the _lower surface of the leaf is provided with stomata_. It is through these organs, then, that water is passed out from the tissues of the leaf. Footnote 14: The "rubber plant" leaf is an easily obtainable and excellent demonstration. [Illustration: Experiment to show through which surface of a leaf water passes off.] Factors in Transpiration.--The amount of water lost from a plant varies greatly under different conditions. The humidity of the air, its temperature, and the temperature of the plant all affect the rate of transpiration. The stomata also tend to close under some conditions, thus helping to prevent evaporation. But there seems to be no certain regulation of this water loss. Consequently plants droop or wilt on hot dry days because they cannot obtain water rapidly enough from the soil to make up for the loss through the leaves. [Illustration: Diagrams of a stoma. _a_, surface view of a closed stoma; _b_, the same stoma opened. (After Hanson.) _c_, diagrams of a transverse section through a stoma, dotted lines indicate the closed position of the guard cells, the heavy lines the open condition. (After Schwendener.)] Green Plants Food Makers.--We have previously stated that green plants are the great food makers for themselves and for animals. We are now ready to attack the problem of how green plants _make_ food.