Water given off by Evaporation from Leaves.--Take some well-watered potted green plant, as a geranium or hydrangea, cover the pot with sheet rubber, fastening the rubber close to the stem of the plant. Next weigh the plant with the pot. Then cover it with a tall bell jar and place the apparatus in the sun. In a few minutes drops of moisture are seen to gather on the _inside_ of the jar. If we now weigh the potted plant, we find it weighs less than before. Obviously the loss comes from the water lost, and evidently this water escapes as vapor from either the stem or leaves. [Illustration: The skeleton of a leaf. _M.R._, the midrib; _P._, the leafstalk; _V._, the veins.] The Structure of a Leaf.--In the experiment with the red ink mentioned above we will find that the fluid has gone out into the skeleton or framework of the leaf. Let us now examine a leaf more carefully. It shows usually (1) a flat, broad _blade_, which may take almost any conceivable shape; (2) a _stem_ which spreads out in the blade (3) in a number of _veins_. [Illustration: Section through the blade of a leaf as seen under the compound microscope. _S_, air spaces, which communicate with the outside air; _V_, vein in cross section; _S.T._, breathing hole (stoma); _E_, outer layer of cells; _P_, green cells.] The Cell Structure of a Leaf.--The under surface of a leaf seen under the microscope usually shows numbers of tiny oval openings. These are called _stomata_ (singular _stoma_). Two cells, usually kidney-shaped, are found, one on each side of the opening. These are the _guard cells_. By change in shape of these cells the opening of the stoma is made larger or smaller. Larger irregular cells form the _epidermis_, or outer covering of the leaf. Study of the leaf in cross section shows that these stomata open directly into air chambers which penetrate between and around the loosely arranged cells composing the underpart of the leaf. The upper surface of leaves sometimes contains stomata, but more often they are lacking. The under surface of an oak leaf of ordinary size contains about 2,000,000 stomata. Under the upper epidermis is a layer of green cells closely packed together (called collectively the _palisade layer_). These cells are more or less columnar in shape. Under these are several rows of rather loosely placed cells just mentioned. These are called collectively the _spongy tissue_. If we happen to have a section cut through a vein, we find this composed of a number of tubes made up of, and strengthened by, thick-walled cells. The veins are evidently a continuation of the tubes of the stem out into the blade of the leaf.