(d) Smear with vaseline the under surface of some of the leaves of such a twig and again expose to sunlight. Do the smeared leaves remain fresh longer than the others? (e) Cut off the end of a twig with a sharp knife whilst it is under water, and leave it exposed to sunlight, dipping in water. The leaves remain fresh. How do you explain these differences? 6. In sunlight, leaves give off water.—Take a piece of cardboard about 4 in. square and make a small hole in the middle. Pass the end of a leafy twig through the hole and make up with wax any chinks between the twig and the card. Put the card on a tumbler containing water, so that the end of the twig dips under water; and invert on the card—covering the leafy end of the twig—a second tumbler which is clean and dry. Put the apparatus in the sunshine and notice the mistiness (or even visible drops of water) forming on the inside of the upper tumbler. Where does this moisture come from? 7. The skin of a leaf is perforated by little pores.—Dip a fresh laurel leaf into boiling water in a beaker or tumbler. Can you see bubbles of air escaping from the leaf? Are they to be seen on both surfaces of the leaf, or only on one? Which? Examine both surfaces of box leaves with a strong lens, and try to see the little dots (pores) on the lower surface. The student who has performed the experiments described in this section, and who has thought about the results obtained, cannot but have gained some insight into the main duties of leaves. The meaning of these results must now be discussed. The formation of starch in leaves.—When the green leaves of a plant are exposed to sunlight in ordinary air—that is in air containing a certain proportion of carbon dioxide—the leaf forms starch in its interior, and the starch can be detected by applying the iodine test (p. 34). When part of a leaf is protected from the light, [Pg 51] as by pinning the halves of a split cork on opposite sides of it (Fig. 31), no starch is formed in the shaded parts, but only in the regions which are exposed to the light. Further, if a variegated leaf is treated in the same way, starch can be detected only in those parts of the leaf which were originally green; the parts which were white are free from starch. It is plain that it is the green colouring matter which puts the energy of the sunlight at the disposal of the leaf and enables it to manufacture starch. A B Fig. 31.—A, Tropæolum leaf, on which have been pinned the halves of a split cork (C). (× ½.) B, the same leaf tested for starch with iodine solution, after exposure to sunlight for an hour. The part shielded from the light remains bleached; the rest of the leaf has turned blue. At least three conditions are therefore necessary for the formation of starch in leaves: (1) the green colouring matter; (2) sunlight; (3) carbon dioxide.