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, 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.
Oxygen is liberated when leaves form starch.—Carbon dioxide gas, which has been seen to be indispensable for the manufacture of starch in leaves, consists of carbon, or charcoal, chemically united with the gas oxygen. The green-stuff of the interior of the leaf makes the starch by causing this carbon to combine with water which has come up from the roots, but it returns to the air the unnecessary oxygen. Water plants, the leaves of which are not directly exposed to the air, use carbon dioxide which the water has dissolved from the air. They also give off the surplus oxygen, after fixing the carbon. This is the explanation of the bubbles of gas which, in sunlight, are often seen rising from the plants in an aquarium. By such an arrangement as is described in Expt. 9, 4, this evolved gas can be collected, and proved to be oxygen.