Fig. 29.—Experiment to prove that green leaves supplied with
carbon dioxide, and exposed to sunlight, give off oxygen gas.		 Fig. 30.—Experiment to prove that tap-water, or
river-water, contains dissolved carbon dioxide.

(b) Repeat the experiment, (i) placing the apparatus in the dark, (ii) without using any plant. No gas collects in the test tube.

(c) To show that the water used contains carbon dioxide in solution, completely fill a gallon can or a large flask with similar water, and attach a cork and a delivery tube which has also been filled with water—dipping the end of the tube into a little clear lime-water ([Fig. 30]). Put the same quantity of lime-water into another vessel for comparison, and then heat the can. Gas is given off, and as it bubbles through the lime-water the liquid is gradually turned milky.

5. Leaves wither in sunlight unless supplied with water.—(a) Cut off a leafy twig and leave it exposed to sunlight for an hour or two; notice the change in the appearance of the leaves.

(b) Put a similar twig in the dark for the same length of time; again notice the leaves. Is the difference due to a difference in light or to one of heat? (c) To test this, keep, if possible, a similar twig in the dark in a warm place. Do the leaves wither as much as in (a)?

(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?