3. Parts of a leaf which are not green do not form starch in sunlight.—Take a variegated leaf from a plant (e.g. the variegated geranium or maple) which has been in bright sunlight for some hours. Apply the usual test for starch. The parts which were originally green contain starch; the originally white parts remain bleached. 4. Leaves supplied with carbon dioxide, and exposed to sunlight, give off oxygen gas.—(a) Take a bunch of fresh watercress or any green water-weed and put it in a beaker or glass jar. Cover the plant with an inverted funnel which is shorter than the beaker. Now fill the beaker with ordinary tap water or river water (not distilled water), so that the end of the neck of the funnel is covered. Completely fill a narrow test tube with water, close it with the thumb, and invert it over the neck of the funnel. If this has been done carefully the test tube will still be full of water. Expose the arrangement (Fig. 29) to bright sunlight, and notice the bubbles of gas which are given off from [Pg 49] the plant and collect at the top of the tube. When a few inches of gas have collected, raise the test tube, close it with the thumb whilst still under water, and hold it mouth upwards. In the meantime, light a splinter of wood with the other hand. When it is well burning, blow out the light, remove the thumb from the test tube, and plunge the glowing splinter into the gas. It bursts into flame again, showing that the gas is oxygen. 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 [Pg 50] 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)?