These little mouths (known as stomata)[6] open in the light and close in the dark. During the daytime, therefore, the air (containing its small proportion of carbon dioxide) has free access to the interior of the leaf through the stomata, and, on the other hand, any water which the leaf does not require can escape in the form of vapour. A leaf requires water not only because all its mineral food (p. 29) is brought to it dissolved in water, but also because water as well as carbon dioxide is required for the manufacture of the starch and other plant-foods. How plant-food is distributed.—The water which comes up from the roots is distributed to the various parts of the leaf through the veins. These are therefore not only supports, which stretch out the soft leaf-stuff to the light and air, but they also form a very complete network of irrigating channels or water pipes. Further, the starch and other foods which a leaf makes are drained off into the stem through other minute channels which are bound up with the water pipes. The starch, for example, is changed into a kind of sugar which dissolves in water and drains away. From the stem, the food solutions are distributed to all the parts where growth is taking place. [Pg 54] EXERCISES ON CHAPTER III. 1. Make drawings of as many cases as possible of economy of leaf surface. 2. Grow various plants, e.g. Tropæolum, Geranium, Fuchsia, Mustard, etc., in the window, and notice the effect which the direction of the light has upon the positions of the leaves. 3. Smear with vaseline the lower surfaces of various growing leaves, and on the following day test the leaves for starch, comparing each with an unsmeared leaf from the same plant. 4. How is the transpiration of water from a green leaf effected and controlled? Discuss the uses of transpiration. (1897) 5. Put the same quantity of water into each of two similar test-tubes, and let the end of a leafy twig dip into one. Weigh the tubes, place them together in the sun for an hour, weigh again, and estimate roughly the weight of water lost by one square inch of leaf surface per hour. Compare various plants in this respect. Repeat the experiments, (a) in a moderate light, (b) in the dark. 6. Make a list of plants in which the leaves are so arranged as (a) to conduct rain-water towards the base of the main stem, (b) to cause rain-water to fall to the ground from the outside of the foliage. Try to discover whether the difference has any relation to the arrangement of the roots. 7. Under what conditions can plants use carbon dioxide as a source of food? Mention experimental and other proofs of the principal statements made. (1905) 8. What part of its food does a green plant obtain from the air? In what form and under what conditions is it taken in? (King’s Scholarship, 1905) [Pg 55] CHAPTER IV. BUDS. THE HISTORY OF A TWIG. 10. THE STRUCTURE AND USES OF BUDS.