3. What part of its food does a green plant obtain from the air? In what form, and under what conditions, is it taken in? (1889) 4. Describe the method of water cultures, and give the general results of a set of experiments. (1898) 5. Give experimental proof that green plants require to be fed with combined nitrogen. (1897) 6. What are the necessary conditions for the formation of starch in a plant? Mention experiments which support your statements. (1896) 7. Explain the influence of light on a growing plant. Illustrate your answer by reference to the changes in a ripening and germinating bean. (King’s Schol. 1903) 8. How can it be proved experimentally that a green plant draws some, but not the whole, of its nourishment from the air? (1904) Fig. 22.—Two mustard seedlings of equal age. E, grown in the dark; N, grown in ordinary daylight. [Pg 37] CHAPTER III. THE FORMS AND DUTIES OF LEAVES. 7. THE FORMS OF LEAVES. 1. The shapes.—Make a collection of the leaves of a large number of different plants, for example, elm, beech, lime, oak, birch, ash, blackberry, pine, yew, horse chestnut, rose, holly, woodsorrel, grass. Lay each in turn flat in your notebook and trace the shape of the leaf blade by passing the point of your pencil round the edge. Measure the length and greatest width; write down these dimensions. Is the greatest width at, above, or below, the middle of the leaf blade? Most of the leaves are flattened green plates. Those of the pine and yew are long and needle-shaped. Do you know any other leaves like these? 2. The veins.—What enables the leaf to keep stretched out? Turn it over and notice the “veins” on the lower side. Do they act like the ribs of an umbrella? Fill in the positions of the main veins in your drawings. Are the veins parallel to each other in any of your leaves? Write a list of as many leaves as you can find which have parallel veins. 3. Skeleton leaves.—Put some leaves in a saucer with a little soft water, and allow them to rot. Clean away the soft stuff from time to time by gently brushing the leaves with an old tooth-brush. Notice the “skeleton” which remains. Skeleton leaves may be made much more quickly by soaking the leaves for some time in a weak solution of bleaching powder. Wash them well before drying. [Pg 38] 4. The colour.—Is the green deeper on the upper or the lower surface of a leaf? Which surface usually receives more light? 5. The apex.—In how many of your leaves is the apex (a) pointed, (b) blunt, (c) rounded? 6. The margin.—Examine the edge or margin of each leaf. In how many is it (a) quite plain, (b) hairy, (c) wavy, (d) saw-edged, (e) doubly saw-edged, (f) spiny? Do you find spines on holly leaves which are so high on the tree as to be out of the reach of cattle? What is the use of the spines?