1. How refreshing and pleasant a tree looks in a wealth of beautiful green foliage. But why does a tree need this covering of green leaves? 2. Plants and trees breathe by means of their leaves. The part of the air taken in by the leaves is a poisonous gas which men and animals breathe out; the other part, which is useful to animals, is breathed out by the leaves. Thus one part of the air helps to feed plants, and the other part to feed animals. 3. Or, put into proper words, this is what happens plants give out oxygen, and take in carbonic acid gas. All the green parts of a plant have the power, if it becomes necessary, to eat carbonic acid gas; but this work is done very much better by the leaves. Leaves are the eating organs of the plant; they are both mouth and stomach. They assist the flow of the sap, and convert carbonic acid and water into the starch which feeds and builds up the plant. 4. The nourishing life blood of an animal is forced through the body by the action of its heart; a plant has no such pumping organ to force along the flow of its sap. The sap is first absorbed into root-hairs and rootlets till their sides are stretched to the full; this water cannot flow back in them, but by their elastic walls it is pressed upwards. At the upper end of the tree - that is, where the leaves are the water in the cells is turned into vapor by the warmth of the sun; or, as we say, it is evaporated. An empty cell at the top then allows the water to flow into it from the next one below; and so the upward movement of the sap is continued and assisted. 5. It may be noticed that a lot of moisture will sometimes collect upon the inside of a glass shade such as is kept upon a window to cover growing plants. This moisture has been drawn out of the pores or tiny breathing holes of the leaves by the action of the sunlight. It is the sunlight, too, which manufactures the starch, and which makes the green coloring of the plant. Plants grown in the dark become white; celery is blanched, or made white, by earthing it up so as to hide its stalks from the sunlight. 6. In order to get as much sunlight as possible, leaves hang in a horizontal position; for the same reason, the lower branches grow out wider than Leaves are always shaped in such a fashion as to get as much sunlight upon them as possible. Plants kept in the dark soon die. upper ones. 1. Considerable variety may be found in the shapes of leaves. Some are lance shaped, like those of a wallflower; spoon shaped, as the English daisy; needle shaped, as the Scotch fir; heart shaped, as the lime, or linden; kidney shaped, as the ground ivy; and so on. 2. Very large trees often have very small leaves; or else they have compound leaves-that is, large leaves divided up into parts to allow them to catch as much of the sunlight as possible; for instance, the leaves of the ash are divided featherwise, and those of the horse-chestnut are LEAVES: 1. LEAF OF CLOVER. 2. LEAF OF ASH TREE. 3. LEAF OF ENG- 7. LEAF OF GROUND-IVY. 9. LEAF OF WALLFLOWER. 8. LEAF parted like the fingers on a hand. Clover is a small plant which has compound leaves; the lettuce is one which bears a simple leaf all in one piece. 3. The veins in a leaf. act as our veins. do-they form the small pipes along which may flow the nourishing life fluid. In some leaves the veins run up all parallel with the center one, as in the grasses; some leaves have branching veins, and in some the veins are like network. 4. On their surfaces leaves also differ very much. The elm leaf has a hairy surface; the foxglove leaf has a downy surface; the coltsfoot leaf a cottony surface; the oak leaf is uneven rough to the touch; and some leaves have a hairy upper surface with a downy under surface. or 5. All these are protections to the leaf a b SKELETON OF LEAVES ΤΟ SHOW (a) BRANCHING VEINS; (b) VEINS PARALLEL TO CENTER. against its enemies; insects feed on the fleshy parts of leaves, and the hairs are intended to hinder the |