Another very important use to man is seen in the fact that leaves, falling to the ground, help to form a rich covering of humus, which acts as a coat to hold in moisture. The forests are our greatest source of water supply. The cutting away of the forest always means a depletion of the reserve water stored in soil, with consequent floods and droughts in alternation. Leaves are used directly by man for food. Examples are cabbage, lettuce, kale, broccoli, and some others. These foods, properly admixed with certain fleshy foods, are of great importance in giving a balance to diet. In a wider sense, all animals depend upon leaves for their food supply either directly, for herbivorous animals feed upon the leaves of plants -or indirectly in foods obtained from roots, stems, seeds, and fruits. For in every case the stored food has been manufactured in the leafy part of the plant and transported within the plant to its place of storage. Even meateating animals are in the long run dependent upon plants, for they feed upon plant eaters. Modified Leaves. In many plants the leaves are reduced to spines or have part of the leaf modified so as to form spines. In some leaves this appears to be for protection against animals, but in some cases, as the cactus, it is a means of protecting the plant against loss of water through evaporation. A cactus, showing the leaves modified into spines. If a cactus is cut open, it will be found to contain a very considerable amount of water. The Indians of the New Mexican desert region, when far from a source of water, sometimes cut off the top of a large cactus, mash up the soft interior of the thickened stem, squeeze out the pulp, and thus obtain several quarts of drinkable water. leaf is covered with a coating of finely branched hairs. Might such a covering be of use to the leaf? In what ways? Leaves may be modified for Storage of Food and Water in Leaves. the storage of food and water. Test an onion, which is a collection of thickened leaves closely wrapped to form what is called a bulb, for starch, Bladderwort, showing finely dissected submerged leaves bearing blades which capture little animals. sugar, and proteid. Squeeze any fleshy leaves and notice the water contained in them. The agave is a desert plant in which the leaves have become greatly thickened as a water and food storage. - Most curious of all are the modifications of the leaf into insect traps. It frequently happens that the habitat of a plant will not furnish the raw food materials necessary to form proteid food and to build protoplasm. Nitrogen is the lacking element. The plant has become adapted to these conditions and obtains nitrogenous food from the bodies of insects which it catches. Examples of insect traps are the common bladderwort (utricularia), the Venus's flytrap (Dionaea muscipula), the sundew (Drosera rotundifolia), and certain of the pitcher plants. Leaf of sundew closing over captured insect. Bladderwort. The simplest contrivance for the taking of animal food by the leaf is seen in the bladderwort. Here certain of the leaves are modified into little bladders provided with trapdoors which open inwards. Small water-swimming crustaceans (as water fleas, etc.) push their way into the trap and there die, perhaps of starvation. Bacteria, causing decay, soon break down their bodies into soluble substances, the nitrogenous portion of which is absorbed by the inner surface of the bladders and used by the plant as food. Venus's Flytrap. In the Venus's flytrap, a curious plant found in our Southern states, the apex of the leaf is peculiarly modified to form an insect trap. Each margin of The the leaf is provided with a Sundew. In the sundew the leaves are covered with long glandular hairs, each of which is extremely sensitive to the stimulus of any nitrogenous substance. These hairs exude a clear, sticky fluid which first renders more difficult the escape of the insect caught in the hairs, and then digests the nitrogenous parts of the insect thus caught. Pitcher plant: a, leaf; b, cross section; c, longitudinal section. Note the insects at the bottom, and the inward-pointing hairs at the top. Pitcher Plants. The common pitcher plant has an urn-shaped leaf which is modified to hold water. Many small flies and other insects find their way into the pitcher and are eventually drowned in the cup. Whether the plant actually makes use of the food thus obtained is a matter unsettled, but some tropical forms undoubtedly do use the caught insects as food. REFERENCE BOOKS ELEMENTARY Sharpe, A Laboratory Manual for the Solution of Problems in Biology, American Andrews, Botany all the Year Round, pages 46-62. American Book Company. ADVANCED Clement, Plant Physiology and Ecology. Henry Holt and Company. Coulter, Barnes, and Cowles, A Textbook of Botany, Part II, and Vol. II. American Book Company. Darwin, Insectivorous Plants. D. Appleton and Company. Goodale, Physiological Botany, pages 337-353 and 409-424. American Book Company. Green, Vegetable Physiology. J. and A. Churchill. Lubbock, Flowers, Fruits, and Leaves, last part. The Macmillan Company. pany. Longmans, Green, and Com Report of the Division of Forestry, U.S. Department of Agriculture, 1899. Strasburger, Noll, Schenck, and Schimper. A Textbook of Botany. The Mac millan Company. Ward, The Oak. D. Appleton and Company. X. OUR FORESTS; THEIR USES AND THE NECESSITY FOR THEIR PROTECTION Problem XIX. Some uses of stems (optional). (Laboratory Manual, Prob. XIX.) (a) Special product from stems. (b) Some woods and their value. (c) Field work in forestry. The Economic Value of Trees. Protection and Regulation of Water Supply. - Trees form a protective covering for the earth's surface. They prevent soil from being washed away, and they hold moisture in the ground. Without trees many of our rivers might go dry in summer, while in the rainy season sudden floods would result. The devastation of immense areas in China and considerable damage by floods in parts of Switzerland, France, and in Pennsylvania has resulted where the forest covering has been removed. No one who has tramped through our Adirondack forest can escape noticing the differences in the condition of streams which flow through areas covered with forest and those from around which trees have been cut. The latter streams often dry up entirely in hot weather, while the forest-shaded stream has a never failing supply of crystal water. Working to prevent erosion after the removal of the forest in the French alps. The city of New York owes much of its importance to its position at the mouth of a great river with a harbor large enough to float the navies of the world. This river is supplied with water |