REFERENCE BOOKS ELEMENTARY Sharpe, A Laboratory Manual for the Solution of Problems in Biology. American Book Company. Andrews, Botany All the Year Round, Chaps. VI, VII. American Book Company. Atkinson, First Studies of Plant Life, Chaps. IV, V, VI, VIII, XXI. Ginn and Company. American Book Company. American Book Company. Dana, Plants and their Children, pages 99-129. Henry Holt and Company. The Macmillan Company. ADVANCED Apgar, Trees of the United States, Chaps. II, V, VI. American Book Company. Coulter, Barnes, and Cowles, A Textbook of Botany, Vol. I. American Book Com pany. Ganong, The Teaching Botanist. The Macmillan Company. Gray, Structural Botany, Chap. V. American Book Company. Kerner-Oliver, Natural History of Plants. Henry Holt and Company. Lubbock, Buds and Stipules. D. Appleton and Company. Strasburger, Noll, Schenck, and Schimper, A Textbook of Botany. The Macmillan Company. Ward, The Oak. D. Appleton and Company. Yearbook, U.S. Department of Agriculture, 1894, 1895, 1898-1910. IX. LEAVES AND THEIR WORK Problem XVIII. A study of leaves in relation to their environment. (Laboratory Manual, Prob. XVIII.) (a) Reactions of stems and leaves to light. (b) Structure. (c) Important functions. (1) Absorption and respiration. (2) Food-making and its by-product. (d) Means of protection (optional). Differences between Roots and Stems. A comparison of the young root and developing stem of a bean seedling show that several marked differences exist: (1) the color of the stem is greenish, while the roots are gray or whitish; (2) the stem has leaves and branches leaving it in a more or less regular manner, while the smaller roots are extremely irregular in their method of growth; (3) the stem grows upward, while the general direction taken by the roots is downward. Effect of Light on Plants. -In young plants which have been grown in total darkness, no green color is found in either stems or leaves, the latter often being reduced to mere scales. The stems are long and more or less reclining. We can explain A pocket garden which has been kept in complete darkness for several weeks. Notice the bleached condition of stems and leaves. the changed condition of the seedling grown in the dark only by assuming that light has some effect on the protoplasm of the seedling and induces the growth of the green part of the plant. Numerous instances could be given in which plants grown in may be killed by its influence. Such are molds, mildews, and some bacteria. Such plants, however, are not green. As a Two stages in an experiment to show that green plants grow toward the light. matter of fact, the stem of a green plant which has but little chlorophyll develops somewhat more rapidly under conditions where it receives no light. Heliotropism. We saw that the stems of the plants kept in the darkness did not always lift themselves erect, as in the case of stems in the light. If seedlings have been growing on a window sill, or where the light comes in from one side, you have doubtless noticed that the stem and leaves of the seedlings incline in the direction from which the light comes. The tendency of young stems and leaves to grow toward sunlight is called positive heliotropism. The experiment pictured on the preceding page shows this effect of light very plainly. A hole was cut in one end of a cigar box and barriers were erected in the interior of the box so that the seeds planted in the sawdust received their light by an indirect course. The young seedling in this case responded to the influence of the stimulus of light so as to grow out finally through the hole in the box into the open air. This growth of the stem to the light is of very great importance to a growing plant, because, as we shall see later, food-making depends largely on the amount of sunlight the leaves receive. Effect of Light. We have already found that seedlings grown in total darkness are almost yellow-white in color, that the leaves are but slightly developed, and that the stem has developed far more than the leaves. We have also seen that a green plant will grow toward the source of light, even against great odds. It is a matter of common knowledge that green leaves turn toward the light. Place growing pea seedlings, oxalis, or any other plants of rapid growth near a window which receives Tall straight stems of the hemlock; the trees reach up toward the source of light. full sunlight. Within a short time the leaves are found to be in positions to receive the most sunlight possible. Arrangement of Leaves. A study of trees in any park, or in the woods, shows that the stems of trees in thick forests are usually tall and straight and that the leaves come out in clusters near the top of the tree. The leaves lower down are often smaller and less numerous than those near the top of the tree. Careful observation of any plant growing outdoors shows us that in almost every case the leaves are so disposed as to get much sunlight. The ivy Good climbing up the wall, the morning-glory, the dandelion, and the burdock all show different arrangements of leaves, each presenting a large surface to the light. Leaves are usually definitely arranged, fitting in between one another so as to present their upper surface to the sun. Such an arrangement is known as a leaf mosaic. examples of such mosaics, or leaf patterns, are seen in trees having leaves which come up alternately, first on one side of a branch, then on the other. Here the leaves turn, by the twisting of their stalks, so that all the leaves present their upper surface to the sun. In the case of the dandelion, a rosette or whorled cluster of leaves is found. In the horse-chestnut, where the leaves come out opposite each other, the older leaves have longer petioles than the |