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IX. LEAVES AND THEIR FUNCTIONS
IN the horse-chestnut bud previously studied the brown scales which cover the green scales of the bud can be shown to be like in structure to a leaf (homologous to a leaf). This is seen better in a near relative of the horse-chestnut, the buckeye, in which a series of changes in form from brown scales to green leaves may be followed.
Stipules. The bud of the tulip tree (Liriodendron tulipifera) is an admirable source of information as to the structure of a leaf.1 In the buds of a tulip tree, however, the scales seem to act as wrappers around the little leaves and not to correspond to the blade of the leaf, but to an outgrowth of the leafstalk or petiole below it. The outgrowths at the base of a leaf are known as stipules. The scales in this case are stipules which have come to protect the bud at a time when the delicate parts need protection most. These stipules are present as scales in many other buds. Frequently, when the leaf becomes able to care for itself, the stipules fall off and disappear. Such a bud is the elm, as can be seen by a careful dissection. Stipules usually are paired. Notice the notched appearance of the scales as you go inward. Most leaves do not show the stipules well, although a complete leaf is supposed to be supplied with them. They are well developed in the rose leaf.
Palmately-veined leaf of the maple.
Leaf of a Dicotyledon.
If we now take up the study of a simple leaf such as the elm, we find that the parts seen in buds of the tulip tree are not always readily found. The petiole or leafstalk
runs into the blade of the leaf as the midrib. Here it divides to form the network of veins found in the leaves of the dicotyledonous plants.1
Leaf of a Monocotyledon. In the monocotyledons, as in Tradescantia, or in the leaves of a grass, there is no midrib, the leaf being traversed by a number of veins which run lengthwise of the blade and give it the name of parallel-veined leaf.2
Parallel-veined leaf of false
Modification of Blade. The blade of a leaf may undergo many modifications in different species of plants. It may have almost any shape, from a thin line to an almost circular outline. It may have an almost smooth margin, as in the leaf of the rubber plant (Ficus elastica), or the margin may be more or less deeply indented, as in the tulip, sassafras, and the oak.
Drawings should be made to show the parts of a typical simple netted-veined leaf and that of a leaf parallel veined. Both drawings should be natural size and should have all parts labeled.
Compound Leaf. If the margin is so deeply indented that the blade is cut down to the midrib, then each part of the blade is said to be a leaflet, and the whole leaf is compound. In most cases it is easy to distinguish a compound leaf: (I) by
1 See Hunter and Valentine, Manual, pages 39-42.
2 Ibid., page 45.
the bud in the axil of the leaf, that is, between the leafstalk and the branch; (2) by the presence of stipules (this is well seen in the rose leaf); (3) by the fact that the arrangement of veins in the leaflet frequently does not follow the same system of branching as would be found in a simple leaf of the same form. -Arrangement of Veins. The shape of the blade of leaves of dicotyledons depends somewhat upon the arrangement of the veins. If the blade is long and thin, the veins will be found to go out from the midrib somewhat like the side parts of a feather; hence the leaf is said to be feather veined (or pinnate). If the blade is nearly circular in outline, the veins will leave the petiole to radiate somewhat like fingers from the palm of the hand. Such venation is said to be pal
The same general arrangement of veins holds true for compound leaves. The pea and locust are examples of pinnately compound leaves, the horse-chestnut of a palmately compound leaf.1
Leaves turn toward the Light. 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 full sunlight. Within a short time the leaves are found to be in positions to receive the most sunlight possible.
Home Experiment. Turn such plants after two or three days so that the leaves are away from the sun. Make observations every hour during some Saturday morning and try to find out just what part of the leaf turns to the light.
Effect of Light. We have already found that seedlings grown in total darkness are almost yellow-white in color, that the leaves
1 For laboratory work on leaves, see Hunter and Valentine, Manual, pages 39-47.
are but slightly developed, and that the stem has developed far more than the leaves. We have also seen (page 99) that a green plant will grow toward the source of light, even against great odds. This effect of light is seen as well in older plants. Any field trip will reveal the fact that in low-lying plants the leaves which are shaded are often yellow and dwarfed. Plants kept in poorly lighted rooms at home show this.
Arrangement of Leaves. - A careful 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 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 each other so as to present their upper surface to the sun. Such an arrangement is known as a leaf mosaic. Good examples of such mosaics, or leaf patterns, are seen in the alternate-leaved trees. Here the leaves turn, by the twisting of the petioles, so that all the leaves present their upper surface to the
A lily, showing long, narrow leaves.
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 young ones. In the mullein the entire plant forms a cone. The old leaves near the bottom have long petioles, and the little ones near the apex come out close to the main stalk. In every case each leaf receives a large amount of light. Other modifica
tions of these forms may easily be found on any field trip. See how many different examples of leaf mosaics you can bring in to record in your notebook.
Effect of Absence of Light upon the Leaves of the Oxalis.1 Place a potted Oxalis in a dark closet or box for two or three hours, having previously exposed it to direct sunlight for a short time. What is the general position of the leaves after removal from the darkness? Compare with the position of the same leaves in the light.
Sleep Movements. The examination of a number of young seedlings of buckwheat or sunflower shows that after a few hours. in darkness the leaves droop. The leaves of the bean and oxalis 1 See Hunter and Valentine, Manual, page 246.