The terms used to designate these different kinds of leaf margins or forms of blade are precise, nearly universally used, but need to be studied only by those who, because of special fondness for the subject, are likely to need them in using books which are beyond the scope of the present one. If, for instance, the reader is interested in finding out what his native roadside plants are, he would need a book describing them, and there are many for different parts of the country. In such books he would find these terms, which say so much in a single word (there are other sets of terms for flowers, fruits, and seeds) totally unfamiliar and quite likely to disgust him at the start. A little study may open up to him that most interesting and easily accessible of recreations, a first-hand familiarity with the wild flowers of one’s own neighborhood.
All leaves are not as simple as the figures show them to be. In many the midrib or principal vein is much elongated and there are small leaflets, sometimes even scores of them, all fastened to a common stalk. Such are called compound leaves ([Figures 36-37]), which may be found in ash, hickory, rosebushes, blackberries, peas, beans, and thousands of other plants.
Fig. 36. Palmately compound leaf, the five leaflets all arising from the tip of the common leafstalk. Fig. 37. Pinnately compound leaf, the leaflets arising from the sides of the common leafstalk. Fig 38. A parallel-veined leaf. All the other leaves figured are netted-veined.
While leaves are literally factories in which one of the most wonderful things in the world is produced, it is so much a part of what plants do or their behavior that the story of it will be given in the chapter on Plant Behavior. Sunlight is absolutely necessary for the process, and to reach this sunlight leaves are attached to their stems in a variety of ways. Some are always opposite each other, as in the common privet, lilac, or honeysuckle; others always alternate, as in the mustard or the rose. There are many variations of these simple arrangements, but in every case the process results in giving each leaf the utmost exposure to the light without which the plant must wither and die. So vital is this exposure to light that in some plants parts of the leaves produce tendrils, as in the case of peas, in order that some near-by support may be used. In one African relative of our lily, this change of leaf form has been so great that its long slender leaf tip is wonderfully adapted to reaching up and catching by its curved tip some support to lift it from the gloom of the tropical forest floor.
Looking down from above on any small plant or bush, or from the sky on a forest, about all that can be seen are the thousands of leaves, all so arranged that it is as though some celestial photographer asked every one of them to so place themselves that they would all be “in the picture.” The competition between leaves on the same plant and between leaves on rival plants is infinitely keener than the friendly pushing of a crowd to get in a picture, and it lasts forever. Furthermore, failure to get in means certain death. So intricate is the method of leaf arrangement, so marvelous the adjustments that all plants must make to insure ample light, that it is not inaptly called leaf mosaic. As we shall see in the chapter on Plant Distribution, particularly in forests, certain variations or partial failures of the process have far-reaching results.