the pistils, but it is often disguised in a very puzzling way, especially by the absence of one or more series of organs. See FLOWER.
Modifications of Leaves exist in other forms than the typical ones of foliage. Scales, such as those which envelop and protect buds in winter, and the seeds in cones, are leaves of simple structure which have no assim ilative powers or functions; they most fre quently originate from an enlarged leaf-base upon which a proper leaf never develops. Bracteal leaves, or bracts, are of similar char acter and grow beneath and about the flowers, of which, when they are colored, they often form the most conspicuous part; but fre quently they are green and are connected with true leaves by intermediate forms. Both scales and bracts have been forced, under experi mental conditions, to develop into true leaves.
The modified leaves which form the flowers of phanerogams are termed "floral and, as has been said, typically succeed one another in whorls from below upward, as sepals, petals, stamens and carpels. The sepals are usually green and much like foliage-leaves; the sepals often retain a likeness, but the interior whorls usually bear no resemblance to leaves, yet occa sionally, in ill-health, revert to a shape which betrays their origin and genetic history.
Periodicity of Leaf Leaves are a temporary part of the plant, arising and dis appearing at more or less regular intervals, usually once a year. This is especially notice able in the higher plants, some of which (an nuals) die in autumn completely, surviving as species only in their seeds; others die down to the roots in the fall and put forth entirely new stems as well as leaves the following spring; while others, as shrubs and tree, die only so far as their leaves are concerned, put ting forth new foliage after the stated period of rest. This period is due to the arrival of
annually recurring unfavorable conditions of temperature or moisture or both, when the activity of life in the plant is suspended and it ceases to feed or grow. In such a state leaves having no function are needless —in fact often harmful — and in many cases die and fall off in so sudden and conspicuous a manner that we say such trees are deciduous; while others, which we call evergreen, retain most of their leaves in a green condition until gradually replaced by new ones, so that the foliage seems to be perpetual. The brilliant colors of the dying leaves of many trees and herbs in autumn are due partly to chemical changes in the decaying chlorophyll and partly to the ex posure of pigment cells previously concealed by the abundance of chorophyll and other features of vital activity. The leaf drops because it no longer receives nourishment from the stem or twig. The cells at its base close up, transmit ting no more sap, and in so doing separate from those in the base of the leaf, which is thus cut off and thrown away.
The morphology, genesis and functions of leaves have been studied most deeply by German students, as Haeckel, Fritz Muller, Gael, Schwender, Marchlewski, Fisher and others. These and other authorities have been well summarized in the English translation by Porter of Strasburger's 'Textbook of Botany' (1903). For the forms and nomen clature of leaves, consult the botanical manuals and textbooks of Gray, Wood and other Amer ican authors. Consult also Atkinson, 'Ele mentary Botany> (New York 1898) ; Coulter, 'Plant Structures> (ib. 1900) ; id. 'Elementary Studies in Botany' (ib. 1913) ; Ganong, 'The Living Plant' (ib. 1913).