. Botany of the living plant. Botany. THE LEAF 69 organisms. Prior to the fall of the leaf its tissues are depleted of all useful materials, whieh are transferred down the petiole to the axis. This is accompanied by changes in the cell-contents which give the varied autumnal tints. What falls away is then little more than an empty skeleton. Its removal leads to a great reduction of the exposed surface of the plant, with the result that there is less loss from evapora- tion; and less resistance to the winter winds. As a whole the plant enters a dormant condition in autumn, partly determined by
. Botany of the living plant. Botany. THE LEAF 69 organisms. Prior to the fall of the leaf its tissues are depleted of all useful materials, whieh are transferred down the petiole to the axis. This is accompanied by changes in the cell-contents which give the varied autumnal tints. What falls away is then little more than an empty skeleton. Its removal leads to a great reduction of the exposed surface of the plant, with the result that there is less loss from evapora- tion; and less resistance to the winter winds. As a whole the plant enters a dormant condition in autumn, partly determined by the climatic conditions, partly by the absence of those organs which play so active a part in its vegetation. But the fall of the leaf is not an inherent feature in any group of plants, nor does the abciss layer form any constant limit between leaf and stem. For example, the British Oak (Queixus robur) is deciduous, that is, it drops its leaves in autumn; but the Holm Oak (Quercus Ilex), which is a native of the Mediter- ranean region, remains evergreen. The common Cherry (Prunus cerasus) drops its leaves in autumn ; but the Cherry Laurel [Prunus lauro-cerasus) is ever- green. Thus though the leaf-fall is a very striking feature of many trees and shrubs in temperate climates, it is really nothing more than a seasonal, and often a specific, adaptation. In many woody plants it does not occur at any regular intervals, while in most herbaceous plants, and especially in annuals such as the Sunflower or Bean, the whole shoot simul- taneously ceases its vegetative activity, leaf and axis remaining connected till they rot. From the description which has been given of the structure of the leaf it will be seen how well that organ is fitted for carrying out the duty of nutrition, while exposed to the ordinary climatic conditions. In the first place a broad expanse of green tissue is required, so as to intercept the light. The larger its area the better. This is a sufficient reason for the
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1919
