. Practical botany. Botany. 66 PRACTICAL BOTANY 60. Self-pruning of leaves and twigs. Many trees and shrubs begin to shed some of their leaves even in the spring, very- soon after tlie leaves are well grown. Examples of this are the lilacs, the syringa (^PhiladelpJius), the Cottonwood, the horse-chestnut, the box elder, and some ILadens. Still more common is the loss of leaves during the summer, which may amount to 30 per cent of the total number of leaves. This leaf fall, coming long before the leaves are cast off in the autumn as a preparation for winter, affects mostly the leaves iiaside th
. Practical botany. Botany. 66 PRACTICAL BOTANY 60. Self-pruning of leaves and twigs. Many trees and shrubs begin to shed some of their leaves even in the spring, very- soon after tlie leaves are well grown. Examples of this are the lilacs, the syringa (^PhiladelpJius), the Cottonwood, the horse-chestnut, the box elder, and some ILadens. Still more common is the loss of leaves during the summer, which may amount to 30 per cent of the total number of leaves. This leaf fall, coming long before the leaves are cast off in the autumn as a preparation for winter, affects mostly the leaves iiaside the crown of the tree, which have such scanty light that they can- not accomplish much photosyn- thesis. Leaves, twigs, and even larger branches which are not getting an adequate sup- ply of light or of water are pruned away by the tree. Were it not fof this, the dense growth in the interior of the tree top and along the trunk would soon render further branchmg me- chanically impossible. What one sees on looking up along tlie trunk into the top of a large tree is mainly dead or dying branches, with few leaves. It is this self-prunmg and pruning by neighboring trees which makes the straight trunks, free from knots and most valuable for timber, in trees grown in woodlands, where they stand moderately close together. In some instances, as the so-called snap willows, the cotton- wood, and the large-toothed aspen, live twigs fall very freely from the tree during wmd or snowstorms, or when it is loaded. Fig. 54. A leaf of acaoia A, as seen by day -B, the same leaf at night. After Darwin. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Bergen, Joseph Y. (Joseph Young), 1851-1917; Caldwell, Otis William, 1869- joint author. Boston, New York [etc. ] Ginn and company
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