. Botany of the living plant. Botany; Plants. THE LEAF 71 as it curves out from the stem is directed upwards (ad-axial), and the phloem downwards (ab-axial), and these relative positions are regularly maintained throughout the leaf (Figs. 20, 47). The petiole appears to be structurally little more than a means of junction between the axis and the blade. Its presence brings two advantages : that the blade is carried some distance outwards from the stem, and thus the probability of one leaf overshadowing another is avoided; and secondly, the narrow petiole allows the blade to yield to the pressu
. Botany of the living plant. Botany; Plants. THE LEAF 71 as it curves out from the stem is directed upwards (ad-axial), and the phloem downwards (ab-axial), and these relative positions are regularly maintained throughout the leaf (Figs. 20, 47). The petiole appears to be structurally little more than a means of junction between the axis and the blade. Its presence brings two advantages : that the blade is carried some distance outwards from the stem, and thus the probability of one leaf overshadowing another is avoided; and secondly, the narrow petiole allows the blade to yield to the pressure of wind, instead of rigidly resisting Fig. 46. Skeleton of the lamina of Ivy. Natural size. It is the blade, or lamina, which is the distinctive feature of the foliage leaf. As shown in such common types as the Sunflower, Dahlia, Cabbage, or Sycamore, the blade consists of a skeleton or framework of thickened and mechanically firm ribs, which support an expanse of relatively thin and delicate tissues. A prominent mid-rib runs up continuously from the leaf-stalk to the tip of the blade, and branch-ribs of successively smaller size pass off from it towards the margin. On removing the softer tissues by reagents, the vascular system can be demonstrated as a leaf-skeleton (Fig. 46). It is then seen that vascular strands of the midrib and of the stronger lateral ribs, give off thinner lateral branches ; that smaller branch-veins pass off from these, and with further ramifica- tions and many fusions form a fine network traversing the thinner areas of the blade. Such a reticulate venation is characteristic of. 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 Bower, F. O. (Frederick Orpen), 1855-1948; Wardlaw, C. W. (Claude Wilson), 1901-. London, Macmillan and Co. , ltd.
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