The natural history of plants, their forms, growth, reproduction, and distribution; . etween the veins, form projecting folds on the concave surface, on the surfacewhich is turned from the sun. They are thus hidden behind the close-pressed layerof ribs as if by a roof, and are consequently protected as efficiently as possible from OLD AND YOUNG LEAVES. 353 the sun and wind. The ribs themselves are composed of celhilar structures whichare not open to the danger of over-transpiration, and the epidermis which coversthem is entirely devoid of stomata. When the leaves at the ends of the youngt
The natural history of plants, their forms, growth, reproduction, and distribution; . etween the veins, form projecting folds on the concave surface, on the surfacewhich is turned from the sun. They are thus hidden behind the close-pressed layerof ribs as if by a roof, and are consequently protected as efficiently as possible from OLD AND YOUNG LEAVES. 353 the sun and wind. The ribs themselves are composed of celhilar structures whichare not open to the danger of over-transpiration, and the epidermis which coversthem is entirely devoid of stomata. When the leaves at the ends of the youngtwigs are opposite, erect, and concave, and their margins are in contact, they forman actual capsule round the apex of the shoot. This occurs in the Wayfaring Tree(Viburnum Lantana), illustrated in fig. 90^. The small folds of green tissueproject into the interior of the capsule, and the still closely-pressed lateral veinsform the outer wall, and at the same time furnish a protective covering for theenlarging green portions of the leaf. As soon as these are fully developed, and the. Fig. 92.—Unfolding of Beech-leaves. The brown bud-scales have been loosened, and the membraneous stipules surrounding the foliage-leaves are visible Further stage of development, the folded foliage-leaves being visible between the stipules. 8 The same twig furtherdeveloped. < Lower surface of a young folded leaf. Portion of the same leaf; the depressions caused by the foldingare bridged over by silky hairs. « Surface view of an unfolded leaf; the stipules are withered and about to fall. Vertical section of a leaf at right angles to the midrib. Vertical section parallel with the midrib. epidermal cells are correspondingly thickened, the projecting folds become smooth,the veins separate from one another, and the whole leaf becomes flat, assumes ahorizontal instead of a vertical position, and turns the upper instead of the lowersurface to the incident light (see fig. 90^). It has already b
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectbotany, bookyear1902
