728. Nature of the leaf. The leaf may be regarded as an expansion of the two outer integuments of the bark, or of the green bark and the epidermis, expanded into a broad, thin surface by a woody framework proceeding from the medullary sheath and the liber.
729. The framework of veins is therefore fibro-vascular, abounding in spiral vessels, and strengthened with liber.
730. The parenchyma exists in two strata more or less distinct. In all those leaves which are ordinarily horizontal in position, one surface being upward and the other downward, these two layers are dissimilar; but in leaves with a vertical lamina (iris), and in phyllodia (§ 307) the two layers are similar.
731. The layers described. The superficial layer of empty tabular cells, belongs to the epidermis. Next beneath this, in the surface on which the sun shines, are one or two layers of oblong cells placed perpendicularly to that surface, and more compact than the cells beneath them, which are pervaded by intercellular passages and by the veins.
732. Place of the stomata. The stomata as a rule belong to the shaded side of the leaf, avoiding the sun's direct rays. On the sunny side there are few comparatively or none. In the submerged leaves of water-plants the epidermal layer is hardly distinguishable, and is wholly destitute of stomata. In such leaves as float upon water (water lilies) stomata are found in the upper surface alone.
733. The chlorophylle. Within all the vesicles of the parenchyma are seen adhering to the walls the green globules of chlorophylle, which give color to the leaf - dark green above, where it is more compact, paler beneath, where the cells are more loose and separate.
734. Vessels of cienchyma pervade the under-layer of parenchyma, returning the elaborated juices through the petiole into the cambium layer.
603, Section of a stem at the origin of a leaf; p, cellular, or pith; a, vascular, the medullary sheath sending off a bundle into the leaf-stalk; d, the swelling (pul-vinus) just below the articulation of the leaf-stalk (J); b, the axillary bud.
604, Minute portion of a leaf of Viola tricolor, viewed in perspective, showing, a, cells of epidermis above; b, compact parenchyma of the upper portion of the leaf; c, loose parenchyma; d, epidermal-cells of the lower surface with stomata, one cut and opening into the intercellular passages. (Magnified 100 diameters.)
735. The structure of bracts sepals, petals, and other organs, which are but modifications of the leaf, hardly requires a separate notice. The same kinds of vessels pervade their parenchyma, but the spiral exist in a larger proportion. In the pistil, the fibro-vascular bundles may be traced to the placenta, and thence into the funiculus and raphe of the ovule. In the more delicate organs chlorophylle is wanting, and the peculiar coloring, matter of whatever other tint, is uniformly diffused through the fluid contents of the cells of parenchyma. The depth of the tint depends on the number of cells thus colored.