. Class-book of botany : being outlines of the structure, physiology, and classification of plants ; with a flora of the United States and Canada . Botany; Botany; Botany. STRUCTURE OF LEAVES. 145 advancing point. This accounts for the wonderful facilUy witli which it penetrates the soil and finds its way uninjured into the hardest earth. 121. DiOTYOQENS. In those few Monocotyledons which bear reticulated leaves (Smilax, Diosoorea), the Dictyogens of Dr. Jjindley, the roots exhibit a structure re- sembling that of exogenous stems. STKUCTUKE OF V28. Natuee of the leaf. The leaf may be
. Class-book of botany : being outlines of the structure, physiology, and classification of plants ; with a flora of the United States and Canada . Botany; Botany; Botany. STRUCTURE OF LEAVES. 145 advancing point. This accounts for the wonderful facilUy witli which it penetrates the soil and finds its way uninjured into the hardest earth. 121. DiOTYOQENS. In those few Monocotyledons which bear reticulated leaves (Smilax, Diosoorea), the Dictyogens of Dr. Jjindley, the roots exhibit a structure re- sembling that of exogenous stems. STKUCTUKE OF V28. Natuee 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 proceed- ing 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 down- ward, 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 ihaded 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 chlorop
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