. Un manuel de botanique pour les collèges et universités ... La botanique. ce qu'on appelle le sclerophylls.. Fig. 937. - Une sec- tion à travers une feuille de l'Osmanthus fragrans {olive parfumée), montrant un T- sclereid {en forme de s), qui à l'ensemble avec l'épaisse cuticule (c) tient compte de la rigidité de la feuille ; notez les trois couches palissadiques (p), a indiqué le xérophytisme relative- ment ; très agrandie. Tel que mentionné ailleurs, le cutinized épidermique externe est un moyen important de raidissement dans ces et d'autres feuilles, complétées dans certains cas par un épaississement des parois épidermiques latéraux (comme dans Ficus, fig.

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. A textbook of botany for colleges and universities ... Botany. the so-called sclerophylls.. Fig. 937. — A cross sec- tion through a leaf of the fragrant olive {Osmanthus fragrans), showing a T- shaped sclereid {s), which to- gether with the thick cuticle (c) accounts for the stiffness of the leaf; note the three palisade layers (p), indicat- ing relative xerophytism; highly magnified. As noted elsewhere, the cutinized outer epidermal wall is an important means of stiffening in these and other leaves, supplemented in some cases by a thickening of the lateral epidermal walls (as in Ficus, fig. 801) or even of the hypodermal walls (as in Pinus, fig. 1039). In the leaves of xerophytic grasses and sedges, patches of thick-walled bast fibers and other mechanical cells may occur just beneath the epidermis (fig. 835), and festoons of such cells often surround the conductive bundles, giving a great amount of strength to the leaves. Some- times (as in Osmanthus, fig. 937) evergreen leaves con- tain isolated mechanical cells (sdereids) extending from the lower to the upper epidermis, apparently acting as supports or braces; occasionally these stiff cells are Y- or T-shaped. The outer cortical layers of most petioles have the collenchymatic thickening charac- teristic of young stems (p. 697). The tearing of leaves at their margins is prevented largely by the presence there of an extra amount of cutinization, while similar protection is afforded in many leaves by marginal veins. Lobed leaves would seem especially subject to tearing at their sinuses; in some cases (as in Ribes, fig. 938) strong arcuate veins Fig. 938.—A portion of a leaf of the golden currant (Ribes aureum), showing ar- cuate veins (v) just below the leaf sinuses, where the danger of tearing otherwise would be considerable.. 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 per

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