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The Role of the Epidermis in Foliar Organization and its Relations to the Minor Venation

Robert B. Wylie
American Journal of Botany
Vol. 30, No. 4 (Apr., 1943), pp. 273-280
Stable URL: http://www.jstor.org/stable/2437454
Page Count: 8
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The Role of the Epidermis in Foliar Organization and its Relations to the Minor Venation
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Abstract

For forty-six species of mesomorphic leaves the epidermal layers constituted 20 per cent of the volume of the blade between veins and, because of the spaces in the mesophyll, made up a greater proportion of its tissue. While the epidermis, protected by the cuticle, is in some degree a barrier over interior tissues, it carries on other functions and seems to be important in these leaves both for support and water conduction in the areas between veins. The epidermal layers are closely connected with smaller veins by means of vein-extensions which for ten species were found along 58 per cent of the total vein length. In these leaves the vein-extensions were only 0.25 mm. apart and areas of blade between supports averaged 0.06 sq. mm., which brought about three-fourths of the epidermis within 60 μ of support and supply. Diffusion through the mesophyll between veins is handicapped by the intercellular spaces. For ten species the lateral contact-area between the epidermal cells was 2.5 to 6 times that for all layers of the spongy mesophyll of the same leaf samples, and the lateral discontinuity of the palisade greatly limits transfer through this zone in the plane of the blade. Experiments with dilute solutions of potassium ferrocyanide showed that this reagent was quickly conducted from veins through vein-extensions into and throughout the epidermal layers of leaves, but it did not directly enter the mesophyll in demonstrable amount. The average distance between veins (of all categories) for these ten species was 109 μ. This span is limited chiefly by mechanical and conduction problems and their solution is aided by the epidermal layers. The living tissues with diminished chlorophyll content (vein-extensions and epidermal layers) contribute to the support of the blade and constitute in some degree a supplementary conductive system in mesomorphic leaves.

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