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Which side of a leaf is the top side and which is the under side,is not always obvious. In general, the side oriented towards the sun bears an extra waxy -and shiny- protective layer: a -thicker- cuticula. Opposite, the lower side facing the shadow, thus the side where the risk of dessication is smaller, bears often relatively more tiny openings (stomata) for gas exchange. This side is often duller (see example of an ivy leaf). If you can dispose of a microscope, you could deduce where the anatomical upper and lower side lay from the orientation of the xylem and phloem vessels in the vascular bundles. The xylem vessels, recognizable as large and hollow tubular structures, are located toward the inner side in the stem, and they protrude to the anatomical upper side (= adaxial side) into the leaf. The phloem is directed to the outer margin in the vascular bundle of the stem, and they extend to the anatomical underside (= the abaxial side) of the leaf. The "adaxial" and "abaxial" side can thus be firmly determined independently of the position of the leaf relative to the sun, and therefore these terms are prevalent in botany.
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