The acorn is an egg-shaped, nut-like fruit (glans), about 18 mm. long and 8–10 mm. broad (Fig. 36); the apex is somewhat pointed with a hard remnant of the stigma, the base is broader, and marked with the circular scar which denotes where it was inserted in the cupule. The trifid character of the stigma can often be observed even on the ripe fruit, which is smooth (or with fine longitudinal striæ), and olive-brown in color when ripe. The ripe acorn may thus be regarded as consisting of the pericarp (to which the calyx or perianth is fused) and the seed.
The pericarp (Fig. 36, p) is a thin, hard shell, comprised of four layers: (1) An epidermis of small, cuboidal cells with their external walls much thickened (Fig. 37, e). (2) Four or five series of very thick-walled and pitted sclerenchyma cells (Fig. 37, 1). (3) Then follow numerous rows of thin-walled parenchyma cells, comprising the chief thickness of the pericarp (Fig. 37). It is in this tissue that the small vascular bundles supplying the pericarp run, and here and there nests of sclerenchyma cells are scattered. The parenchyma cells may contain minute starch grains, in addition to the remains of chlorophyll corpuscles, even when ripe; they also contain tannin, and, here and there, crystals of calcium oxalate. (4) The internal epidermis consists of elongated cells in one layer.
The seed proper fills up the entire cavity inclosed by the fruit-wall above described. It consists of a relatively very thin testa, or seed-coat, closely enveloping the large,
