132 THE OAK 
developed from the fertilised egg-cell fills up the interior 
of the latter, as described in Chapter II. 
The ripe fruit is the acorn, and we may regard it 
apart from the cupuie ; it contains the seed. 
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 striae) and olive-brown in colour 
when ripe. The ripe acorn may thus be regarded as 
consisting of the pericarp (to which the calyx or peri- 
anth is fused) and the seed. 
The pericarp (fig. 36, p) is a thin, hard shell, com- 
prised of four layers:—(1) An epidermis of small, 
cuboidal cells with their external walls much thickened 
(fig. 37,8). (2) Four or five series of very thick-walled 
and pitted sclerenchyma cells (fig. 37, 1). (8) 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. 
