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 cupule ; 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, 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. 



