132 THE OAK. 



The acorn is an egg-shaped, nut-like fruit (ylans}, 

 about 18 mm. long and 8-10 mm. broad (Fig. 3G) ; the 

 apex is somewhat pointed with a hard remnant of the 

 stigma, the base is broader, and marked with the cir- 

 cular 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 color 

 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, com- 

 prising the chief thickness of the pericarp (Fig. 37). It 

 is in this tissue that the small vascular bundles supply- 

 ing the pericarp run, and here and there nests of scleren- 

 chyma 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 elon- 

 gated 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, 



