422 



A TEXT-BOOK OF BOTANY. 



The middle layer, which is composed of parenchyma, may con- 

 tain protoplasm, starch, sugars, calcium oxalate, coloring princi- 

 ples, alkaloids and other principles, and it may also have oil-secre- 

 tion cells, as in cubeb and pepper, or oil-secretion canals, as in 

 ^orange (Fig. 121) and the fruits of the Umbelliferae, in the latter 



FIG. 246. Rhamnus cathartica. A, cross-section through wall of the pericarp. E, epi- 

 carp; F, sarcocarp; H, endocarp; e, epidermis; o, calcium oxalate in cells of hypodermis; p. 

 parenchyma; h, secretion cells containing a substance which is insoluble in alcohol or chloral 

 solutions, soluble in solutions of potassium hydroxide, and colored reddish brown or green- 

 ish with ferric chloride solutions; c, calcium oxalate cells of endocarp; w, sclerotic cells; f, 

 stereome cells. B, cross-section of entire fruit, showing one seed; E, F. H, g, f, w, as in A; 

 S, seed-coat; S 1 , outer wall of seed-coat; End, endosperm; c, cotyledons; g, vascular bundle. 

 C, cross-section of a seed: S 1 , S 2 , S 3 , different layers of the seed-coat; R, vascular bundle of 

 raphe; t, position of vessels of mestome strand; g, mestome strand; Rf, cleft in which raphe 

 is situated; End, endosperm; C cotyledons; Sv, cells with thick walls ;Sp,parenchymatous 

 cells. After Meyer. 



of which they are known as vittae (see Volume IT) ; milk vessels 

 sometimes occur, as in poppy ; a collenchymatous layer is some- 

 times developed beneath the epidermis, as in capsicum ; in some 

 cases sclerenchymatous cells may be present, as in pimenta and 

 cubeb (Fig. 135) ; and in still other instances the entire pericarp 

 may be made up of stone cells, as in the nuts. 



