202 THE ELEMENTS OF VEGETABLE HISTOLOGY 



Mesocarp Tissue. — The different parts of this 

 layer show considerable differentiation. Immediately 

 beneath the exocarp is a layer or two of thick-walled, 

 rectangular, porous, yellow or brownish stone cells 

 with their narrow sides toward the thin exocarp 

 which they reinforce. Irregular, angled, thin-walled 

 parenchyma cells form the greater part of the meso- 

 carp tissues; but many of these become transformed 

 into secretion cells and contain volatile oil. The 

 parenchyma cells near the endocarp are very small 

 and are divided into two zones by an irregular layer 

 of large oil-secretion cells. The fibro-vascular bundles, 

 consisting of spiral vessels and small amounts of fibrous 

 tissue, are scattered through the mesocarp layer. 

 In powdered materials the cells of this layer are appar- 

 ent on surface view. The stone cells are usually con- 

 solidated with fragments of the epidermis, and are 

 irregular in form with thick, porous walls (Plate 67). 

 As the parenchyma is thin-walled, it is usually broken 

 in powdering. The compound starch masses (Plate 

 67), contained in these cells, remain more or less 

 unbroken. Several starch masses may adhere, thus 

 preserving the form and often the walls of the par- 

 enchyma cells. The oil cells are present as circular 

 or polygonal thin-walled elements, each cell contain- 

 ing one or more highly refractive oil globules. The 

 fibro-vascular tissues are apparent in longitudinal 

 view, and consist of spiral vessels, usually in combi- 

 nation with rather thick-walled, porous fibers. 



Endocarp Tissues. — The endocarp consists of a 

 single layer of stone cells. The individual cells are 

 rectangular, porous, and thickened on three sides. 

 In powdered pepper these stone cells may be apparent 



