I50 THE STRUCTURE OF ECONOMIC PLANTS 



(i) the endosperm ; (3) the embryo. There are usually five layers 

 or zones of cells in the pericarp. These in order from the outer 

 surface are : (i) the epicarp or outer epidermis ; (x) the hypodermal 

 parenchyma; (3) the cross cells; (4) the intermediate cells; and 

 (5) the tube cells. 



The cells of the epicarp are elongated parallel to the long axis of 

 the grain, and have pitted walls that are much thickened and 

 cutinized, appearing beaded in surface view. At the apex, the 

 cells are more polygonal; and some of them develop as epidermal 

 hairs which are thick-walled, pointed, and broad or bulbous at 



Fig. 6i. The grain of wheat and early stages in germination. 



the base. Beneath the epicarp are one or two layers of elongated 

 hypodermal cells which resemble those of the former. The cells 

 of both epicarp and hypodermis become much compressed, and their 

 walls are so heavily thickened at maturity that the cell cavities 

 are not easily distinguishable. Underlying this layer, there may 

 be some thin-walled intermediate cells associated with cross cells, 

 the latter being so named because they are oriented with their 

 long axes at right angles to the outer layers of the pericarp. The 

 cross cells have characteristic transverse pits and contain chloro- 

 phyll when the grain is young. The tube cells make up the inner 

 epidermis of the pericarp, and underlie the cross cells, forming a 

 spongy parenchyma in which there are numerous intercellular 



