8 



THE WHEAT PLANT 



In most parts the pericarp of the ripe grain is 45-50 ^ thick and com- 

 posed of four or five layers of cells, namely, an outer and inner epidermis 

 with two or three layers of parenchyma between. 



The outer epidermis is formed chiefly of elongated cells from 125 to 

 210 IJL long and 25-30 //, broad ; the cell walls, which are about 5-7 ^ thick, 

 are perforated with pores, and in surface view appear beaded (Fig. i). 

 The cells are arranged lengthwise from base to apex of the grain ; near 

 the apex they become shorter, some of them being almost square. 



Belonging to the epidermis are the hairs of the " brush '' ; these are 

 single, fine-pointed cells -5 mm. to i mm. long, each with a more or less 



FIG. i. a, Cells of the outer 

 epidermis of caryopsis( x 260) ; 

 b, wall of cell showing thicken- 

 ing ( 



FlG. 2. Hairs of the "brush" 

 at the apex of the caryopsis 

 (x 105). 



bulbous base ; their cell cavities are of smaller diameter than the cell 

 walls, which have a thickness of about 6 p, (Fig. 2). 



Beneath the epidermis are one or two layers of thick- walled parenchyma 

 and an irregular layer of similar tissue, the cells of which have thin walls, 

 and are crushed and often disorganised. 



Succeeding these is a well-defined layer consisting of elongated cells, 

 the largest diameters of which are arranged at right angles to those of the 

 epidermal cells, and therefore across the grain. The cells of this " cross 

 layer " in the young grain contain chlorophyll, but in the ripe grain are 

 empty like the rest of the pericarp cells. Except at the apex and base of 

 the grain, where they are shorter, they are usually from 100 to 150 p, long 

 and 15-20 p, broad, with walls 5-7^ thick, in which are irregular slits or 

 pores. 



In adapting themselves to the shrinking grain when ripening takes 



