138 THE WHEAT PLANT 



The wall of the ovary, up to the time of fertilisation of the ovum, is 

 composed of thin-walled parenchyma enclosed between an outer and an 

 inner epidermis, but changes take place in the thickness and character of 

 its tissues as growth proceeds. 



In a half - grown grain certain portions of the wall have become 

 differentiated from the rest. The outer epidermis is then composed of 

 cells three or four times as long as broad, stretched longitudinally, with 

 lateral walls showing irregular beaded thickenings and a cuticle on the 

 external surface. 



The inner epidermis consists of long thin-walled cells parallel with 

 those covering the exterior. 



Between these outer and inner layers is loosely compacted parenchyma, 

 in transverse section, three or four cells across. The tissue is colourless 

 with the exception of a differentiated chlorophyllous " cross layer " one 

 cell thick, which lies immediately within the inner epidermis. The cells of 

 this layer are cylindrical, five or six times as long as broad, with rounded 

 ends, their long axes arranged at right angles to the long axis of the 

 grain ; each cell has an elongated nucleus and vacuolated cytoplasm, in 

 which are imbedded a number of chloroplasts about 4 p, in diameter, 

 which usually contain minute starch grains measuring, when fully grown, 

 about 3 /J, by i -4 p. 



Chloroplasts are found also in some of the outer subepidermal cells in 

 the ventral furrow of the young pericarp. 



In Abyssinian purple-grained wheat, the cell-sap of the cross layer 

 and a few of the cells of the parenchyma adjacent to it contain anthocyan. 



As the grain matures the minute starch grains (1-2 p in diameter) 

 present in the parenchyma of the young pericarp disappear gradually 

 from the apex of the ovary towards the base, remaining longest in the 

 portion covering the embryo and in the ventral furrow : the cuticle and 

 thickening of the outer epidermal cell walls then become more marked. 



The cells of the inner epidermis do not keep pace with the increase 

 in area of the inner surface of the ovary, and are torn asunder, appearing 

 in the ripe pericarp as a series of isolated sinuous cells the " tube-cells " 

 of Vogl (Fig. 5) which in some parts are widely separated from each 

 other. 



The cells of the " cross layer " lose their green colour, their contents 

 disappear, and their walls increase greatly in thickness and develop narrow 

 transverse pits (Figs. 3, 109). 



The rest of the parenchyma of the pericarp shrivels, and ultimately 

 forms an irregular crushed layer between the strong epidermis and the 

 thick-walled " cross layer " (Fig. 3). 



