510 THE STRUCTURE OF ECONOMIC PLANTS 



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layer of epidermal cells; (i) an intermediate zone which he 

 divides into an external and internal region; and (3) an internal 

 or digestive zone which forms the innermost layer of cells 

 covering the endosperm. (Fig. uy^, B, C.) 



As the seed develops 

 and approaches maturity, 

 definite changes occur in 

 the character of the in- 

 tegument. The inner 

 digestive layer acts as 

 an haustorial zone, and 

 there is a gradual diges- 

 tion of the inner region 

 of the middle layer 

 ,'-0 1 which progresses centrif- 

 y-ml ugally until nothing is 

 left of the entire middle 

 zone except a narrow 

 band of much-crushed 

 tissue. While the diges- 

 tion of the intermediate 

 zone of the integument 

 is going on, a special 

 type of wall thickening 

 is taking place in the 

 cells of the external 

 layer. There is a dep- 

 , , osition of cellulose on 



Fig. 175. A, median section through seed showing . ■ill 



position of embryo; B, transection of portion of in- the inner tangential wall 



tegument showing layers and part of endosperm; C, q( Qa.ch epidermal Cell 



surface view of external layer of seed coat : cot, cotyle- J i f ' C 



dons ; eW, endosperm ;/■/, inner digestive layer of seed ^^^ ^i"^^ lOrmatlOn OI 



coat; m I, middle partially digested and crushed layer; longitudinal bands ofcel- 



I, outer layer showing hair-like projections of cell i • i • i ^ „^ c .„ 



1 ru Ar A u <i - A c M,^ lulose which extend from 



walls. {B and C, redrawn after Soueges, Ann. Set. Nat.) 



the base of each cell 

 toward the surface. The cellulose bands are finally isolated 

 through a gelatinization of the intervening wall substance with 

 the result that hairs are formed on the surface of the mature seed. 

 These are not true epidermal hairs, but the thickened portions of 

 the epidermal cell walls which have been separated from one an- 

 other. 



