176 THE STRUCTURE OF ECONOMIC PLANTS 



centripetally. The initial development of the endosperm occurs 

 at the expense of the antipodal cells and the surrounding nucellar 

 tissue, some of which is absorbed. 



Brenchley (4) has investigated the development of the endosperm 

 and points out that the differentiation of the cells on the ventral 

 side opposite the furrow results in the formation of a peripheral 

 layer of aleurone cells. The deposition of starch begins at about 

 the tenth to eleventh day, when the endosperm cells are completely 

 formed, and proceeds for five or six weeks until the conclusion of 

 the ripening period. Starch is deposited first on the flanks of the 

 endosperm, and none can be detected on the portion of the endo- 

 sperm beneath the furrow connecting the two flanks until later in 

 ontogeny. Finally, the cells become so filled with starch that the 

 nuclei and cytoplasm of the cells are affected, and in those which 

 contain large amounts of starch, the protoplasmic contents and the 

 nuclei become disorganized. 



The Integuments. — While the embryo and endosperm are 

 undergoing progressive development and change, the two integu- 

 ments are also being modified. Initially, each integument consists 

 of two cell layers; but at about the time that the zygote divides, 

 the cells of the outer one begin to disintegrate. This is evidenced 

 in a loss of turgidity and a degeneration of the protoplasts; until, 

 finally, the cell contents disappear completely and the cell walls 

 are crushed and obliterated. (Fig. 80, A, B, C.) The inner integ- 

 ument retains its form, and its cells remain distinct, increasing in 

 size until about the milk-ripe stage, when there is a collapse of the 

 cells of the outer layer of the inner integument owing to a loss of 

 cell contents. The cells of the inner layer of the inner integument 

 persist for some time longer and in them are the pigments which 

 give color to the yellow and red grained wheats. Finally, as the 

 grain dries and shrinks in ripening, the radial walls of the inner 

 layer become crushed and partially dissolved, so that the collapsed 

 remains of the outer and inner walls represent all that is left of the 

 seed coat at maturity. (Fig. 80, D.) 



The Pericarp. — The pericarp or ovary wall keeps pace with 

 the development of the enclosed ovule. It consists chiefly of 

 parenchymatous cells which lie between an outer and inner epi- 

 dermis. As maturation proceeds, there is some differentiation and 

 the cells of its outer epidermis begin to exhibit the pits and beaded 

 thickenings described for the mature grain. The parenchyma is 



