6 BULLETIlSr 183, U, S. DEPARTMENT OF AGEICULTURE. 



green color. The chlorophyll-bearing cells themselves become some- 

 what thickened and remain plainly discernible as a layer, even at 

 maturity. The inner integument does not disappear as does the 

 outer one. The cell walls undergo considerable modification and the 

 layer becomes crushed, so that it exists in the ripened fruit only as 

 a dull brown line. It is in the investing membrane of the nucellus, 

 however, that the greatest functional transformation occurs. The 

 cell walls of this delicate tissue become thickened, and the layer 

 quickly assumes, either independently or in conjunction with the 

 inner integument, the functions of a selective semipermeable mem- 

 brane. Brenchley states that even before the glumes are gi'own fast 

 to the fruit it is necessary to prick through this membrane to bring 

 about the entrance of killing fluids. The absorption of the nucellar 

 tissue begins almost immediately after fertilization has taken place. 

 It proceeds rapidly and is almost total, a small body persisting near 

 the funicle and fragments of collapsed cell walls in chance locations 

 elsewhere. 



Of the tissues which envelop the seed, the semipermeable membrane 

 is the only one performing more than mechanical service. On the 

 other hand, all the growth within the embryo sac is of physiological 

 purpose in one or more ways. There are three important structures — 

 the aleurone layer, the embryo, and the starch endosperm (PI. II, 

 fig. 1). The three are associated in development and are intimately 

 related in later germination and growth. Preliminary to fertili- 

 zation, the nucleus of the embryo sac divides and redivides until 

 eight nuclei are present. The egg cell and two companion cells locate 

 near the micropyle. Two others unite to form a large nucleus near 

 the center, while the remaining three, the antipodal cells, pass to the 

 end opposite the egg cell. 



Following fertilization, the egg cell begins the initial divisions of the 

 growth of the embryo, but its development is less rapid than that of 

 the large central nucleus, which is the source of the endosperm. The 

 earliest and most vigorous cell division of the endosperm-forming 

 tissue occurs near the distal end of the gram upon the ventral side and 

 adjacent to the antipodal cells which, according to Johannsen,^ have 

 by this time increased to the number of 30 or thereabouts. After the 

 first layer of endosperm cells has been formed, the growth continues 

 radially toward the center of the grain. The mature stage is always 

 found on the ventral side and particularly in the flanks of the 

 furrow. It is only after the embryo sac is entirely filled with cells 

 that the outer layer of the endosperm begins to differentiate. Al- 

 though at this time starch grains are infrequent throughout the 

 endosperm, its external layer begins to exhibit the proteolytic 



iJohannsen, W. Cm Fr0hviden og dens Udvilcling hos Byg. Meddelelser, Carlsberg Laboratoriet, 

 Bd. 2, Hefte 3, p. 103-133, 3 pi., 1884. (French ed., p. 60-77.) 



