2.80 THE STRUCTURE OF ECONOMIC PLANTS 



sis. The zygote undergoes a period of rest, then divides trans- 

 versely, and the two daughter cells again divide in the same plane 

 so that a linear tetrad of cells is formed. Of these, the basal one 

 gives rise to the suspensor and the other three produce the embryo 

 proper. Artschwager and Starrett (3) have described the subse- 

 quent stages in the embryogeny and point out that "after the forma- 

 tion of the quadrant, no strict law governs the sequence of cell 

 division"; although "a certain balance seems to be maintained so 

 that the final product is remarkably uniform." 



Fig. 139. A-0, series of diagrammatic drawings showing sequence of cell division in devel- 

 opment of young embryo. Cell a represents apical cell of tetrad and cell d basal one. (Re- 

 drawn after Artschwager and Starrett, Jour. Agr. Res.') 



In most cases, each cell of the tetrad divides once to form an 

 eight-celled embryo. Cell d usually divides first, followed by cell 

 c, or they may divide simultaneously; and this is also true of cells 

 a and b. (Fig. 139) Cell a always divides vertically; but b may 

 divide either vertically or transversely, and this is the case with cell 

 c, so that the octant stage may be five, six, or seven tiered. Sub- 

 sequent divisions produce the i6-celled embryo which, like the 

 octant, varies in its number of tiers depending upon the plane of 

 the division walls. At this time, the embryo begins to be club- 

 shaped; and, in about five days, becomes globular. The cotyle- 

 dons are differentiated approximately two days later and the 

 embryo is usually mature by the twelfth to fourteenth day. 



Concurrent with the development of the young embryo, there is 

 free nuclear division of the endosperm nuclei which occupy a 



