630 THE STRUCTURE OF ECONOMIC PLANTS 



stage is reached about 15 hours after pollination; and, as in the 

 development of the latter, the cells of tier a are the first to divide 

 so that an octant is formed and the embryo is temporarily in a 

 li-celled stage. No further divisions occur in tier a until all the 

 lower tiers have undergone division. This occurs in sequence, 

 tier c dividing first, followed by tier e, and the transverse division 

 of cell / to form the daughter cells, g and /», thus completing the 

 formation of the i6-celled embryo. (Fig. 3x8, D.) 



— 'phm 



e - ^^ I ^/y phm in 



'dgn 



Fig. 318. Stages in embryogeny : A, two-celled embryo in process of division, 9 hours 

 after pollination ; B, four-celled embryo at ii hours ; C, eight-celled embryo at 2.0 hours ; 

 D, sixteen-celled embryo at i6 hours ; E, embryo showing differentiation of histogens at 

 46 hours ; F, initial lobing preceding formation of cotyledons at 3 days ; G, lower portion of 

 embryo showing differentiation of primary root. Letters a to h refer to the succession of cell 

 divisions (description in text) : dgn, dermatogen ; dgn in, dermatogen initial ; phm, periblem ; 

 /»^w /■«, periblem initial ; /ic/, pericycle; /^Z, plerome; />/ i«, plerome initial ; r c, cells of root 

 cap (this includes all shaded cells except those of suspensor, sus^. (After Jones, Hilgardia.') 



Beyond the i6-celled stage, it is difficult to follow the sequence 

 of cell divisions; but the next development is the cutting off of 

 definite histogens (dermatogen, periblem, and plerome) by the 

 formation of tangential and vertical walls. (Fig. 3x8, £.) Three 

 days after anthesis, slight elevations can be observed in tier a 

 which later become the cotyledons. (Fig. 318, F.) Cell g under- 

 goes transverse and longitudinal division to form two layers of 

 cells, and those of the upper layer constitute the dermatogen initials 

 while the lower ones contribute to the root cap. The derivatives 



