198 



MORPHOLOGY OF ANGIOSPERMS 



bryo. In the octant stage the dermatogen begins to be differ- 

 entiated, the periclinal divisions appearing first in the terminal 

 octants and proceeding toward the root end of the embryo. The 

 differentiation, however, is almost simultaneous, so that the 

 dermatogen is soon completed except that of the root-tip, which 

 is derived from the adjacent cell of the suspensor, and appears 

 comparatively late. The periblem and plerome are differen- 

 tiated early from the tissue within the dermato«;en. The stem- 

 tip and cotyledons are derived from the four apical octants, and 

 the bulk of the hypocotyl from the four basal octants. The 

 root-tip, however, is completed by the adjacent cell of the sus- 



Fig. 90. — Cipsella Bursa-pastoris. Series showing contribution of upper cell of suspen- 

 sor to embryo (plerome and dermatogen shaded) : s, upper cell of suspensor; A, 

 hypophysis; d t dermatogen ; d\ portion of dermatogen derived from hypophysis; 

 pi, plerome ; p, periblem ; p' , portion of periblem derived from hypophysis ; x 400. 



pensor (Fig. 90, s). This cell divides transversely, the basal 

 daughter-cell taking no part in the formation of the embryo, 

 but the other . daughter-cell (hypophysis of Hanstein) filling 

 out the periblem and dermatogen of the root-tip. The hypophy- 

 sis divides transversely, the daughter-cell next the embryo com- 

 pleting the periblem of the root. The other daughter-cell by 

 two longitudinal divisions gives rise to a plate of four cells, 

 each of which divides transversely, the plate of four cells toward 

 the embryo completing the dermatogen of the root-tip, and the 

 other plate constituting the first layer of the root-cap. 



