310 Plant Physiology 



hypocotyledonary regions of the embryo. In the octant 

 stage the dermatogen begins to be differentiated, the 

 periclinal divisions appearing first in the terminal oc- 

 tants 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 differentiated early from the tissue within 

 the dermatogen. The stem-tip and cotyledons are de- 

 rived 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 suspensor. 

 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 

 hypophysis divides transversely, the daughter-cell next 

 the embryo completing 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 trans- 

 versely, the plate of four cells toward the embryo com- 

 pleting the dermatogen of the root-tip, and the other plate 

 constituting the first layer of the root-cap." 



178. Polarity. - This term denotes a differentiation of 

 the two poles of a growing cell or organ. Any part of 

 any seed-plant or member is therefore recognized as hav- 

 ing an apical pole and a basal pole, the apical being the 

 direction of growth of the shoot, and the basal the direc- 

 tion of growth of the root. From the preceding description 

 of the growth and differentiation of the embryo the sig- 



