276 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



cells derived from the terminal cell ca and two superposed cells ci 

 and m derived from the basal cell cb (Fig. 148 A-C). In the follow- 

 ing stage, each of the four cells divides again so that the terminal 

 tier now comprises the quadrant cells q, the middle tier comprises 

 the two juxtaposed cells at m, and ci divides transversely to form 

 the daughter cells n and n' (Fig. 148D). Thus the upper three tiers 

 of this stage owe their origin to the basal cell cb, and the lowest tier 

 of four cells to the terminal cell ca of the two-celled proembryo. 

 The four cells of the tier q divide to form the octant stage, the walls 

 segmenting the quadrant cells being oriented more or less diagonally ; 

 the two cells of the tier m undergo a vertical division to give rise to 

 four cells lying directly above the octants; n also divides by a 

 vertical wall ; and n' divides by a transverse wall to form o and p 



Fig. 148. Development of embryo in Lactuca sativa. {After Jones, 1927.) 



(Fig. 14&E-G). At the same time tangential walls are laid down 

 in the tiers q and m to cut off an outer layer of dermatogen cells from 

 the inner cells which undergo further divisions to give rise to the 

 periblem and plerome (Fig. 148G). Regarding further develop- 

 ment, the cell p gives rise to a suspensor consisting of a variable 

 number of cells; o to the root cap and dermatogen of the root; n to 

 the remaining part of the root tip; m to the hypocotyledonary 

 region; and q to the cotyledons and stem tip. 



Geum urbanum (Soueges, 19236) offers a significant variation from 

 the above scheme in the early demarcation of a special cell called 

 the epiphysis initial. 5 After the two-celled stage (Fig. 149A) the 

 first wall in ca is markedly oblique, resulting in two unequal cells 

 a and b (Fig. 149B-C). Of these, a divides to cut off a wedge-shaped 

 cell e, which is called the epiphysis (Fig. 149 D,E). At the same 

 time the middle cell m divides vertically and ci divides transversely, 

 so that there are now four tiers of cells in all, designated as q, m, n y 



6 For further information on the epiphysis, see Soueges (19346). 



