THE EMBRYO 



275 



1475). This is the four-celled stage of the proembryo. At the 

 third cell generation, the two cells of the tier ca divide by a vertical 

 wall, at right angles to the first, to give rise to the quadrant stage 

 (Fig. 147C). The quadrants q divide transversely to produce a 

 group of eight cells called the octants (I and V) and ci gives rise to 

 the two suspensor cells n and n' (Fig. 147 D,E). The wall in the 

 middle cell m, which functions as the hypophysis, is curved and 

 joined on both sides to the transverse wall which originally sepa- 

 rated ca from cb, and now separates the tiers V and m (Fig. 147 F). 



G^— l-^ h * ^ I 



Fig. 147. Development of embryo in Ludwigia pdustris. (After Soueges, 19S5a.\ 



Of the two daughter cells formed from m, the lower, which is lentic- 

 ular in shape, gives rise to the initials of the root tip while the upper 

 gives rise to the root cap. The cells of the tier I and V give rise to 

 the cotyledons and the stem tip. Figure 147 G-I shows some of the 

 stages in the differentiation of the dermatogen de, periblem pc, 

 plerome pi, the cells pco which are destined to form the root cap, 

 and the cells iec, which are destined to form the root cortex. 



Asterad Type. Lactuca saliva (Jones, 1927) may be used as an 

 illustration of the Asterad type which has been based on the studies 

 of Carano (1915) and Soueges (1920c) on various members of the 

 Compositae. The four-celled proembryo consists of two juxtaposed 



