•i74 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



less cordate in longitudinal section (Fig. 146 A, B). The hypocotyl 

 as well as the cotyledons soon elongate in size, mostly by transverse 

 divisions of their cells (Fig. 146C,Z)). During further development 

 the ovule becomes curved like a horseshoe and the growing coty- 

 ledons also conform to this shape for spatial reasons (Fig. 14QE,F). 



B ^ E F 



Fig. 146. Older stages in development of embryo of Capsella bursa-pastoris. 

 (After Schaffner, 190G.) 



Although Capsella has been used for over fifty years to illustrate 

 the embryogeny of the dicotyledons, the embryos of the Onagraceae 

 are of a simpler and more uniform type and Ludwigia palustris 

 (Soueges, 1935a) may be used as an illustration. The first division 

 of the zygote is transverse (Fig. 147 A) after which the terminal cell 

 ca divides longitudinally to form two juxtaposed cells and the basal 

 cell cb divides transversely to form the two cells ci and m (Fig. 



