266 



EMBRYOGENESIS IN PLANTS 



of a single large siispensor cell and a large ellipsoidal embryo, Fig. 69, 

 the latter continuing to elongate till it fills the embryo sac. This body 

 consists mainly of a massive cotyledon, the cells of which are filled 

 with starch, the shoot apex and root being differentiated relatively late 

 in the lower region of the embryo. These bulky embryos are of interest 



Fig. 68. Embryonic development in a monocotyledon, 



Lininophyton obtusifoUiim 



( X 284, after Johri) 



in relation to the embryonic developments found in primitive dicoty- 

 ledons (p. 252). They also suggest that the single cotyledon is not likely 

 to be explained in terms of the abortion or suppression of one of the 

 primordia in an initially or potentially dicotyledonous organisation. 



If we assume that the ancestors of the monocotyledons had two 

 (or more) cotyledons, and that a critical genetical change, or series of 

 changes, resulted in the monocotyledonous condition, then the ultimate 

 problem is to consider how, at a certain stage in the dicotyledonous 



