FACTORS IN EMBRYOGENESIS 



23 



division is evidence of differences in metabolic activity at the two poles. 

 Indeed, it may well be that the inception of polarity is due to a drift or 

 trend towards metabolic heterogeneity in an initially homogeneous 

 system. There are many indications that the suspensor and embryonic 



a 



A 



S 



Fig. 5. The first division of the zygote in various encapsulated 



embryos (diagrammatic) 



A, as in most liverworts and mosses. B, Aiithoceros. C, Psilotum, Tmesipteris and 

 Equisetum. D, Isoetes. E, Selaginella and Lycopodium. F, Ophioglossiim vulgatiiin. 

 G, AdiaiUiim, Onoclea and other leptosporangiate ferns. H, as in flowering plants. 

 The distal, epibasal, apical or embryonic cell (or embryonic region), is stippled; s, 

 suspensor; //, hypobasal cell ; 5'>', synergids; a, archegonium neck ; w, micropyle. 



ends are physiologically different; for whereas the latter continues to 

 grow and divide, yielding the meristematic apical region of the embryo, 

 the suspensor and the cells adjacent to it enlarge and become 

 differentiated as parenchymatous cells {see Fig. 7). 



distal one as the epibasal cell. As the suspensor is always the cell lying adjacent to the neck of the 

 archegonium, the embryo during its growth thrusts itself into the tissue of the prothallus. Such an 

 embryogeny is said to be endoscopic. Where no suspensor is present, as in Psilotum, the epibasal 

 cell lies next the neck of the archegonium, the shoot apex grows out through the neck, and the 

 embryogeny is said to be exoscopic. The Marattiaceae, however, afford examples of endoscopic 

 embryogeny, even though no suspensor is present. In the embryogenesis of flowering plants a 

 suspensor is of very general occurrence. 



