EMBRYOGENESIS IN FLOWERING PLANTS 303 



using the basic medium given above, most proembryos did not grow, 

 though some developed into undifferentiated callus-hke bodies. It is 

 thus possible to induce at will typical or atypical growth. In the embryo 

 factor we have a chemical substance which makes for, or sustains, 

 meristematic activity. Embryos kept for a few days without this factor 

 were unable to respond to subsequent applications and soon lost their 

 viability. Before it can affect the growth of embryos, the embryo factor 

 must apparently reach a certain threshold concentration : the younger 

 the embryo, the higher is this threshold value. In other words, as an 

 embryo becomes older it is increasingly able to synthesise its own 

 growth factor, the mature embryo being entirely autotrophic. 



The condition of the plant from which embryos are obtained 

 affects the behaviour of the embryos in culture. Embryos from plants 

 kept in a warm but poorly lighted greenhouse failed to develop on the 

 basic medium, whereas those from plants growing outside at relatively 

 low temperatures in an abundance of light grew on the basic medium 

 alone. Van Overbeek concludes that the latter conditions are probably 

 favourable for the formation of embryo factor in the parent plant, the 

 factor being probably a hormone-like substance. 



That the watery endosperm of the coconut contains the growth- 

 promoting 'coconut-milk factor' at all stages of development has been 

 shown by Steward and Caplin (1952). Some activity is also shown by 

 parts of the immature embryo but not by the solid endosperm. Growth- 

 promoting substances of analogous activity have been detected in the 

 immature milky endosperm of maize, in the gelatinous material in 

 immature walnut fruits, and in the young gametophyte of Ginkgo biloba. 

 The tissues from which these substances have been obtained appear to 

 grow at the expense of, or to be nourished by, the nucellar tissue. In 

 Cocos, Zea, and Juglans it is the immature embryo that eventually 

 benefits from the growth factor. The conditions that are conducive to 

 the accumulation of the substance seem to involve, or to be associated 

 with, a delay in the growth of the embryo but a precocious formation 

 of the endosperm. 



When immature embryos of Hordeiim vulgare were excised and 

 cultured in a medium containing 12-5 per cent sucrose, minerals and 

 agar, considerably improved growth was obtained when Hordeum 

 endosperms were placed on the medium surrounding the embryos 

 (Ziebur and Brink, 1951). These endosperms also stimulate the growth 

 of immature embryos of other genera. Coconut milk and malt extract, 

 both of which promote the growth of some dicotyledonous embryos in 

 culture, had no favourable effect on Hordeum embryos. 



Immature sedge embryos (species of Carex) can be excised and 

 successfully grown in Whites' standard nutrient solution (Lee, 1952). 



