316 EMBRYOGENESIS IN PLANTS 



is normally determined by the entry of the male nuclei into the embryo 

 sac and their fusion with the egg and polar nuclei, physiological in- 

 vestigations of the relevant cytogenetical phenomena seem likely to 

 make for important advances in our knowledge. If the gametophyte is 

 of the normal type, the zygote and primary endosperm nuclei, with 2x 

 and 3x chromosomes respectively, are alike qualitatively, except that the 

 endosperm nucleus has two gene complexes of megaspore origin. In 

 the Oenothera (Onagraceae) type of development, the zygote and 

 primary endosperm nuclei are identical. But in gametophytes of 

 bisporic and tetrasporic origin, the gene complexes of the polar nuclei 

 are unlike and, on fertilisation, the zygote and primary endosperm 

 nuclei will differ qualitatively. These nuclei will also differ from those 

 of the adjacent nucellar tissue. Different chemical reactions may thus 

 result from the development of the endosperm in different instances, 

 and these may affect the embryonic development. Brink and Cooper 

 (1947), to whom the reader is referred for a valuable review of these 

 matters, have pointed out that the genetical constitution of the endo- 

 sperm, e.g. in the Gramineae, can be varied in known ways by controlling 

 the kind of pollen used in fertilisation, and that this approach has 

 proved of great value in investigating the properties of endosperm and 

 its effects on the developing embryo. They have also indicated, by 

 reference to the relevant literature, that several kinds of hereditary 

 modification of the balance between the embryo, endosperm, and 

 maternal tissues may lead to seed abortion. These cytogenetic and 

 physiological phenomena, including that of hybrid vigour, seem likely 

 to occupy an important place in future studies of embryogenesis. 



In concluding this Chapter on the experimental investigation of 

 angiosperm embryology, it may be noted that the most promising field 

 of work appears to lie in physiological genetics; for the basic pheno- 

 menon of the normal embryonic development of any particular species 

 is that there is a regulated specific growth development of the zygote 

 into a progressively larger, more complex and highly differentiated 

 entity. This growth development is the result of the characteristic 

 metabolism of the germ itself and of its environment, both of which 

 are gene-determined. As we have seen, modern biochemical techniques 

 and experimental ingenuity have enabled a beginning to be made in the 

 study of these very important and interesting embryological phenomena. 



