EMBRYOGENESIS IN FLOWERING PLANTS 227 



as Nemec (1910), the composition of a hybrid endosperm will be such 

 as to be adjusted to, or compatible with, the needs of the developing 

 hybrid embryo, the implication being that the nutrition provided by 

 the nucellus and embryo sac alone might somehow be inadequate. 

 Much could obviously be said both for and against this view. Some 

 young embryos may require a very delicately balanced supply of 

 growth-regulating substances and other metabolites, but it should not 

 be overlooked that the endosperm also draws all its nutrients from the 

 embryo sac and adjacent tissues. Certainly, where no endosperm is 

 formed the embryo soon stops developing and degenerates. By its 

 active nuclear division, cell formation and growth, the endosperm is 

 able to draw, like all actively growing tissues, on the materials in the 

 adjacent cells ; and this has the effect of surrounding the embryo with 

 a richly proteinaceous tissue on which it in turn can draw for its 

 further growth. Merely to indicate some of the elements of the problem 

 at once shows what a complex process embryogenesis really is. More- 

 over, while the embryo is developing, maturation is also proceeding in 

 the tissues surrounding the embryo sac, i.e. there is competition for 

 nutrients between the tissues within and outside the embryo sac (Brink 

 and Cooper, 1940, 1947). 



During the development of the embryo and endosperm, the surface 

 of the embryo sac becomes actively absorptive, breaking down and 

 utilising the materials from the adjacent nucellar tissue. In various 

 species, the embryo sac may elongate into haustorial structures of a 

 remarkable kind. In these developments we may perhaps see some of 

 the innovations by which the flowering plants have become the most 

 advanced and highly elaborated members of the Plant Kingdom. In 

 some species, the endosperm forms extensive haustorium-hke structures. 

 (For a review of the many and varied embryo sac developments, the 

 reader is referred to Maheshwari, 1950.) How these diverse, post- 

 fertilisation embryo sac developments affect the embryogeny are 

 clearly matters to which plant embryologists might well devote much 

 attention and thought. 



Rao (1938) has suggested that in those plants in which the embryo 

 grows rapidly from the outset and is early differentiated, the endosperm 

 tends to be non-cellular, cell formation taking place only at a later 

 stage. But where the embryo grows slowly, and where the seed typically 

 contains an immature and undifferentiated embryo, the endosperm is 

 cellular from the beginning, or quickly becomes so. Various examples 

 are cited of undifferentiated embryos which are surrounded by cellular 

 endosperm, e.g. Peperomia pellucida, Magnolia, Linaria vulgaris, 

 Sar codes sanguinea (Oliver, 1890) and Striga lutea (Mitchell, 1915), 

 the last two species being parasites. Many species show early formation 



