EMBRYOGENESIS IN FLOWERING PLANTS 291 



(Urticaceae) (Fagerlind, 1944), Fig, 80f-h. The further development 

 of antipodal embryos, however, has not been determined and it is 

 not known if they are viable. The presence of antipodal embryos, 

 with their inverted orientation, raises interesting questions regarding 

 the relation of the polarity of the embryo sac to the embryonic 

 development. 



Adventive embryos afford further examples of polyembryony. In 

 the orchid species Spirarithes australis, some of the constituent races 

 yield normal zygotic embryos, but others form two to six adventive 

 embryos from the inner integument (Swamy, 1949). 



Polyembryony may be due to the presence of multiple embryo sacs 

 within the ovule {see Maheshwari, 1951, for the relevant literature). 

 In some species, polyembryony may be induced simultaneously by 

 several of the methods indicated above. In Allium odor urn one third 

 to one half of the ovules may contain multiple embryos of ovum, 

 synergid and antipodal origin (Tretjakow, 1895, and Hegelmaier, 1897); 

 and Haberlandt (1923, 1925) has shown that, even in castrated flowers, 

 embryos with diploid nuclei are formed from these nuclei, as also from 

 cells of the inner integument. The innate complexity of this species has 

 been further indicated by Modilewski (1925, 1930, 1931); for it appears 

 that some embryo sacs contain haploid nuclei and some diploid nuclei. 

 In diploid embryo sacs, the polar nuclei are fertihsed and yield an 

 endosperm with pentaploid nuclei; the embryos have their inception 

 in the unfertilised diploid egg and antipodal cells. In haploid embryo 

 sacs, the viable embryos are formed from haploid ova after normal 

 fertilisation. The several developmental situations have been sum- 

 marised as follows by Maheshwari (1950, p. 348): 



'(i) In a haploid and normally fertilised embryo sac, embryos 

 may begin to develop from all cells of the embryo sac and even from 

 the adjacent integumentary cells, but only the zygotic embryo 

 survives so that the mature seeds contain a single embryo, (ii) 

 Embryos may also begin to form from one or more cells of the 

 haploid and unfertilised embryo sac, but owing to the lack of an 

 endosperm, which can arise only after triple fusion, their growth is 

 soon arrested and they become non-viable, (iii) In a diploid but 

 unfertilised embryo sac, any of its cells (also the cells of the inner 

 integument) may begin to form an embryo, but eventually they all 

 degenerate owing to the absence of an endosperm, (iv) In diploid 

 embryo sacs, in which the secondary nucleus is fertilised, endosperm 

 formation proceeds actively and all the cells of the sac are capable 

 of giving rise to embryos, but only the egg embryo usually attains 

 maturity.' 



