348 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



Bacchi (1943) sometimes found more than one embryo sac in an 

 ovule of Citrus resulting in two zygotic embryos, and Nielsen (1946) 

 has recorded the same in Poa pratensis. 2 



A Few Special Cases. In some plants multiple embryos are pro- 

 duced by the simultaneous operation of more than one of the 

 methods named above. One of the most interesting of them is 

 Allium odorum. Many years ago, Tretjakow (1895) and Hegel- 

 maier (1897) recorded the occurrence of synergid and antipodal 

 embryos in as many as one-third to one-half of the ovules of this 

 species. Later, Haberlandt (1923, 1925) reported that even in 

 castrated flowers there is an increase in the size of the ovules, accom- 

 panied by the production of embryos from several sources — egg, 

 synergids, antipodals, and the cells of the inner integument. He 

 found the diploid number of chromosomes in all the embryos. 

 Modilewski (1925, 1930, 1931), who made a further study of the 

 plant, found that it forms two kinds of embryo sacs, some haploid 

 and others diploid. In the diploid embryo sacs only the polar nuclei 

 are fertilized, resulting in a pentaploid endosperm; embryos arise 

 from the unfertilized but diploid egg and antipodal cells. In hap- 

 loid embryo sacs, on the other hand, viable embryos are formed only 

 after fertilization. In conclusion, four possibilities are mentioned: 



1. In a haploid and normally fertilized 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. 



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

 haploid and unfertilized 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 nonviable. 



3. In a diploid but unfertilized 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. 



4. In diploid embryo sacs, in which the secondary nucleus is 

 fertilized, 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. 



2 Species of the genus Poa show other abnormalities also, for which a reference 

 may be made to the works of Tinney (1940), Engelbert (1941), Akerberg (1939, 1943), 

 and Hakansson (1943, 1944). 



