346 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



Origin of Embryos from Cells of the Embryo Sac Other than the 

 Egg. Besides the zygotic embryo produced from the egg, embryos 

 may also be produced from other parts of the embryo sac. The 

 most common source is the synergids which frequently become egg- 

 like and may be fertilized by sperms from an additional pollen tube 

 (Fig. 116E) or develop without fertilization (Fig. 178). Several 

 cases of both kinds are known and have already been discussed in 

 Chaps. 6 and 9. 



Production of embryos from antipodal cells is much rarer. Shat- 

 tuck (1905) noted that the antipodal cells of Ulmus americana often 

 present an egg-like appearance and in some cases he actually found 

 embryos in this position. Ekdahl (1941) has confirmed this in 

 U. glabra (Fig. 196 A-C), and Modilewski (1931), Mauritzon (1933), 

 and Fagerlind (1944) have figured similar cases in Allium odorum, 

 (Fig. 196D), Sedum fabaria, and Elatostema sinuatum eusinuatum 

 (Fig. 196.Z?). The further fate of these antipodal embryos has not 

 been determined, however, and it is not known whether they are 

 viable. It is to be noted that the egg-like antipodal cells which 

 have been recorded in some other plants like Plumbagella (Dahlgren, 

 1916) and Rudbeckia (Maheshwari and Srinivasan, 1944) have not 

 been observed to give rise to embryos. Nor have the laterally 

 situated egg-like cells of Penaea (Stephens, 1909), Plumbago (Dahl- 

 gren, 1937), Vogelia (Mathur and Khan, 1941), and Acalypha 

 (Maheshwari and Johri, 1941) ever shown such a behavior. 



Embryos Arising from Cells outside Embryo Sac. The develop- 

 ment of embryos from the cells of the nucellus and integument has 

 already been considered in Chap. 9. Citrus, Eugenia, and Mangif- 

 era are well-known examples of this type of polyembryony. The 

 embryos, although initiated outside the embryo sac, subsequently 

 come to lie inside it and are nourished by the endosperm (Fig. 192). 



Recent work indicates that the occurrence of adventive embryony 

 may not be a constant feature of all the individuals of a species. 

 Swamy (1948) has found that the orchid Spiranthes cernua com- 

 prises three races. The first shows normal sexual reproduction and 

 a single zygotic embryo is produced in each seed. In the second 

 race, which is apomictic, the male and female gametophytes are 

 both functionless. Fertilization does not occur but the cells of the 

 inner layer of the inner integument give rise to adventive embryos of 

 which two to six may mature in a seed. In the third race, which is of 



