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INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



late to be able to effect fertilization. Under the two last-named 

 conditions, the unfertilized egg was frequently found to divide and 

 give rise to a haploid embryo ("facultative parthenogenesis") (Fig. 

 180). 6 In Orchis, Listera, and Platanthera instances of a different 

 nature were also noted, in which the egg was fertilized and gave rise 

 to a normal embryo, but simultaneously an unfertilized synergid 

 also began to develop, so that two embryos were formed, one diploid 



A B D 



Fig. 179. Supernumerary pollen tubes in embryo sacs of Epipadis latifolia. A,B, 

 double fertilization, showing one sperm nucleus fusing with egg and the other with 

 secondary nucleus ; note presence of second undischarged pollen tube. C, micropy- 

 lar part of embryo sac, showing two pollen tubes. D, micropylar part of another 

 embryo sac, showing five sperm nuclei in proximity to egg. (After Hagerup, 194-5.) 



and the other haploid. Further, in one ovule of Orchis in which the 

 pollen tube had not yet entered the embryo sac, the egg as well 

 as one of the synergids had begun to divide, thus indicating the 

 possibility of a production of twin haploid embryos (Fig. 181C). 7 



6 In some embryo sacs of another orchid, Spiranthes australis, the pollen tube 

 was found to have entered the embryo sac but the male gametes were still undis- 

 charged, while embryo formation had already commenced (Maheshwari and 

 Narayanaswami, 1950). 



7 Rarely the egg cell of Orchis was found to receive two sperms (Fig. 181 A) and 

 give rise to a triploid embryo (Fig. 1815), or both the egg and one synergid were 

 fertilized to give rise to twin diploid embryos (Fig. 181 D). 



