318 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



and is therefore believed to have arisen from a male gamete of 

 N. tabacum. 



The third androgenic haploid was reported in Crepis by Geras- 

 simova (1936). She castrated the flowers of plants of C. tectorum 

 having certain dominant characters and treated them with X-rays. 

 They were then crossed with plants of the same species having 

 recessive characters. One haploid plant of the recessive type was 



B 



D 



Fig. 181. Presence of twin embryos and other abnormalities in Orchis maculatus. 

 A, four male nuclei in embryo sac; two of these are in close contact with the egg, 

 suggesting the possibility of both fusing with it. Note also secondary nucleus and 

 nucleus of a displaced synergid (?). B, embryo sac has received two pollen tubes; 

 presence of 60 chromosomes (n = 20) in embryo seems to indicate that egg nucleus 

 had fused with two male nuclei. C, unfertilized embryo sac, showing division of 

 haploid egg and a synergid. D, embryo sac with twin embryos. (After Hagerup, 

 19U-) 



obtained. It is believed that the X-ray treatment killed the egg 

 nucleus and the haploid embryo arose from the sperm nucleus only. 

 Summing up, haploid embryos may result in various ways. The 

 first and most important source is the unfertilized egg. Failure 

 of fertilization may be due to any of the following causes: (1) absence 

 of a pollen tube, (2) inability of the tube to discharge its contents, 



(3) an insufficient attraction between the male and female nuclei, 



(4) an early degeneration of the sperms, and (5) a discordance in the 



