POLY EM BRYONY 351 



about their origin. Of special interest are the diploid -diploid, hap- 

 loid-diploid, diploid-triploid, and haploid-triploid twins. 3 



Considering the diploid-diploid twins first, Randolph (1936) saw 

 two embryos lying parallel to each other in some kernels of Zea 

 mays, and the twin plants resulting from such kernels were found to 

 be genetically identical even in heterozygous stocks. He also saw 

 occasional seedlings with two plumules and a single radicle. Skov- 

 sted (1939) reported twins in Trifolium pratense, with both members 

 having a chromosome fragment, and in Medicago sativa, with both 

 having an extra chromosome. In all these cases the seedlings are 

 interpreted as having originated by the cleavage of a single embryo. 

 While this is probably true, it must be noted that cytologically simi- 

 lar diploid seedlings may also arise in other ways, the most frequent 

 source being the fertilization of more than one cell of the embryo 

 sac. 4 Diploid-diploid twins may also arise from embryos produced 

 in two separate embryo sacs in an ovule or by nucellar budding. 

 Also, it is possible that sometimes a haploid cell of the embryo sac 

 may give rise to a diploid embryo by a process of "endoduplication" 

 of the chromosomes. A monozygotic origin may, therefore, be 

 assumed only when the seedlings are completely identical in all 

 essential respects. 6 



Haploid-diploid twins were reported by Kappert (1933) in Linum 

 usitatissimum, and by Ramiah et al. (1933, 1935) in Oryza sativa. 

 Subsequently, they have been recorded in several other plants, 

 viz., Triticum durum (Kihara, 1936), Solarium tuberosum, Phleum 

 pratense (Muntzing, 1937), Triticum vulgare (Kasparayan, 1938), 

 Secale cereale (Kostoff, 1939), Capsicum annuum (Christensen and 

 Bamford, 1943), Dactylis glomerata (Muntzing, 1943), and Gossyp- 

 ium barbadense (Harland, 1936; Webber, 1938; Skovsted, 1939; 

 Silow and Stephens, 1944). Kappert (1933) explained his twins of 

 Linum on the basis that the diploid member of the complex was 

 derived from the fertilized egg and the haploid member from an 

 unfertilized cell of the same embryo sac. Ramiah et al. (1933) also 



3 See in this connection Webber's (1940) review of "polyembryony." 



4 Except in bi- and tetrasporic embryo sacs, all the nuclei of an embryo sac are 

 genetically identical. 



5 Identical seedlings may also arise if the egg and one synergid are fertilized by 

 the two male gametes discharged from the same pollen tube. Possibilities of such 

 an occurrence seem to be indicated in some orchids (Hagerup, 1947). 



