EMBRYOGENESIS IN FLOWERING PLANTS 293 



embryos, the embryos showing the same or muUiple chromosome 

 numbers (Webber, 1940). Separate twin embryos, diploid and geneti- 

 cally identical even in heterozygous stocks, and embryos with two 

 plumules but a common radicle, have been observed in Zea mays seeds 

 (Randolph, 1936). In Thfolium pratense, twin embryos each with a 

 chromosome fragment, and in Medicago sativa, twins each with an 

 extra chromosome, have been observed (Skovsted, 1939). All these 

 associated embryos appear to be identical twins, i.e. originating from 

 a single fertilised ovum. Cytologically and genetically identical diploid 

 embryos could also result from the fertilisation of an ovum and one of 

 its synergids. Diploid twins could also arise by nucellar budding and 

 by the inception of an embryo in two separate embryo sacs in an ovule. 

 Haploid-diploid twins have also been reported in a number of species 

 {see Maheshwari, 1951). The explanation of this phenomenon is that 

 the ovum is normally fertilised and that another nucleus of the embryo 

 sac is stimulated to develop; or that there are two embryo sacs in the 

 ovule, and that fertilisation in one induces parthogenesis in the other. 

 In the case of diploid-triploid twins, and haploid-triploid twins, several 

 instances of which have been reported (Maheshwari, 1951), the several 

 evident cytological explanations have been advanced. It is often 

 difficult, if not impossible, to determine the cytological events that have 

 led to the formation of twin embryos of different ploidy. 



Indications of a useful methodological approach, and of the 

 valuable data that may accrue from critical studies of polyembryony, 

 have been given by Randall and Rick (1945). In cytological investi- 

 gations oi Asparagus officinalis they found that of 405 multiple seedlings, 

 97 per cent were twins, 1 1 were triplets and there was one set of quad- 

 ruplets. Diploid twins (with 2n = 20) were the most frequent, but 

 haploids, triploids, tetraploids and trisomic individuals, with 10, 30, 40 

 and 21 chromosomes respectively, were also found. In the haploid- 

 diploid twins, the haploid member was always much smaller; with this 

 exception, however, size differences were no guide to the chromosome 

 constitution of the seedlings. The several embryonic conditions are 

 variously attributed to cleavage polyembryony and to the fertilisation 

 or stimulation of two cells in the same or in contiguous embryo sacs. 

 Although there are serious technical difficulties to be overcome, com- 

 prehensive investigations combining morphological, cytological and 

 genetical observations, and embryo culture of polyembryonic species, 

 seem likely to advance materially our knowledge of the factors in 

 embryogenesis. 



