312 EMBRYOGENESIS IN PLANTS 



(1953) has attributed the abortion of the embryo in an aneuploid apple 

 to genetical factors. 



When Datura stramonium is crossed with D. metel, the embryo 

 degenerates after undergoing only two or three divisions (Satina and 

 Blakeslee, 1935). In the cross D. prunosa X D. metel, the primary 

 endosperm nucleus undergoes several divisions but the zygote usually 

 remains undivided. In two other crosses, D. meteloides x D. discolor 

 and D. ceratocaula x D. meteloides (Sachet, 1948), most of the ovules 

 are fertilised and the endosperm and embryo at first develop normally, 

 though rather slowly. When the embryo has reached the spherical 

 stage, the endosperm begins to disintegrate but there is enlargement 

 and active division of the cells of the integumentary tapetum, or 

 endothelium. The voluminous tissue mass which is formed invades the 

 embryo sac and replaces the endosperm. The embryo survives in this 

 mass for a few days but does not undergo further divisions. Ultimately 

 it degenerates and disappears and hence viable seeds are not produced. 



Satina, Rappaport and Blakeslee (1950) have pointed out that if, in 

 attempted interspecific crosses, the pre-fertilisation barriers can be over- 

 come, so that the male nucleus comes into contact with the egg cell, 

 fertilisation will usually take place. In many such crosses, however, the 

 zygote may fail to develop into a viable embryo, and potentially 

 interesting hybrids are thus not obtained. This difficulty can some- 

 times be overcome by the excision and artificial culture of young 

 embryos. Our information on the factors which cause the arrested 

 growth of hybrid embryos, or bring about their early abortion, is still 

 very inadequate. The tumour-like tissue mentioned above is consistently 

 present in Datura ovules showing embryo abortion, i.e. in incompatible 

 crosses. This incompatibility between species, which is of primary 

 importance in evolution, has been the subject of important investigations 

 by Blakeslee et al. They consider that fuller investigation of the action 

 of tumoral growth in hybrid ovules may eventually lead to techniques 

 enabling wider species and even generic crosses to be made. 



Abnormal histological developments leading to embryo abortion 

 in ovules have been reported in other plants. Intrusive growths into 

 the embryo sac have been reported in Oenothera crosses by Renner 

 (1914), in Epilobium crosses by Michaelis (1925), in Medicago by Brink 

 and Cooper (1940), in Nicotiana x Petunia crosses by Kostoff (1930), 

 in reciprocal diploid and tetraploid Lycopersicon crosses by Cooper 

 (1945), and in various Nicotiana species crosses by KostolT (1930) and 

 by Brink and Cooper (1940). The cell proliferations observed in these 

 materials are generally comparable with those in Datura. The arrest 

 of embryo growth and the collapse of seeds have been reviewed in 

 detail by Brink and Cooper (1947). Different views have been advanced 



