EMBRYOGENESIS IN FLOWERING PLANTS 301 



among others that crosses between higher plants, which appear to 

 yield non-viable or abortive seed, may yet be made to yield growing 

 embryos and eventually seedlings and mature plants. Thus Tukey 

 (1944) showed that the early-aborting embryos o{ Primus hybrids could, 

 if dissected out and maintained in culture, be grown to mature and 

 vigorous fruiting trees. Earlier, Jergensen (1928) had used the method 

 to obtain hybrids between Solanum nigrum and S. luteum. Other 

 records are those of Bensley (1930) using Gossypium hirsutum and 

 G. herbaceum; Skirm (1942): species of /'nww^' and of L/7/ww ; Brink, 

 Cooper and Ausherman (1944): Hordeum jubatum and Secale cereale; 

 Smith (1944): Lycopersicon esculentum and L. peruvianum; Iwanow- 

 skaya (1946): Triticum durum and Elymus arenarius; and Sanders 

 (1948): several species of Datura. 



In crosses between Hordeum jubatum and Secale cereale, fertilisation 

 takes place within four hours of pollination but the hybrid seeds 

 collapse and fail to germinate. If, however, the 9-12 days old hybrid 

 embryos are dissected out and placed in an artificial culture medium, 

 they will grow. One such seedling was maintained into the flowering 

 stage. Although no seeds were set by this plant, the important point 

 had been demonstrated that intergeneric crosses were sometimes possible 

 and that the death of the resuhing embryo was not necessarily due to 

 inherent lethal factors but to unknown factors in the environment 

 during development (Brink, Cooper and Ausherman, 1944). It has 

 been a fairly general experience that hybrid embryos tend to abort 

 before the cotyledons are differentiated, i.e. some metabolic factor (or 

 complex of factors) becomes critically important at this stage. The 

 nature of this factor has not yet been ascertained. The older the 

 embryo the more nearly autotrophic it is. The main technical problem 

 in embryo culture, therefore, is to establish the conditions required by 

 very young embryos; for these either die or develop into a mass of 

 callus. 



Embryo culture is important economically because it enables 

 potentially valuable hybrids to be reared. It also affords a means of 

 curtailing the time required to obtain plants of species in which the 

 seeds normally have a long dormant period prior to germination. In 

 this connection. Barton and Crocker (1948) have used the method of 

 embryo culture to obtain early information on the viability or germina- 

 tion-capacity of seeds: non-viable germs soon become discoloured, 

 whereas viable ones enlarge and become green and autotrophic. 



Embryo culture is also important in that it affords an experimental 

 approach to the action of various nutrients in morphogenetic processes. 

 Thus Doerpinghaus (1947) has examined the differences between 

 species of Datura in their capacity to utilise different carbohydrates. 



