104 W. GORDON WHALEY 



Ashby, 1930, 1932, 1937; East, 1936; Sprague, 1936; Copeland, 1940; Mur- 

 doch, 1940; Kempton and McLane, 1942; Whaley, 1944, 1950). 



Most of the investigations have dealt with mature seed and embryo size. 

 The evidence shows that in many instances hybrid vigor is associated with a 

 high embryo weight. In some cases the initially high-weight embryo is found 

 in a relatively large seed. There is, however, by no means a consistent correla- 

 tion between either high embryo weight or large seed size and heterosis. 



The results of studies on corn inbreds and hybrids in our own laboratory 

 (Whaley, 1950) seem representative of the general findings. Among some ten 

 inbred lines there occurred a great deal of variation from one line to another 

 as to both embryo weight and seed weight. There was somewhat more varia- 

 tion with respect to embryo weight. Among the Fi hybrids, all of which 

 exhibited considerable vigor under central Texas conditions, there were a 

 few with embryo weights which exceeded those of the larger-embryo parent. 

 For the most part, the embryo weights were intermediate, and in one or two 

 cases they were as low as that of the smaller-embryo parent. The weight of 

 the seed tissues other than the embryo tended to follow that of the pistillate 

 parent, but was generally somewhat higher. Double crosses which had vigor- 

 ous Fi hybrids as pistillate parents characteristically had large seeds with 

 what were classified as medium-weight embryos. 



The few reports, such as Copeland's (1940), concerning the development 

 of embryos in inbred and hybrid corn, suggest that at the earlier stages of 

 development some hybrid vigor is apparent in the hybrid embryos. The 

 observations of hybrid vigor during early development of embryos and the 

 absence of any size advantage at the time of seed maturity are not necessari- 

 ly conflicting. In most plants, embryo and seed maturation represent fairly 

 definite stages at which a certain degree of physiological maturity and of 

 structural development has been attained. It is probably to be anticipated 

 that even though certain heterotic hybrids show early embryo development 

 advantages, these advantages may be ironed out by the time the embryo 

 and the seed mature. The size of both the embryo and the other seed tissues 

 is conditioned not only by the genotype of these tissues themselves, but also 

 by the nutritional background furnished them by the plant on which they 

 grow. 



It is quite possible that this genotype-to-background relationship is an 

 important consideration in the determination of whether or not hybrid vigor 

 is exhibited in the development of the embryo and seed. The background 

 provided by the pistillate parent might be such as to preclude the develop- 

 ment of embryo vigor, even though the embryo genotype were of a definitely 

 heterotic constitution. The fact that hybrid vigor is apparent during certain 

 stages of embryo and seed development may or may not be related to an 

 embryo or seed size advantage at maturity. Because of this, it seems doubt- 

 ful that embryo or seed size is a reliable measure of hybrid vigor; and that 



