EMBRYOLOGICAL DEVELOPMENT OF INBRED AND HYBRID ZEA MAYS L.

THE EARLY researches in maize genetics before the rediscovery of Mendel's laws have been reviewed by Singleton (1935). The early literature on hybrid vigor has been discussed by Jones ( 1918) and development of hybrid corn in relation to heterosis has been summarized by Singleton (1941). The problem of heterosis has been critically reviewed by East (1936) and more recently by Whaley (1944), so that only a few ideas most pertinent to the present problem need be mentioned. Shull (1908, 1909, 1910, 1911) was the first to attempt to give a genetic explanation of hvbrid vigor in maize. He introduced the term "heterozygosis," from which "heterosis" was derived. Almost at the same time East (1908, 1909), who was also working on maize, contributed to knowledge of the genetic nature of hybridization. Bruce (1910) believed that hybrid vigor is the result of the number of "dominant" genes rather than of the "heterozygous" genes present in the hybrid, as Shull (1908) visualized. Keeble and Pellew (1910) reached the same conclusion as Bruce, while East and Hayes (1912) and Shull (1914) concluded that the stimulus to development through hybridization is directly derived from heterozygosity. Later Jones (1917) suggested "the dominance of linked genes hypothesis" based on the idea of dominant favorable genes of Bruce (1910), Keeble and Pellew (1910) and Morgan's (1915) discovery of linkage. Up to 1930 Jones' theory forthe explanation of heterosis had been generally accepted. In 1930 Ashby (1930, 1932, 1936) began to publish a series of physiological investigations on the immediate cause of hybrid vigor. He studied the growth rate of inbred and hybrid seedlings in maize for a period of 20-70 days after germination of the seeds and found no difference in the rate of growth. Ashby concluded that hybrid vigor is due to the difference in initial embryo size the so-called "initial capital hypothesis." He further said that the growth rate in his plants seemed to be determined by a single gene. In a second series of Ashby's (1937) and in Luckwill's (1937) studies dealing with hybrid vigor in tomatoes the same conclusion was reached, but modified the concept of "original capital" to apply to the axis of the embrvo rather than the embryo as a whole. East (1936) criticized Ashby's work in many details. Lindstrom (1935) failed to substantiate Ashby's thesis as his experimental results showed that decapitated vigorous F1 maize hybrid seedlings, deprived of some of the 1 Received for publication August 9, 1946. The author wishes to express his sincere appreciation to Professor J. T. Buchholz, under whose direction this work was initiated and carried out. He is also grateful to Professor C. M. Woodworth of the University of Illinois for his kind help in offering the use of experimental material. 2 Present address: Institute of Botany, Academia Sinica, 320 Yoyang Road, Shanghai. "capital," still exceeded the parental strains in dry weight at maturity. Sprague (1936) and Mlurdoch ( 1940) studied the weights of embryo axes of maize. Sprague concluded that "hybrid vigor can not be attributed to the maintenance of an initial difference in embryo size," while Murdoch found that the embryo axes of the two hybrids were significantly heavier than those of their parents. Whaley (1939) made extensive studies in tomatoes and concluded that "the presence of the heterosis is not always accompanied in the hybrid by a larger embryo" and "no relation was found, either in the embryo or during development, between the volume of the apical meristem and heterosis." Shafer (1944), in his studies of the relation of embryo axis weights to heterosis, reported: "some hybrids grow at a much faster rate than either parent" and "some hvbrids show heterosis in spite of having embryo axes no heavier than those of their parents." More recently Buchholz (1945) found that the growth rate of hybrid pine embryos is greater than that of the selfed embryos of the parents and that there is no difference in embryo size or meristem size at comparable morphological stages. The purpose of the present investigation was to study the growth rate of the embryo and endosperm of inbred and hybrid maize during the period between fertilization and the maturing of the seed and its relation to heterosis. Randolph (1936) studied the developmental morphology of the caryopsis in maize based upon the observations of the sectioned slides of the dent variety Pride of Michigan. Copeland (1940) published an abstract in which he pointed out that some hybrid maize embryos showed added vigor at the 4-10 day stages. Even before Ashby, Kiesselbach (1926) in his studies of the immediate effect of gametic relationship and of parental type upon the kernel weight of corn reported that "nineteen inbred strains of two varieties whose kernel weights were increased an average of 10.1 per cent by crosspollination in 1923 had their embryos, endosperms, and pericarps increased 11.8, 10.4, and 3.2 per cent, respectively." However, the growth rate of inbred and hybrid maize for the period between fertilization and the maturing of the seed is still not known. MATERIAL AND METHODS.-The plants for this investigation were grown at the University Experimental Farm. Three well-established inbred lines were used for experiments: two yellow lines, 38-11 and WF9, and one white line, K-64. The inbred lines were recommended by Professor C. M. Woodworth, Agronomist, and were so chosen because they are being widely used in breeding commercial hvbrid corn. The ears of eight to ten plants in each line were bagged as usual before silking. Just before pollination the group of silks was carefully separated into two halves; the separation into right and