Embryo Development in Nicotiana Species Hybrids.

I n a study of the compatibilities of certain Nicotiana species, several crosses were reported which produce seeds unable to germinate. (MCCRAY 1932). Many others are recorded in the literature on this genus. Some of these seeds are parthenocarpic; others have embryos not able to develop. In still other cases the hybrid germinates but is unable to carry on further. All degrees of vigor have been observed from these conditions up to heterosis. Several interesting questions might be raised in connection with the development of the hybrid embryos displaying such different degrees of viability and vigor. Does the hybrid begin its development as soon after pollination as does the new individual of pure species? That is to say, does fertilization take place as readily in.one case as in the other? Do the cell divisions in the embryo and its consequent growth proceed as rapidly in the hybrid as in the pure species? If so, when does growth in the non-viable embryos stop? If not, what sort of growth rate does the hybrid have as compared with its pure parent? These are the questions which the present study attempts to answer. Strangely, considering all the work which biologists, especially botanists, have done on hybridization, the scientific literature sheds very little light on these problems. It is nearly an unworked field. So far as I have been able to find only two studies have any bearing on the problem in hand, those of KOSTOFF (1930) and BRIEGER (1928), both on Nicotiana. To be sure KRUMHOLZ (1930) compared the size of embryos of interspecific Oenothera hybrids with that of embryos of the pure species, but he measured only the fully matured embryos, and says nothing about their growth. Incidentally it might be noted here that he found, in reciprocal crosses, the embryo size to be practically determined by the female used in the cross. If the female was of a large seeded species, the hybrid embryo was large and vice versa. KOSTOFF describes several experiments on the formation and development of hybrid embryos. He points out the fact that in some crosses the embryo develops for a few (up to 15) days, then stops, followed by the death of the ovule; he mentions other cases in which the young hybrid persists for a longer period, although not able to mature to the point where germination is possible. He also observed that parthenocarpic seeds and