1. The investigation was undertaken in order to determine in what type of hybrid, and at what stage in the life cycle of such hybrids, heterosis was manifested.
2. Populations of seven inbred and twelve hybrid lines ofLycopersicum spp. were grown and sampled at 16, 29 and 145 days after sowing.
3. The hybrids did not germinate more quickly than their parents.
4. There was found to be very little correspondence between the presence of heterosis in the seed and in the mature hybrid, and the reasons for this are discussed.
5. No size differences were observed, either at 16 or 145 days, between reciprocal hybrids from seeds of different weight.
6. In certain hybrids heterosis was already established in the young shoot primordium by the 16th day and was maintained on a relative basis until the 145th day. In others heterosis arose subsequent to the 16th day.
7. The heights of the plants on the 139th day bore no very close relationships to their weights; several hybrids which exhibited weight heterosis did not show height heterosis and vice versa.
8. The time of flowering of the hybrids was intermediate between that of their parents, except in intraspecific crosses which differed in a major growth factor (d orbr), when dominance of early flowering was shown.
9. Heterosis in certain intraspecific hybrids is explicable on Jones’s theory of the dominance of linked size-determining factors, but in the case of interspecific hybrids it is considered that the hypothesis proposed by East gives a better interpretation of the facts.
KeywordsSeed Weight Hybrid Seed Maternal Parent Hybrid Line Hybrid Vigour
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