Abstract
(1) In 1952 and 1955, we observed, with a view to find out the fundamental knowledge of breeding works, the manifestation of hybrid vigour between varieties and strains, of second inbred generation in our improvement work of Numbu cabbage and of spring' sowing cabbages. (2) F1 plants could be distinguished from their parental varieties by their hybrid vigour and morphological characters, already on th. eir planting date (42 days after sowing), although the degree of hybrid vigour manifested was low at that time. (3) Hybrid vigou, r increased earliness of hybrids, though the effect was different more or less between the combinations of each varieties and strains. Numbu-strains and certain mid-season varieties showed greater hybrid vigour, when they partook one or both of parent of hybrid. The rate of acceleration of earliness was performed more by late variety combinations than by combinations of early varieties.
