Abstract
(1) Pentaploid hybrids between tetraploid and hexaploid Avena species are completely or highly sterile. This sterility is clearly due to the gametic abortion which is caused by the abnormal maturation divisions.
(2) The cytological observation was extensively made on selfed (F2-F4) and back-crossed progenies (BF1-BF5). Their chromosome number changes in a very wide range, 2n=42-67. Also the chromosome configuration is usually quite variable in the same plant, and varies even more from plant to plant, but there were obtained hyperhexaploids possessing 22II, 24II and 25II in the back-crossed offspring and also an octoploid plant with 28II in the F4 progeny. The offspring with 22II bred true, but the others did not, because some chromosomes occasionally or often failed to pair at meiosis. Besides, the author get two constant types, one is the euhexaploid plant (n=21), and the remaining is the unexpected diploid plant (n=7) as reported in his previous paper (NISHIYAMA 1933).
(3) Hybrid progenies showed a wide variation in their fertility and plant-vigour. It is especially mentioned that hexaploid derivatives with 21II and hyperhexaploids with 22II are highly fertile, but those with 24II and 25II are low or semi- sterile. All of these hyper-hexaploids show the semi-dwarf habit.
In conclusion, I have great pleasure in recording my gratitude to Professor H. KIHARA for much helpful criticism and guidance.
