Abstract
Oenothera erythrosepala used to grow on the floodplain of the Tama River; however, it is scarcely found nowadays. Oenothera erythrosepala showed a wide distribution compared to Oenothera biennis and Oenothera laciniata, having the same genus. Their germination properties were studied to determine the factor for the above facts and elucidate the relationship between changes in vegetation on the Tama River floodplain and an increase and a decrease of the plants of the genus. At our germination experiments it is presumed that Oenothera erythrosepala has no gap-detecting mechanism unlike Oenothera biennis and Oenothera laciniate.It is reasonably concluded that the reproductive strategy of Oenothera erythrosepala cannot cope with the current situation of the floodplain, where bare land has decreased and vegetation has increased, and it has resulted in its decline.
