RAINFALL PERIODICITY AS A MAJOR FACTOR IN THE FORMATION OF FLOWERING RACES OF CAMPHORWEED (HETEROTHECA SUBAXILLARIS)

Plants of camphorweed (Heterotheca subaxillaris sensu latiore) were raised in a uniform garden at Northampton, Mass., from seed taken from 19 sites in different parts of the range of the species. Flowering races were observed. Flowering time at Northampton correlated with periodicities of rainfall at the seed source in most instances. The earliest-flowering plants were obtained from areas where above-average amounts of rainfall occur in spring, while the later-flowering plants came from sites where above-average rainfall occurs in mid- to late summer. Other workers have observed a correlation between the frost-free season and the formation of flowering races in prairie grasses, Eupatorium rugosum, and other plants. There is little correlation between the frost-free season and the formation of flowering races in Heterotheca and it is postulated that, within the range of camphorweed, periodicity of rainfall is a more critically limiting factor.