Photoperiodic Adaptation to Latitude in Xanthium Strumarium

Photoperiodic responses of collections of Xanthium strumarium L., grown from seed obtained in nature at various localities in North America between latitudes 20° and 45.5° N, were examined. The critical night length was found to vary noticeably with latitude of origin, from about 7.5 hr in the northernmost strains to above 10 hr in the southern strains. These differences are considered to represent genetic adaptation of the reproductive system to environmental variables as a result of natural selection. Several strains showed a quantitative short day response rather than the more usual qualitative response. Strains from Hawaii exhibited a surprisingly tardy and erratic short‐day response with a critical night length of about 11 hr, which may be an adaptation to a tropical climate.