Photoperiod evidence in the introduction of xanthium (cocklebur) to australia.

North American populations of cocklebur Xanthium strumarium L. are diverse in form and show critical dark periods ranging from 7.75 to 10.75 hours (critical photoperiods of 13.25 to 16.25 hours). South American cockleburs also are diverse in form and response. American cockleburs are adapted to diverse ecosystems, which suggests that they are indigenous. In contrast, the widespread Australian populations known as Noogoora Burr are all of the chinense morphological complex and respond uniformly to photoperiod. Plants from southern Louisiana and Australia are nearly identical in external form and in critical dark period of 10.5 hours. Probably the Australian Noogoora Burr and the Indian monsoon form arrived in their present countries after 1800 as contaminants in cottonseed from the Mississippi Delta region.