Uptake of CO2 by Roots of Xanthium Plants

1. The magnitude of CO2 uptake and fixation by Xanthium plant roots was reinvestigated in an attempt to explain unexpectedly low C14/C12 ratios of tree leaves with regard to fixation of radiocarbon of nuclear-test origin. 2. It was found that plants derive relatively insignificant amounts of carbon from CO2 via the roots, and the minute amounts that are incorporated are approximately one hundred times less than the amount lost by root respiration. 3. The results permit the conclusion that the previously noted C14/C12 ratios of the tree leaves were most likely obtained by incorporation of carbon from sources stored in the tree.