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We employed high and low pollination treatments to examine the effect of varying offspring number on the allocation of resources to individual seeds in 5 female clones of the dioecious perennial, Canada thistle (Cirsium arvense (L.) Scop.). In comparison to high pollination plants, low pollination plants displayed a significantly reduced seed set, an unchanged rate of seed abortion, a significant increase in mean achene weight, and an altered pattern of achene weight allocation across primary, secondary, and tertiary flowerheads. These data do not support theoretical predictions for a uniform level of provisioning to individual seeds and are most consistent with the hypothesis that limitation of available resources constrains the level of provisioning to individual seeds. We discuss some of the assumptions underlying offspring provisioning theory in light of our results and discuss the usefulness ofthis theory as a descriptor of plant systems. We are indebted to the following people for their constructive comments on an earlier draft of this manuscript: R. V. Cartar, E. L. Charnov, I. Fleming, M. McGinley, and M. L. Reid. Thanks also go to M. Mather, B. Seehra, J. Brooks, and M. Boyes who helped in the processing of the flowerheads. This work was supported by a Natural Sciences and Engineering Research Council operating grant to BDR and an NSERC postgraduate scholarship to RGL.