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Flowers of 65 Canadian weeds were bagged to determine if external pollinators were needed for seed set. All of the 33 annual weeds studied, and 21 of the 23 biennials and caespitose perennials, set viable seed within pollination bags. None of the rhizomatous or stoloniferous perennial weeds studied set viable seed beneath the bags. It was concluded that self-fertilization in annual, biennial, and caespitose perennial weeds, and in a few instances, agamospermy in caespitose perennials, are factors that help make them successful weeds. Perennial weeds that can persist and spread in newly created disturbed habitats by virtue of their superior powers of vegetative reproduction are not self-compatible.

The chromosome numbers of 58 Canadian weeds are reported. Counts obtained on material of Ambrosia psilostachya DC. and Epilobium angustifolium L. differ from those obtained by other workers. The chromosome numbers of the following species had not previously been reported: Silene cserei Baumg. 2n = 24; Axyris amaranthoides L. 2n = 18; Erigeron philadelphicus L. 2n = 18; Lepidium ramosissimum Nels. 2n = 64; Dracocephalum parviflorum Nutt. 2n = 14; Rumex fennicus Murb. 2n = 40; R. occidentalis Wats. 2n = ca. 140; R. stenophyllus Ledeb. 2n = 60. The significance of some of these chromosome numbers is discussed.