Date of Award


Degree Type


Degree Name

Doctor of Philosophy


The bull thistle, Cirsium vulgare (Savi) Ten. is listed as a noxious weed in Ontario and in Australia. It is widespread and found in many disturbed habitats such as roadsides, quarries, construction sites and pastures. Successful weeds are considered to demonstrate, among others, the following features: the production of a large number of small seeds, long-lived seeds, the presence of an efficient dispersal mechanism, and self-compatibility. Although seed production is substantial and Cirsium vulgare is self-compatible, the species has several characteristics that should constrain its demographic success. Seed dispersal is limited and in past studies, this species had been dismissed as only having a persistent seed bank at depths of 15 cm. Furthermore, seeds do not show innate dormancy and are capable of germinating under light or dark conditions. I have shown that seeds can acquire an induced light requirement for germination under certain simulated overwintering conditions. Furthermore, this species is capable of forming a persistent seed bank at a depth of 3 cm or at the soil surface in highly vegetated areas. This is fortuitous, since the bull thistle, a monocarpic perennial, cannot reproduce in arable situations where the soil is cultivated on a yearly basis. Preliminary observations of unusually coloured (black) seeds from some populations in southwestern Ontario led to the development of the present investigation. Genetic polymorphism for seed colour is described for the first time. Most plants produce the typical seed as characterized in the literature, yellow or gray with longitudinal dark striae. Black-seeded plants were observed in three of the four populations surveyed in southwestern Ontario. The frequency in these three populations ranged from 1.2% to 8.3%. Chemical assays showed that the total phenolic content is not associated with seed colour, but only black seeds contain tannin. Black seeds may act as allelopathic agents. Preliminary experiments show that extracts from black seed coats may inhibit germination and reduce the germination rate in other plant species.



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