GENECOLOGICAL STUDIES OF GROUNDSEL (SENECIO-VULGARIS L) .3. POPULATION VARIATION AND ITS MAINTENANCE IN THE UNIVERSITY-BOTANICAL-GARDEN, CAMBRIDGE

In order to investigate population variation and its maintenance in groundsel (Senecio vulgaris L.) growing in the Cambridge University Botanic Garden, experiments have been carried out to study heritable differences in the rate of development of progenies of seed-parents from sites of different history and land use in and round the Garden. There were significant differences between progenies of seed-parents and between samples from different flowerbeds but the oldest beds did not have the quickest developing plants. Such a pattern of behaviour may be related to the fact that all the beds have been well maintained since the 1950s. Since its establishment, two distinctive genetic variants have entered the Garden. On average, progenies from radiate and from simazine-resistant seed-parents in the study area were significantly slower to develop than non-radiate simazine-sensitive plants but, in both cases, we also found plants of intermediate rate of development, which might be the result of hybridization between the 'immigrants' and the precociously developing non-radiate, herbicide-sensitive plants. Progenies from seed-parents collected at two sites just outside the boundaries of the Botanic Garden were slower in their rate of development than those from the intensively hand-weeded flowerbeds of the Garden. Samples collected from the same area in different seasons, and grown at the same time, did not differ overall in rate of development, but there were significant differences within flowerbeds. Subsets of progenies of a set of seed-parents were grown both in summer and winter. The rank order of maturity was not the same in the two parts of the experiment, indicating that the progeny of different seed-parents have different relative fitness in different times of year. This behaviour, in a species which is capable of reproducing at all seasons of the year, could be a major factor in the maintenance of population variation in the Garden proper.