Patterns of Change in Seed Weight Over Time on Individual Plants

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Abstract
Seeds from individual plants of 8 herbaceous species [Amaranthus blitoides, A. retroflexus, Chenopodium album, Diplotaxis tenuifolia, Melilotus alba, Onopordum acanthium, Verbascum blattaria, V. thapsus] exhibited the same general pattern over a period of time; the largest seeds were produced first and then progressively smaller seeds ripened as the plants aged. The difference between the mean weight of the largest seeds and the mean weight of the smallest was significant (P < 0.05) for all 8 spp. Such differences averaged as much as 25% for 1 spp. (M. alba) and were much greater for the seeds of some individual plants within certain species. All 8 spp. are short-lived weeds, all grew in London, Canada, and all but 1 were in naturally occurring populations. For some species (e.g., D. tenuifolia) all individual plants exhibited more or less the same patterns of seed weight change over the season, but for other species (e.g., O. acanthium) there were great differences between plants in seed weight patterns. In a separate study also made in London, Ontario, Canada, 10 spaced plants of A. retroflexus were grown in a garden free of interference from other plants. Ripe seeds were collected on 7 occasions for each plant. With all plants there was a significant decrease in mean weight per 100 seeds over the growing season. Some differences in seed weight between populations reported by other authors could be partly or wholly accounted for by differences in the maturity of the plants being sampled. The great differences between individuals with respect to seed weight patterns suggest that any comparisons of seed weight between populations should be based on collections from a large number of plants in each habitat.