Seed mass is considered to be the least plastic component of reproductive yield. Yet, in invasive populations of garlic mustard, Alliaria petiolata, seed mass was highly variable (eightfold among populations, 2.5-7.5 fold within populations, two-threefold within individuals, and 1.4-1.8 fold within fruits). Variation in seed mass among populations explained nearly half of the total variance. Variation among seeds within fruits accounted for a further 25% of variance. Individual seed mass within a plant decreased with increased distance from the main stem, suggesting that access to parental resources limits seed size in a predictable manner. MANOVAs and Roy-Bargmann stepdown analyses revealed significant effects of seed mass, but not seed position (within a fruit or within an infructescence), on an array of subsequent seedling traits. Smaller seeds germinated significantly earlier, and seedlings from small seeds produced their first primary leaves significantly later and grew significantly taller. After accounting for seed mass as a covariate, only one seedling trait, date of first leaf emergence, was affected by seed position in a fruit. Differences in seed mass may therefore affect seedling recruitment via effects on early seedling growth in this weedy species.

This study reports the bioconcentration and the uptake and elimination kinetics of a series of nonreactive, hydrophobic organic substances in the submerged aquatic macrophyte Myriophyllum spicatum. The tested substances represent a wide range of aqueous solubilities and 1-octanol-water partition coefficients (/fow). The plantwater bioconcentration factor is shown to follow a linear relationship with the octanol-water partition coefficient for all chemicals, including the superhydrophobic chemicals with log Kow up to 8.3. The uptake and elimination rate constants tend to follow a "biphasic" relationship with 0 . A kinetic model is developed for organic chemical bioconcentration in submerged aquatic macrophyte species. This model is applied to the Detroit River and Lake St. Clair to illustrate the role of aquatic macrophytes in chemical dynamics in aquatic systems.

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The population genetic structure and spatial genetic structure of the dioecious aquatic macrophyte Vallisneria americana was examined near Turkey Island in the Detroit River in late summer of 1991. The density and distribution of male, female, and vegetative ramets was determined for three transects parallel, and three transects perpendicular to the island, at mean water depths of 38–306 cm. A strong male‐biased sex ratio was observed in shallow water, while sex ratios became increasingly female‐biased, and plant density increased, at greater water depths. Cellulose acetate gel electrophoresis was used to characterize the allozyme phenotype of each ramet that had been screened for seven enzyme systems. Overall, 91 allozyme phenotypes were identified. A single allozyme phenotype accounted for 33%–55% of all the ramets (depending on the transect), indicating extensive regional vegetative growth. However, the population as a whole displayed high genetic diversity (HT = 0.3403), with most of the diversity occurring within transects (Hs = 0.3297) rather than between transects (DST = 0.0106). The possible factors accounting for the high small‐scale genetic diversity of this clonal aquatic plant species are discussed.

Arisaema dracontium (green dragon) is a perennial herb that is widely distributed in eastern North America. However, in Canada, at the northern edge of its distribution, the species is designated as "vulnerable" with respect to conservation status. In natural populations, seedlings are uncommon; the present study was undertaken in order to characterize seed and seedling properties in green dragon. Seeds were sampled from five sites, ranging from Ontario at the northern limit of the distribution range, to Louisiana in the south. Seed germinability ranged from 25 to 55%, depending upon source. Experiments indicated that neither the hard seed coat nor a water-soluble exudate from the seed was responsible for inducing or maintaining dormancy. Patterns of seed germination appear to reflect general climatic conditions at the sites where seeds had originated. Cold stratification at 3°C produced significantly greater relative germinability in all seed collections except the most southerly one, from Baton Rouge. These seeds also had a slower overall speed of germination. In contrast, germination of seeds from the most northerly site was promoted by cold stratification and occurred over a relatively brief period. Germination in alternating light and dark conditions decreased the speed of germination compared to germination in the dark, however exposure to light changed the phenology of germination by promoting development of adventitious roots and primary leaves in these seedlings.

The effects of population density on module demography were studied in Trapa natans L., an annual aquatic macrophyte capable of extensive clonal propagation. At low density, the floating plants produced ten times as many ramets (clonal offshoots) as those at high density. Module mortality occurred at three levels: leaf, ramet (shoot), and genet (genetic individual). There was 100‐fold variation in the size of nuts containing germinable seeds. In early summer there was a highly significant linear relationship between dry mass of nuts and the total mass of ramets that each had produced. In early summer most (73–83%) of the variation in total plant biomass was attributable to variation in initial nut size. However the significance of initial nut size was diminished later in the season. The great success of the exotic weed T. natans at colonizing and monopolizing an aquatic habitat is a function of its highly productive clonal growth response to low‐density conditions, combined with greater proportional allocation of biomass to reproductive structures, resulting in greatly increased nut production at low initial density. The species appears able to develop and maintain a population at extremely high density: plant buoyancy and the production of large, well‐protected nuts allow rapid early growth from the sediment each year and early pre‐emption of the water surface.

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