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We conducted a four-year field competition experiment involving Lythrum salicaria and the North American wetland species that it occasionally displaces, Typha angustifolia. The design placed Lythrum and Typha in a modified replacement series experiment having four starting densities (64, 36, 16, and 1 plant(s) per quadrat) and four relative proportions of each species (i.e., 0.25, 0.50, 0.75, or 1, [for densities 64, 36 and 16 only]). Results of a repeated measures ANOVA indicate that overall rate of ramet production (ORP) differed significantly between species, between density treatments and between years. We also found evidence of significant intra-specific competition in both species. In the first year of the experiment, ORP in Typha was greater than that in Lythrum. However, from the second year onward, this situation was reversed and ORP in Typha was much lower than that in Lythrum. Analysis of covariance indicates a significant effect of species proportion on ORP, suggesting that both intra-, and inter-specific competition were occurring. Log-transformed ratios of input/output (i.e. proportions of Lythrum/Typha established at the beginning of the experiment, divided by the proportions found at the end of each growing season) demonstrated that in the first year Typha gained advantage in the mixtures, while by the second and third years the output ratio line crossed the 45° threshold, suggesting the two species could coexist. However, by the fourth year, the output ratio line was entirely above the 45° line, suggesting an overall advantage to Lythrum in the mixtures. The experiment clearly demonstrates temporal development in competitive behaviour, and suggests that Lythrum may take at least four years before becoming competitively dominant in a wetland community. The results also suggest that short-term experiments should be interpreted with care, if they are to be used to forecast the long-term outcome of competition in nature.

Summary 1. The demographic effects of an organochlorine contaminant on the dioecious aquatic macrophyte Vallisneria americana are reported. Plants were sampled from two natural populations in the Huron-Erie corridor of the Great Lakes Basin, and placed in experimental treatments of high (396 mg per litre), medium (132 mg per litre), low (66 mg per litre) and zero concentrations of trichloroethylene (TCE). Plant responses were determined in terms of genet mortality (i.e. death of the entire genetic individual), ramet production (clonal growth) and mortality, leaf birth- and death rates, leaf area and sexual reproduction. 2. TCE in the water caused significant ramet and genet mortality, and reduced the growth of surviving plants. Control plants produced significantly more leaves than contaminated plants. Contaminant-exposed plants continued producing their leaves for 4-6 weeks longer than controls, and showed greater rates of leaf death. The total leaf area of genets exposed to TCE was significantly lower than that of unexposed plants. TCE was associated with a significant decrease in flower production. 3. The concentration of TCE was determined in plant tissues, water and sediment. Levels of TCE were greatest in the sediment, and sediment originating from each site adsorbed TCE to differing degrees. TCE was more concentrated in the underground storage tissues (roots and turions) than it was in leaves. Root and turion tissues of plants from one site contained significantly less TCE than those of plants from the other site, despite an identical concentration of TCE in the water; differences may have been due to contrasting sediment affinities for TCE, and/or different rates of TCE accumulation by the plants from each site. 4. TCE was bioaccumulated, particularly in roots. This appeared to be attributable to the fact that TCE accumulated, in the first instance, in sediments. The logIo of TCE concentrations in sediment were linearly correlated with TCE concentrations in roots, suggesting that TCE uptake is through the root system rather than the leaves, and that uptake is driven by the concentration gradient between sediment and plant. 5. Plants that survived the TCE treatment appeared to be resistant to the chemical; they were as likely to flower as surviving untreated plants. Males were more likely than females to survive TCE treatment. 6. It is concluded that modular demography can be used to characterize the performance of plants that are stressed by exposure to various concentrations of the organochlorine solvent trichloroethylene, and that plants from different sites differ in their ability to survive contaminant stress.

CONCEPTUAL ISSUES IN PLANT BREEDING SYSTEMS: J. and L. Lovett-Doust: Sociology of plants - an emerging synthesis Robert Bertin: Paternity in Plants David Haig and Mark Westoby: Inclusive fitness, seed resources, and maternal care Paul Cox: Monomorphic and dimorphic sexual strategies - a modular approach Spencer Barrett: The evolution, maintenance, and loss of self-incompatability systems Mark Schlessman: Gender diphasy ("sex choice") ECOLOGICAL FORCES: Michael Zimmerman: Nectar production, flowering phenology, and strategies for pollination Tom Lee: Patterns of fruit and seed production Don Waller: Plant morphology and reproduction Jacob Weiner: The influence of competition on plant reproduction Steve Hendrix: Herbivory and its impact on plant reproduction REPRODUCTIVE STRATEGIES OF NON-ANGIOSPERMS: Robert De Wreede and Terry Klinger: Reproductive strategies in algae Brent Mishler: Reproductive ecology of bryophytes Michael Cousens: Reproductive strategies of pteridophytes.

SUMMARY (1) The density and distribution of European canker (Nectria galligena) on all combinations of four scion and interstock cultivars in an orchard of 10 yr old trees in the Lower Mainland of British Columbia were determined. (2) There was no effect of interstock cultivar upon tree mortality, but there were large scion cultivar effects. The numbers of cankers and degree of aggregation of cankers on surviving trees followed a pattern similar to that of tree mortality. Trees of the cultivars 'Summerland Red McIntosh' and 'Harrold's Delicious' were quite susceptible to buildup of canker and subsequent death, while 'Spartan' and 'Golden Delicious' proved relatively resistant. (3) Multiple regression analysis indicated that the number of cankers per tree was significantly related to the sum of the cankers on the eight surrounding neighbours of each tree. (4) Trunk cross-sectional area was significantly correlated with the number of cankers per tree on McIntosh and Spartan trees. However, it was shown that differences in tree size alone did not account for the differences in numbers of cankers on the four scion cultivars. (5) It was concluded generally that scion cultivar and European canker may act in conjunction and result in a heterogeneously stunted apple orchard.