No abstract

No abstract

There is widespread uncertainty about the nature and role of morphological plasticity in resource competition in plant communities. We have assayed the foraging characteristics of leaf canopies and root systems of eight herbaceous plants of contrasted ecology using new techniques to create controlled patchiness in light and mineral nutrient supply. The results are compared with those of a conventional competition experiment. Measurements of dry matter partitioning and growth in patchy conditions indicate a consistent positive association between the foraging characteristics of roots and shoots, supporting the hypothesis of strong interdependence of competitive abilities for light and mineral nutrients. Differences are identified in the abilities of dominant and subordinate plants to forage on coarse and fine scalcs. It is suggested that a trade-off exists in the scale ("high" in dominants) and precision (high in subordinates) with which resources are intercepted and that this trade-off contributes to diversity in communities of competing plants.

Comparative experiments measuring the form and extent of plant responses to environmental factors allow an ecologically important distinction to be made between morphological plasticity and cellular acclimation. The two phenomena are associated with different sets of plant traits with which they occur with highest fidelity and potency at opposite ends of productivity gradients. A trade-off between scale and precision occurs with respect to the responses of plants to patchiness in resource supply and this provides valuable predictors of the status achieved by species in plant communities.

SUMMARYVariation in seed size is an ecological paradox. Seed size, an important determinant of ecological distribution, is subject to considerable selection pressure, yet intra-family variation in seed size in the Sbeffield region of Central England is less than expected.Taxonomically-linked constraints of variation result from: number of ovules per carpel (solitary or numerous); placentation type (apical, basal or axile, parietal); presence or absence of endosperm in the ripe seed and the pattern of embryogenesis (Asterad, Chenopodiad or Caryophyllad, Onagrad, Solanad). These character states are not readily modified and tend to be conservatively expressed in families. The smaller-and larger-seeded families of the local flora are characterized by different combinations of these features. Larger-seeded families may be further subdivided into tbose from moist or relatively dry habitats. It is suggested that the presence of a starchy endosperm, an unusual feature in the dicotyledons, may allow the development of a high root/shoot ratio in the young seedling. From extrapolation, it is suggested that (a) optimization of ecologically important characteristics and (h) adaptive radiation of major taxa during the evolutionary history of the angiosperms may have been restricted by these relatively stable groups of characters.