THE ECOLOGICAL SPECIALIZATION OF DICOTYLEDONOUS FAMILIES WITHIN A LOCAL FLORA: SOME FACTORS CONSTRAINING OPTIMIZATION OF SEED SIZE AND THEIR POSSIBLE EVOLUTIONARY SIGNIFICANCE

Abstract

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 Sheffield 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 those 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 (b) adaptive radiation of major taxa during the evolutionary history of the angiosperms may have been restricted by these relatively stable groups of characters.