Hybridization between Senecio smallii and S. tomentosus (Compositae) on the granitic flatrocks of the Southeastern United States
Senecio smallii andS. tomentosus occasionally hybridize on the granitic flatrocks of the Piedmont of the Southeastern United States. These hybrids are intersectional crosses between sectionsAurei andTomentosi. The parental species have different chromosome numbers withn = 22 inS. smallii andn = 23 inS. tomentosus. Meiosis in the hybrids is abnormal with 22ns and li or 2iis and 3is, and the percentage of stainable pollen is greatly reduced over that of the parents. The hybrid pollen grains are structurally abortive as demonstrated by the scanning electron microscope. Origin of the natural hybrids was documented by the production of artificial hybrids including F1’s, F2’s, and BC’s. Very few of the F2 seeds germinated, and all of the F2 seedlings later died, indicating hybrid breakdown. Seeds of the F1 and BC generations germinated well, and the seedlings were vigorous. Population samples indicate that there is some variation within the natural population systems; however, no evidence of introgression was detected. The isolating mechanisms and possible evolutionary effects of this natural hybridization are discussed.
Reproductive ecology of granite outcrop plants from the south-eastern United States
Most of the Piedmont Physiographic Province of the southeastern United States is covered with mixed mesophytic forest of oaks, hickories, and pines. Within this “sea”, however, are “islands” of exposed granite and gneiss. A characteristic, and largely endemic, assemblage of plants has adapted to the environmental extremes that bare rock provides by strongly altering their morphology, physiology, and life history. With respect to their reproductive ecology, however, these plants appear very similar to their congeners and to the Piedmont flora as a whole. Except for ant-pollinated Diamorpha smallii, most species show the expected range of pollen vectors, including wind, bees, flies, butterflies, moths and one species of hummingbird. Fruit and/or seed dispersal appears to be highly localised and effected primarily by wind and water. If anything, most species appear to possess adaptations against long-distance dispersal, which would carry propagules into the inhospitable matrix of oak-hickory-pine forest. Mating systems are variable, including examples of both self-compatible and self-incompatible taxa. Consistent with the expectation of low gene flow between populations on isolated outcrops, genetic data show strong differentiation and suggest the potential for genetic drift and/or natural selection to result in divergence. Some of the endemic species on granite outcrops have originated by allopolyploidy, whereas others appear to represent products of more gradual divergence in geographical isolation. There is reason to believe that some weedy species of early successful sites were originally restricted to granite outcrops and spread more recently to sites disturbed by human activities.
Investigations on the rôle of organic matter in plant nutrition
1. As the result of decomposition of green manure in a swamp soil, the coarse fractions (including coarse and fine sand) of the surface layer (first 3 inches) decreased by 11·2 per cent. while the fine ones (silt+clay) increased by 9·8 per cent. in the course of 43 days. The effect was less pronounced in the manured soil maintained at 60 per cent. saturation, while no alteration was to be noticed in the case of the unmanured specimens. 2. Treatment with dilute hydrochloric acid prior to dispersion had no appreciable effect on the estimates of mechanical composition obtained in the different cases. Loss in solution was also comparatively small. 3. The silica-sesquioxide ratio increased slightly as the result of decomposition of green manure in swamp soil. The composition of the fine fractions was also altered, the total percentages of the oxides of iron, aluminium and silicon increasing from 90·5 to 99·3 in the course of 71 days. 4. The practical significance of the above and related observations has been discussed.