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.
Plant Succession on Central Texas Granite
The following study of plant succession was made on Enchanted Rock, a hill in the southern part of Llano County. This hill is one of several large pre-Cambrian intrusions of coarse-grained granite in the Central Mineral Region of Texas. A similar distribution of the vegetation has been found on all such intrusions visited. According to Paige ('I2), the reddish tone of the granite is largely due to potash feldspars in the f orm of microcline, orthoclase, and albite-oligoclase. In some of these, sodium 'replaces the potassium. Biotite, an iron-magnesia mica, and quartz are commonly present, while accessory minerals, such as magnetite, apatite, and titanite, usually occur. In general the region has a rather mild climate with a mean temperature of 67.30 F., ranging from 48.70 in January to 85.20 in July. Frequent " northers " occur from November until March, often with very warm intervening periods. The average length of the growing season is 247 days, usually with no killing frosts between March i6 and November i8. The altitude, which is only 1824 feet, is not a significant factor in temperature changes. The rainfall varies considerably, with an annual mean of 23.18 inches from i890 to i930. The lowest amount recorded was 9.9i inches in i9io and the highest 49.88 in i919. The prevailing winds of the region are from the southeast. Bray ('oi) discussed the general features of the vegetation of granitic areas and rocky slopes, but did not give a detailed analysis of the succession on granite.