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.
Granite Outcrop Communities of the Piedmont Plateau in Georgia
Communities in depressions, occurring as islands surrounded by naturally exposed granitic rock, were classified by the correlated of maximum soil depth and characteristic vegetation cover into: diamorpha communities, 2—6 cm; lichen—annual herb communities 16—39 cm; and herb—shrub communities, 40—50 cm. The total flora of the 40 island communities, included 76 species, but only 39 of these occurred in five or more communities and were considered characteristic. The extreme environmental conditions of high light intensity and low soil moisture content were indicated by the fact that of the 20 plants present in over 50% of the communities, 18 were lichens, mosses, and flowering annuals and only two were perennial flowering plants. The flora of the island communities is distinct from that of border vegetation. Only three endemics are among the 39 characteristics plants. Seasonal aspects of the outcrop vegetation are striking. During the winter months, November through February, mosses and lichens are most con...
Physiological Factors Affecting Composition of Soybeans. I. Correlation of Temperatures During Certain Portions of the Pod Filling Stage with Oil Percentage in Mature Beans1
THE rapid increase in the importance of the soybean as an agricultural product in recent years has been due in considerable measure to its high oil content and to the usefulness of this oil as a constituent of food and industrial products. While there have been extensive research programs to develop new varieties that will produce more oil, and to develop new uses for the oil, there has been relatively little study of the physiological factors that affect oil formation, i.e., the response of the oil forming mechanism to environment. That elements of the climate might affect the composition of soybeans was suggested many years ago by Garner, et al. (2), who concluded that under usual conditions climate is a more potent factor than soil in controlling size and oil content of soybean seeds. Viljoen (9) studied the relation of temperature during the growing season to oil content at maturity. He found no correlation between maximum temperatures and oil percentage. He did, however, find a highly significant correlation between mean minimum temperature and oil percentage, and between mean temperature and oil percentage. These correlations existed for two varieties and two growing seasons at from 9 to 12 field locations in South Africa. The temperatures of the different locations ranged from a monthly mean maximum of 91.3° to a monthly mean minimum of 46.0°. The extreme temperatures for single days were 102.2° and 34.5°. There was also a wide variation in rainfall at the different locations, the total for the growing season ranging from 1.88 in. to 25.67 in. Dillman and Hopper (1) determined correlations between oil percentages of four varieties of flax and July temperatures at a group of nonirrigated and irrigated stations. For the nonirrigated stations, the correlation coefficients of oil percentage with average maximum, average minimum, and average mean were negative and were significant at the .01 probability level for all four varieties. For the irrigated stations, the only significant correlation coefficients were negative coefficients for two varieties with average minimum (P = .01) and the same two varieties with average mean (P = .05). It thus appeared that the