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941.
Previous tree-ring studies indicated that the total area affected by drought in the western United States has rhythmically expanded and contracted over the past 300 years, with a period near the 18.6-year lunar nodal and 22-year double-sunspot cycles. Recently collected tree-ring data from the U.S. Corn Belt for the years 1680 to 1980 were examined for evidence of either of these cycles on a regional scale. Spectral analysis indicated no periodicity in the eastern part of the Corn Belt, but a significant 18.33-year period in the western part. The period length changed from 17.60 to 20.95 years between the first 150 years and the last 151. High-resolution frequency analysis showed that the structure of the 18.33-year spectral peak was complex, with contributions from several frequencies near both the lunar nodal and double-sunspot periods. A t-test of difference of means in reconstructed annual precipitation weakly corroborated a previous finding of an association between drought area and the phase of the double-sunspot cycle. Both the high-resolution frequency analysis and the t-test results indicate that the periodic component of drought near 20 years is too weak and irregular to be of use in drought forecasting for the Corn Belt. 相似文献
942.
Diploid and diploid-triploid mosaic individuals of Platemys platycephala were found in natural populations. In mosaic specimens, the blood, spleen, liver, and testis contained both diploid and triploid cells. The ratio of triploid to diploid cells was more variable among individuals than among somatic tissues within an individual. Only diploid cells underwent meiosis in males; haploid gametes were produced. There appears to be geographic variation for mosaicism in that only diploids were found in Bolivia, whereas diploids and diploid-triploid mosaics occured in Surinam. 相似文献
943.
Maugh TH 《Science (New York, N.Y.)》1985,227(4694):1570-1571
944.
Maugh TH 《Science (New York, N.Y.)》1985,227(4694):1569-1570
945.
946.
Sun M 《Science (New York, N.Y.)》1985,230(4727):738
947.
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950.
Stable carbon isotope ratios of organic matter in rock varnishes of Holocene age from western North America and the Middle East show a strong association with the environment. This isotopic variability reflects the abundance of plants with different photosynthetic pathways in adjacent vegetation. Analyses of different layers of varnish on late Pleistocene desert landforms indicate that the carbon isotopic composition of varnish organic matter is a paleoenvironmental indicator. 相似文献