首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   152篇
  免费   2篇
  2013年   2篇
  2012年   3篇
  2011年   4篇
  2009年   3篇
  2008年   3篇
  2007年   3篇
  2003年   1篇
  2000年   4篇
  1999年   1篇
  1998年   4篇
  1997年   4篇
  1996年   1篇
  1995年   4篇
  1994年   7篇
  1993年   1篇
  1992年   2篇
  1990年   4篇
  1989年   1篇
  1988年   1篇
  1987年   2篇
  1986年   1篇
  1985年   1篇
  1984年   1篇
  1982年   3篇
  1980年   4篇
  1979年   3篇
  1978年   1篇
  1977年   1篇
  1976年   1篇
  1975年   1篇
  1973年   3篇
  1972年   4篇
  1971年   2篇
  1968年   1篇
  1966年   1篇
  1961年   1篇
  1960年   1篇
  1959年   4篇
  1958年   5篇
  1957年   4篇
  1956年   7篇
  1955年   3篇
  1954年   8篇
  1953年   2篇
  1952年   5篇
  1951年   10篇
  1950年   7篇
  1949年   4篇
  1948年   5篇
  1946年   3篇
排序方式: 共有154条查询结果,搜索用时 15 毫秒
101.
Historians and anthropologists have credited the invention of trapping beaver with castoreum bait and steel traps to the Indians of southeastern Canada in the 1790s. However, English trappers in Virginia used this technique before 1728, and the Indians of southeastern Canada were using it by 1756. It is suggested that the widespread adoption of castoreum and steel traps by Northeastern Indians in the 1790's was due to the collapse of the world fur market after the French Revolution.  相似文献   
102.
103.
104.
Intertidal molluscs on sandflats in Oyster Harbour and PeelInlet, Western Australia, were compared using transects. Twenty-fourspecies of molluscs were recorded in Oyster Harbour and only7 in Peel Inlet. Two groupings were foundin Oyster Harbour:a midlittoral component and infralittoral fringe. There wasno vertical community structure in Peel Inlet. Molluscs retainedon a 1 mm sieve had about the same mean density and biomassin the two areas but molluscs retained on a 2 mm sieve had amean density of 203/m2 and a biomass of 132 g/m2 in Oyster Harbourand were absent in Peel Inlet. Several possible reasons forthe absence of large molluscs in Peel Inlet are discussed. PeelInlet is on the west coast in a faunal overlap zone where thenumber of species is reduced. The mean temperature in Peel Inletis 2-3°C higher than in Oyster Harbour and salinity variationsare much more pronounced. (Received 20 November 1979;  相似文献   
105.
106.
107.
Ko?odziej, B., Golubic, S., Bucur, I.I., Radtke, G. & Tribollet, A. 2011: Early Cretaceous record of microboring organisms in skeletons of growing corals. Lethaia, Vol. 45, pp. 34–45. A spectacularly preserved assemblage of microbial euendoliths, penetrating into skeletons of growing scleractinian corals, has been recognized in Early Aptian (Early Cretaceous) reef limestone of the Rar?u Mountains (East Carpathians, NE Romania). Microboring euendolithic filaments were found in five coral colonies of the suborder Microsolenina. They remained in part well‐preserved, often impregnated with iron oxides, which made them visible even in strongly recrystallized parts of coral skeletons. Filaments of a wide range of sizes (2–40 μm in diameter) were concentrated within medium parts of coral septa, oriented along the septa in the direction of the coral growth. The larger filaments were tubular, occurring in bundles and branched into finer, often tapering branches. Their behaviour and organization were quite similar to the modern euendolithic siphonalean chlorophyte Ostreobium. Filament diameters exceeded those reported for the modern species, but covered a similarly wide size range. Narrower frequently branching filaments, 4–8 μm in diameter, resemble distal branching patterns of modern Ostreobium quekettii. Some very thin filaments (ca. 1–2 μm) observed within skeleton or inside the large tubular filaments, sometimes associated with globular swellings, may represent euendolithic fungi. The recrystallization of coral skeleton had limited effect on preservation of euendoliths due to their impregnation with iron oxides; microbial euendoliths were subjected to different taphonomic changes. □Chlorophytes, Early Cretaceous, fungi, microbial euendoliths, Romania, scleractinian corals.  相似文献   
108.
109.
We report the phylogeographic pattern of the Patagonian and Subantarctic plant Hypochaeris incana endemic to southeastern South America. We applied amplified fragment length polymorphism (AFLP) and chloroplast DNA (cpDNA) analysis to 28 and 32 populations, respectively, throughout its distributional range and assessed ploidy levels using flow cytometry. While cpDNA data suggest repeated or simultaneous parallel colonization of Patagonia and Tierra del Fuego by several haplotypes and/or hybridization, AFLPs reveal three clusters corresponding to geographic regions. The central and northern Patagonian clusters (∼38–51° S), which are closer to the outgroup, contain mainly tetraploid, isolated and highly differentiated populations with low genetic diversity. To the contrary, the southern Patagonian and Fuegian cluster (∼51–55° S) contains mainly diploid populations with high genetic diversity and connected by high levels of gene flow. The data suggest that H. incana originated at the diploid level in central or northern Patagonia, from where it migrated south. All three areas, northern, central and southern, have similar levels of rare and private AFLP bands, suggesting that all three served as refugia for H. incana during glacial times. In southern Patagonia and Tierra del Fuego, the species seems to have expanded its populational system in postglacial times, when the climate became warmer and more humid. In central and northern Patagonia, the populations seem to have become restricted to favourable sites with increasing temperature and decreasing moisture and there was a parallel replacement of diploids by tetraploids in local populations.  相似文献   
110.
  • 1 This paper develops a framework of spatial and temporal variability for a habitat typology of the Upper Rhône River (France) and its alluvial floodplain that is based on about 17 years of data collection and analysis. The aim was to provide a scale of spatial-temporal variability for river habitat templet predictions on trends in species traits and species richness.
  • 2 In developing this framework, eight physical-chemical variables were available and could be considered for twenty-two habitat types: seventeen superficial (surface) and five interstitial (0.5 m below the substrate surface). These habitat types were selected in two areas (Jons and Brégnier-Cordon) after geomorphological considerations and because of differences in their biological characteristics.
  • 3 The data sets used were processed by a ‘fuzzy coding’ method using, for each variable, the frequency distribution (by modalities = categories) of all measurements and monthly means over an annual scale. Two tables were produced; the first corresponded to an expression of the total variability, and the second represented an evaluation of the temporal variability.
  • 4 Each of these tables was analysed by correspondence analysis, which provided factorial scores that were used to calculate, by habitat type and by variable, a total variability and a temporal variability in terms of cumulated variability of factorial scores for the eight physical–chemical variables. The rationale in describing variability from these two tables is that total variability equals temporal variability plus spatial variability. The spatial variability was then determined by the difference between total and temporal variability. From this procedure, a positioning of the twenty-two habitat types on the spatial and temporal variability axes was obtained.
  • 5 The estimate of spatial variability did not consider any error term that may have occurred in the above model; it was then tested by an independent assessment of the spatial variability using thirteen variables in nine major habitat types. A high correlation between the two ways of assessing spatial variability (r = 0.85, P < 0.004) underscored the reliability of the spatial variability that was calculated previously.
  • 6 The river habitat templet obtained for the Upper Rhône and its alluvial floodplain appears to be appropriate to test the predictions on patterns of species traits and species richness in the framework of spatial and temporal variability.
  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号