全文获取类型
收费全文 | 442篇 |
免费 | 14篇 |
专业分类
林业 | 8篇 |
农学 | 33篇 |
基础科学 | 2篇 |
56篇 | |
综合类 | 78篇 |
农作物 | 26篇 |
水产渔业 | 19篇 |
畜牧兽医 | 212篇 |
园艺 | 5篇 |
植物保护 | 17篇 |
出版年
2020年 | 9篇 |
2019年 | 4篇 |
2018年 | 12篇 |
2017年 | 6篇 |
2015年 | 9篇 |
2014年 | 6篇 |
2013年 | 13篇 |
2012年 | 16篇 |
2011年 | 23篇 |
2010年 | 19篇 |
2009年 | 9篇 |
2008年 | 15篇 |
2007年 | 13篇 |
2006年 | 13篇 |
2005年 | 11篇 |
2004年 | 9篇 |
2003年 | 14篇 |
2002年 | 11篇 |
2001年 | 14篇 |
2000年 | 17篇 |
1999年 | 9篇 |
1998年 | 5篇 |
1997年 | 5篇 |
1996年 | 5篇 |
1995年 | 6篇 |
1992年 | 13篇 |
1991年 | 11篇 |
1990年 | 10篇 |
1989年 | 6篇 |
1988年 | 13篇 |
1987年 | 8篇 |
1986年 | 6篇 |
1985年 | 10篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1976年 | 3篇 |
1975年 | 3篇 |
1973年 | 4篇 |
1972年 | 5篇 |
1971年 | 4篇 |
1970年 | 4篇 |
1969年 | 6篇 |
1967年 | 5篇 |
1966年 | 4篇 |
1963年 | 4篇 |
1941年 | 3篇 |
1939年 | 5篇 |
排序方式: 共有456条查询结果,搜索用时 203 毫秒
441.
Bauder Javan M. Chandler Houston C. Elmore Michele L. Jenkins Christopher L. 《Landscape Ecology》2022,37(10):2519-2533
Landscape Ecology - Wildlife distributions are often subdivided into discrete conservation units to aid in implementing management and conservation objectives. Habitat suitability models,... 相似文献
442.
TC Johnson CA Scholz MR Talbot K Kelts RD Ricketts G Ngobi K Beuning I Ssemmanda JW McGill 《Science (New York, N.Y.)》1996,273(5278):1091-1093
Lake Victoria is the largest lake in Africa and harbors more than 300 endemic species of haplochromine cichlid fish. Seismic reflection profiles and piston cores show that the lake not only was at a low stand but dried up completely during the Late Pleistocene, before 12,400 carbon-14 years before the present. These results imply that the rate of speciation of cichlid fish in this tropical lake has been extremely rapid. 相似文献
443.
Jenkins RV 《Science (New York, N.Y.)》1976,192(4240):657-658
444.
445.
Jenkins MM 《Science (New York, N.Y.)》1963,142(3596):1187
Four long bipolar planarians were discovered in a stock culture of asexual Dugesia dorotocephala. Such worms have not been previously reported. Eyespots, auricles, and responses to stimuli were normal in each head-end, and functional pharynges were present. Worms in the stock culture had not been subjected to temperature variations, chemicals, or other experimentation. It is suggested that the axial gradient was effectively flattened by a normal head-inhibiting factor being unable to diffuse to the posterior end. 相似文献
446.
Accurate face recognition is critical for many security applications. Current automatic face-recognition systems are defeated by natural changes in lighting and pose, which often affect face images more profoundly than changes in identity. The only system that can reliably cope with such variability is a human observer who is familiar with the faces concerned. We modeled human familiarity by using image averaging to derive stable face representations from naturally varying photographs. This simple procedure increased the accuracy of an industry standard face-recognition algorithm from 54% to 100%, bringing the robust performance of a familiar human to an automated system. 相似文献
447.
The discovery of a turtle in the Early Jurassic(185 million years before present) Kayenta Formation of northeastern Arizona provides significant evidence about the origin of modern turtles. This new taxon possesses many of the primitive features expected in the hypothetical common ancestor of pleurodires and cryptodires, the two groups of modern turtles. It is identified as the oldest known cryptodire because of the presence of a distinctive cryptodiran jaw mechanism consisting of a trochlea over the otic chamber that redirects the line of action of the adductor muscle. Aquatic habits appear to have developed very early in turtle evolution. Kayentachelys extends the known record of cryptodires back at least 45 million years and documents a very early stage in the evolution of modern turtles. 相似文献
448.
Caspersen JP Pacala SW Jenkins JC Hurtt GC Moorcroft PR Birdsey RA 《Science (New York, N.Y.)》2000,290(5494):1148-1151
Carbon accumulation in forests has been attributed to historical changes in land use and the enhancement of tree growth by CO2 fertilization, N deposition, and climate change. The relative contribution of land use and growth enhancement is estimated by using inventory data from five states spanning a latitudinal gradient in the eastern United States. Land use is the dominant factor governing the rate of carbon accumulation in these states, with growth enhancement contributing far less than previously reported. The estimated fraction of aboveground net ecosystem production due to growth enhancement is 2.0 +/- 4.4%, with the remainder due to land use. 相似文献
449.
Ma'ayan A Jenkins SL Neves S Hasseldine A Grace E Dubin-Thaler B Eungdamrong NJ Weng G Ram PT Rice JJ Kershenbaum A Stolovitzky GA Blitzer RD Iyengar R 《Science (New York, N.Y.)》2005,309(5737):1078-1083
We developed a model of 545 components (nodes) and 1259 interactions representing signaling pathways and cellular machines in the hippocampal CA1 neuron. Using graph theory methods, we analyzed ligand-induced signal flow through the system. Specification of input and output nodes allowed us to identify functional modules. Networking resulted in the emergence of regulatory motifs, such as positive and negative feedback and feedforward loops, that process information. Key regulators of plasticity were highly connected nodes required for the formation of regulatory motifs, indicating the potential importance of such motifs in determining cellular choices between homeostasis and plasticity. 相似文献
450.
Doyle LR Carter JA Fabrycky DC Slawson RW Howell SB Winn JN Orosz JA Prša A Welsh WF Quinn SN Latham D Torres G Buchhave LA Marcy GW Fortney JJ Shporer A Ford EB Lissauer JJ Ragozzine D Rucker M Batalha N Jenkins JM Borucki WJ Koch D Middour CK Hall JR McCauliff S Fanelli MN Quintana EV Holman MJ Caldwell DA Still M Stefanik RP Brown WR Esquerdo GA Tang S Furesz G Geary JC Berlind P Calkins ML Short DR Steffen JH Sasselov D Dunham EW Cochran WD Boss A Haas MR Buzasi D Fischer D 《Science (New York, N.Y.)》2011,333(6049):1602-1606
We report the detection of a planet whose orbit surrounds a pair of low-mass stars. Data from the Kepler spacecraft reveal transits of the planet across both stars, in addition to the mutual eclipses of the stars, giving precise constraints on the absolute dimensions of all three bodies. The planet is comparable to Saturn in mass and size and is on a nearly circular 229-day orbit around its two parent stars. The eclipsing stars are 20 and 69% as massive as the Sun and have an eccentric 41-day orbit. The motions of all three bodies are confined to within 0.5° of a single plane, suggesting that the planet formed within a circumbinary disk. 相似文献