全文获取类型
收费全文 | 934篇 |
免费 | 71篇 |
国内免费 | 2篇 |
专业分类
林业 | 57篇 |
农学 | 23篇 |
基础科学 | 7篇 |
147篇 | |
综合类 | 128篇 |
农作物 | 37篇 |
水产渔业 | 120篇 |
畜牧兽医 | 413篇 |
园艺 | 19篇 |
植物保护 | 56篇 |
出版年
2022年 | 6篇 |
2021年 | 18篇 |
2020年 | 10篇 |
2019年 | 23篇 |
2018年 | 18篇 |
2017年 | 25篇 |
2016年 | 28篇 |
2015年 | 23篇 |
2014年 | 29篇 |
2013年 | 79篇 |
2012年 | 44篇 |
2011年 | 43篇 |
2010年 | 32篇 |
2009年 | 24篇 |
2008年 | 46篇 |
2007年 | 35篇 |
2006年 | 37篇 |
2005年 | 31篇 |
2004年 | 24篇 |
2003年 | 36篇 |
2002年 | 33篇 |
2001年 | 27篇 |
2000年 | 30篇 |
1999年 | 29篇 |
1998年 | 12篇 |
1997年 | 6篇 |
1995年 | 7篇 |
1994年 | 6篇 |
1993年 | 10篇 |
1992年 | 20篇 |
1991年 | 6篇 |
1990年 | 11篇 |
1989年 | 16篇 |
1988年 | 12篇 |
1987年 | 6篇 |
1986年 | 6篇 |
1985年 | 11篇 |
1983年 | 8篇 |
1982年 | 6篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1978年 | 9篇 |
1977年 | 7篇 |
1976年 | 6篇 |
1974年 | 7篇 |
1973年 | 9篇 |
1971年 | 8篇 |
1970年 | 9篇 |
1969年 | 5篇 |
1968年 | 5篇 |
排序方式: 共有1007条查询结果,搜索用时 15 毫秒
81.
Hansen TA 《Science (New York, N.Y.)》1978,199(4331):885-887
Species longevity in Lower Tertiary volutids (Gastropoda) is primarily controlled by a combination of developmental type and environmental tolerance. Larval dispersal may be an important factor in molluskan evolutionary rates. 相似文献
82.
83.
Fluorogenic substrate detection of viable intracellular and extracellular pathogenic protozoa 总被引:24,自引:0,他引:24
Viable Leishmania promastigotes and amastigotes were detected by epifluorescence microscopy with fluorescein diacetate being used to mark living parasites and the nucleic acid-binding compound ethidium bromide to stain dead cells. This procedure is superior to other assays because it is faster and detects viable intracellular as well as extracellular Leishmania. Furthermore, destruction of intracellular pathogens by macrophages is more accurately determined with fluorescein diacetate than with other stains. The procedure may have applications in programs to develop drugs and vaccines against protozoa responsible for human and animal disease. 相似文献
84.
Climate forcing by anthropogenic aerosols 总被引:14,自引:0,他引:14
Charlson RJ Schwartz SE Hales JM Cess RD Coakley JA Hansen JE Hofmann DJ 《Science (New York, N.Y.)》1992,255(5043):423-430
Although long considered to be of marginal importance to global climate change, tropospheric aerosol contributes substantially to radiative forcing, and anthropogenic sulfate aerosol in particular has imposed a major perturbation to this forcing. Both the direct scattering of shortwavelength solar radiation and the modification of the shortwave reflective properties of clouds by sulfate aerosol particles increase planetary albedo, thereby exerting a cooling influence on the planet. Current climate forcing due to anthropogenic sulfate is estimated to be -1 to -2 watts per square meter, globally averaged. This perturbation is comparable in magnitude to current anthropogenic greenhouse gas forcing but opposite in sign. Thus, the aerosol forcing has likely offset global greenhouse warming to a substantial degree. However, differences in geographical and seasonal distributions of these forcings preclude any simple compensation. Aerosol effects must be taken into account in evaluating anthropogenic influences on past, current, and projected future climate and in formulating policy regarding controls on emission of greenhouse gases and sulfur dioxide. Resolution of such policy issues requires integrated research on the magnitude and geographical distribution of aerosol climate forcing and on the controlling chemical and physical processes. 相似文献
85.
Nitrous oxide, methane, ammonia, and a number of other trace constituents in the earth's atmosphere have infrared absorption bands in the spectral region 7 to 14 microm and contribute to the atmospheric greenhouse effect. The concentrations of these trace gases may undergo substantial changes because of man's activities. Extensive use of chemical fertilizers and combustion of fossil fuels may perturb the nitrogen cycle, leading to increases in atmospheric N(2)O, and the same perturbing processes may increase the amounts of atmospheric CH(4) and NH(3). We use a one-dimensional radiative-convective model for the atmospheric thermal structure to compute the change in the surface temperature of the earth for large assumed increases in the trace gas concentrations; doubling the N(2)O, CH(4), and NH(3) concentrations is found to cause additive increases in the surface temperature of 0.7 degrees , 0.3 degrees , and 0.1 degrees K, respectively. These systematic effects on the earth's radiation budget would have substantial climatic significance. It is therefore important that the abundances of these trace gases be accurately monitored to determine the actual trends of their concentrations. 相似文献
86.
A comparison of the size of polar coronal holes with the cosmic-ray intensity observed during the most recent sunspot cycle shows close correspondence. This lends support to recent suggestions that the well-known sunspot-cycle modulation of cosmic rays is a three-dimensional effect, probably related to the global character of the interplanetary magnetic field. 相似文献
87.
McEwen AS Hansen CJ Delamere WA Eliason EM Herkenhoff KE Keszthelyi L Gulick VC Kirk RL Mellon MT Grant JA Thomas N Weitz CM Squyres SW Bridges NT Murchie SL Seelos F Seelos K Okubo CH Milazzo MP Tornabene LL Jaeger WL Byrne S Russell PS Griffes JL Martínez-Alonso S Davatzes A Chuang FC Thomson BJ Fishbaugh KE Dundas CM Kolb KJ Banks ME Wray JJ 《Science (New York, N.Y.)》2007,317(5845):1706-1709
Water has supposedly marked the surface of Mars and produced characteristic landforms. To understand the history of water on Mars, we take a close look at key locations with the High-Resolution Imaging Science Experiment on board the Mars Reconnaissance Orbiter, reaching fine spatial scales of 25 to 32 centimeters per pixel. Boulders ranging up to approximately 2 meters in diameter are ubiquitous in the middle to high latitudes, which include deposits previously interpreted as finegrained ocean sediments or dusty snow. Bright gully deposits identify six locations with very recent activity, but these lie on steep (20 degrees to 35 degrees) slopes where dry mass wasting could occur. Thus, we cannot confirm the reality of ancient oceans or water in active gullies but do see evidence of fluvial modification of geologically recent mid-latitude gullies and equatorial impact craters. 相似文献
88.
Phosphorylation of ULK1 (hATG1) by AMP-activated protein kinase connects energy sensing to mitophagy
Egan DF Shackelford DB Mihaylova MM Gelino S Kohnz RA Mair W Vasquez DS Joshi A Gwinn DM Taylor R Asara JM Fitzpatrick J Dillin A Viollet B Kundu M Hansen M Shaw RJ 《Science (New York, N.Y.)》2011,331(6016):456-461
Adenosine monophosphate-activated protein kinase (AMPK) is a conserved sensor of intracellular energy activated in response to low nutrient availability and environmental stress. In a screen for conserved substrates of AMPK, we identified ULK1 and ULK2, mammalian orthologs of the yeast protein kinase Atg1, which is required for autophagy. Genetic analysis of AMPK or ULK1 in mammalian liver and Caenorhabditis elegans revealed a requirement for these kinases in autophagy. In mammals, loss of AMPK or ULK1 resulted in aberrant accumulation of the autophagy adaptor p62 and defective mitophagy. Reconstitution of ULK1-deficient cells with a mutant ULK1 that cannot be phosphorylated by AMPK revealed that such phosphorylation is required for mitochondrial homeostasis and cell survival during starvation. These findings uncover a conserved biochemical mechanism coupling nutrient status with autophagy and cell survival. 相似文献
89.
Marilyn Holly Carla A. Kirts Yvonne Everett Jan L. Flora David O. Hansen Cornelia Butler Flora John A. Hayward David Haenke Alessandro Bonanno 《Agriculture and Human Values》1988,5(4):57-78
Marilyn Holly is an Associate Professor of Philosophy at the University of Florida. Her special interests include political philosophy, ethics and ecology, Jungian thought, and Native American spirituality in its relation to the environment. She is currently at work on a monograph-length piece on the latter topic. She has taught an undergraduate course in ethics and ecology, and plans to do so again. The following essay emerged from her search for a suitable text for such a course. 相似文献
90.
The linear polarization of sunlight multiply scattered by the atmosphere, and cloud particles of Venus has been computed and compared with observations over the wavelength range from the ultraviolet to the infrared region. The following properties of the visible cloud layer are derived: the refractive index of the cloud particles is 1.45 +/- 0.02 at a wavelength of 0.55 micron, and there is an indication of a slight decrease in the value from the ultraviolet to the near-infrared region; the mean particle radius is very near 1 micron, and most of the particles are spherical; the cloud layer occurs high in the atmosphere where the pressure is about 50 millibars (equivalent to an altitude of approximately 20 kilometers on the earth). The results for the index of refraction eliminate the possibility that the visible clouds are composed of pure water or ice. 相似文献