全文获取类型
收费全文 | 293520篇 |
免费 | 16014篇 |
国内免费 | 1410篇 |
专业分类
林业 | 24276篇 |
农学 | 14438篇 |
基础科学 | 3215篇 |
45588篇 | |
综合类 | 38076篇 |
农作物 | 17562篇 |
水产渔业 | 17666篇 |
畜牧兽医 | 118698篇 |
园艺 | 6662篇 |
植物保护 | 24763篇 |
出版年
2020年 | 3074篇 |
2019年 | 3846篇 |
2018年 | 4279篇 |
2017年 | 4706篇 |
2016年 | 5012篇 |
2015年 | 4620篇 |
2014年 | 5949篇 |
2013年 | 16624篇 |
2012年 | 7494篇 |
2011年 | 9653篇 |
2010年 | 8081篇 |
2009年 | 8443篇 |
2008年 | 8776篇 |
2007年 | 7955篇 |
2006年 | 8339篇 |
2005年 | 7603篇 |
2004年 | 7306篇 |
2003年 | 7143篇 |
2002年 | 6486篇 |
2001年 | 7138篇 |
2000年 | 6992篇 |
1999年 | 6256篇 |
1998年 | 4274篇 |
1997年 | 4303篇 |
1996年 | 4003篇 |
1995年 | 4562篇 |
1994年 | 4007篇 |
1993年 | 3719篇 |
1992年 | 5035篇 |
1991年 | 5375篇 |
1990年 | 5127篇 |
1989年 | 5061篇 |
1988年 | 4554篇 |
1987年 | 4658篇 |
1986年 | 4543篇 |
1985年 | 4701篇 |
1984年 | 4293篇 |
1983年 | 4009篇 |
1982年 | 3176篇 |
1981年 | 3021篇 |
1980年 | 3001篇 |
1979年 | 3856篇 |
1978年 | 3361篇 |
1977年 | 3041篇 |
1976年 | 2929篇 |
1974年 | 3328篇 |
1973年 | 3256篇 |
1972年 | 3040篇 |
1971年 | 3002篇 |
1970年 | 2839篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
Maugh TH 《Science (New York, N.Y.)》1981,211(4487):1142
994.
995.
Archeological deposits have been discovered in three pimple mounds on the upper Texas coast. Such incorporation indicates that (i) these particular mounds are aggradational rather than erosional in origin; (ii) mound formation continued until at least A.D. 1100 to 1200; (iii) each mound took 300 to 500 years to form; (iv) mounds within the same mound field did not form simultaneously; (v) certain Gulf Coast pimple mounds have archeological research potential; and (vi) archeological data also offer a means for investigating mound formation. 相似文献
996.
Values of the oxygen isotope ratios (delta(18)O) in tree-ring cellulose closely reflect the delta(18)O values in atmospheric precipitation and hence mean annual temperature. The change in delta(18)O in cellulose is 0.41 per mil per degree Celsius for selected near-coastal stations. The values of delta(18)O in precipitation and cellulose also change with altitude, as demonstrated for Mount Rainier, Washington. A temperature lapse rate of 5.2 degrees +/- 0.5 degrees C per 1000 meters calculated from cellulose delta(18)O values agrees with the accepted mean annual lapse rate of 5 degrees C per 1000 meters for this region. Cellulose delta(18)O values and delta(18)O values of carbon dioxide equilibrated with leaf water differ by a fixed 16 per mil. 相似文献
997.
998.
Heathcock CH 《Science (New York, N.Y.)》1981,214(4519):395-400
For the scientist who wishes to synthesize complex organic compounds, the most difficult problem is often establishing the correct configuration at the various chiral centers as the synthesis is being carried out. In the past decade, there has been an increasing effort to find direct solutions to this problem, which is particularly acute in the synthesis of acyclic and other conformationally flexible molecules. One of the oldest organic reactions, the aldol condensation, is emerging as a powerful tool for use in achieving such stereocontrol. 相似文献
999.
1000.
Pruett RL 《Science (New York, N.Y.)》1981,211(4477):11-16
Rapid increases in the price of imported crude oil have accelerated a shift in the raw material base for chemical feedstocks from natural gas to crude oil to coal. Widespread use of ethylene as a feedstock has depended on the availability at attractive prices of natural gas and petroleum. As the resource base shifts from natural gas and petroleum to coal, ethylene is being replaced by synthesis gas (a mixture of hydrogen and carbon monoxide of varying composition), which can be manufactured directly from any of these carbonaceous sources. This trend is expected to accelerate in the 1980's. Organics likely to be produced from synthesis gas include ethanol, ethylene glycol, and vinyl acetate. 相似文献