全文获取类型
收费全文 | 1220127篇 |
免费 | 101491篇 |
国内免费 | 4027篇 |
专业分类
1325645篇 |
出版年
2021年 | 19263篇 |
2020年 | 13633篇 |
2019年 | 17377篇 |
2018年 | 18085篇 |
2017年 | 16507篇 |
2016年 | 28697篇 |
2015年 | 43867篇 |
2014年 | 52229篇 |
2013年 | 78626篇 |
2012年 | 34067篇 |
2011年 | 20832篇 |
2010年 | 42848篇 |
2009年 | 44795篇 |
2008年 | 19526篇 |
2007年 | 16549篇 |
2006年 | 22839篇 |
2005年 | 23735篇 |
2004年 | 22848篇 |
2003年 | 20604篇 |
2002年 | 18636篇 |
2001年 | 28585篇 |
2000年 | 25349篇 |
1999年 | 27061篇 |
1998年 | 24781篇 |
1997年 | 24628篇 |
1996年 | 24241篇 |
1995年 | 22320篇 |
1994年 | 22221篇 |
1993年 | 21103篇 |
1992年 | 22835篇 |
1991年 | 21009篇 |
1990年 | 19316篇 |
1989年 | 20469篇 |
1988年 | 18628篇 |
1987年 | 17269篇 |
1986年 | 16461篇 |
1985年 | 18462篇 |
1984年 | 19200篇 |
1983年 | 17096篇 |
1982年 | 18787篇 |
1981年 | 18161篇 |
1980年 | 16913篇 |
1979年 | 14937篇 |
1978年 | 15042篇 |
1977年 | 14613篇 |
1976年 | 13850篇 |
1975年 | 12879篇 |
1974年 | 12920篇 |
1973年 | 13433篇 |
1972年 | 11085篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
231.
232.
James B. Reid 《Animal behaviour》1982,30(4):1212-1216
A young captive rook, Corvus frugilegus, inserted a plug into a plug-hole in its aviary floor so that a pool of water formed, which was used by all four rooks in the aviary for drinking and bathing. The bird was selective about which, of six holes, it chose for insertion of the tool, choosing the appropriate one with respect to the water source. Days on which attempted or successful tool-use occurred were drier than other days, and days of successful tool-uses were also warmer. Availability of fresh water to the birds did not influence the occurrence of tool-use. 相似文献
233.
234.
J. D. MUMFORD 《The Annals of applied biology》1981,97(2):243-252
The results of a survey of sugar beet growers are reported which provide subjective information on farmer perceptions of the hazard from pests and diseases on sugar beet, the control methods available and used, farmers' estimates of the costs and effect of controls, their objectives for pest control, and their attitudes to advice. In addition, objective information from research and field observations on losses and control methods is provided. The basis of pest control decisions and implications from comparisons of subjective and objective perceptions of the various problems and solutions are discussed. 相似文献
235.
236.
237.
238.
239.
Cynthia L. Deitrick Richard E. Katholi David J. Huddleston Kathy Hardiek Lucienne Burrus 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》2001,751(2)
Vitamin B6, measured as pyridoxal 5′-phosphate (PLP), is a co-enzyme in the transsulfuration pathway of homocysteine metabolism. Since depletion of PLP has been suggested as an independent risk factor for coronary artery disease, PLP is frequently measured to guide patient care. By a change and utilization of an Aquasil C18 column and the addition of an acetonitrile clean-up gradient to the potassium phosphate, with sodium perchlorate and bisulfite buffer between samples we report the modification of a previously described method for analysis of PLP. The result is a more practical, efficient, reliable and robust method for daily clinical use. We also determined and report that it is critical to protect freshly prepared standard PLP samples from light exposure during assay preparation. 相似文献
240.