全文获取类型
收费全文 | 961534篇 |
免费 | 115927篇 |
国内免费 | 545篇 |
出版年
2016年 | 10277篇 |
2015年 | 15688篇 |
2014年 | 18004篇 |
2013年 | 25396篇 |
2012年 | 28896篇 |
2011年 | 28811篇 |
2010年 | 19633篇 |
2009年 | 18562篇 |
2008年 | 26351篇 |
2007年 | 27216篇 |
2006年 | 25448篇 |
2005年 | 24689篇 |
2004年 | 24338篇 |
2003年 | 23800篇 |
2002年 | 23108篇 |
2001年 | 39834篇 |
2000年 | 40234篇 |
1999年 | 32558篇 |
1998年 | 12489篇 |
1997年 | 13098篇 |
1996年 | 12594篇 |
1995年 | 12310篇 |
1994年 | 12178篇 |
1993年 | 12035篇 |
1992年 | 27997篇 |
1991年 | 27234篇 |
1990年 | 26694篇 |
1989年 | 26061篇 |
1988年 | 24201篇 |
1987年 | 23836篇 |
1986年 | 21859篇 |
1985年 | 22157篇 |
1984年 | 18573篇 |
1983年 | 16259篇 |
1982年 | 13098篇 |
1981年 | 11938篇 |
1980年 | 11233篇 |
1979年 | 18226篇 |
1978年 | 14629篇 |
1977年 | 13315篇 |
1976年 | 12707篇 |
1975年 | 13782篇 |
1974年 | 14838篇 |
1973年 | 14677篇 |
1972年 | 13244篇 |
1971年 | 12248篇 |
1970年 | 10534篇 |
1969年 | 10139篇 |
1968年 | 9118篇 |
1967年 | 8159篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Uracil-DNA glycosylase in insects. Drosophila and the locust 总被引:3,自引:0,他引:3
It has been reported that Drosophila lacks a uracil-DNA glycosylase but that a direct incising activity on uracil-containing DNA appeared developmentally only in third instar larvae. In contrast we have found by two independent assays, that uracil-DNA glycosylase exists in both Drosophila eggs as well as in third instar larvae. The first assay shows the liberation of [3H] uracil from a d(AT)n polymer randomly substituted with [3H]uracil by its synthesis in the presence of [3H] dUTP. The second fluorometric assay for uracil-DNA glycosylase depends on the unique topological properties of circular DNAs and has the advantage of detecting apyrimidinic/apurinic (AP) endonuclease activity as well. To test one other insect, locust eggs were also assayed for uracil-DNA glycosylase. The amount of uracil-DNA glycosylase correlated well with the amount of DNA in actively replicating cells. 相似文献
102.
A two-step process to synthesize racemic lorcaserin was developed from 2-(4-chlorophenyl)ethanol via formation of bromide or tosylate derivatives. These derivatives were reacted with allylamine in neat conditions to provide pure N-(4-chlorophenethyl)allylammonium chloride. This compound was cyclized in neat conditions using aluminum or zinc chloride to give racemic lorcaserin. After resolution of enantiomers, the wrong enantiomer was racemized and recycled to give new R-lorcaserin. 相似文献
103.
Field test of an automated radio‐telemetry system: tracking local space use of aerial insectivores 下载免费PDF全文
Documenting local space use of birds that move rapidly, but are too small to carry GPS tags, such as swallows and swifts, can be challenging. For these species, tracking methods such as manual radio‐telemetry and visual observation are either inadequate or labor‐ and time‐intensive. Another option is use of an automated telemetry system, but equipment for such systems can be costly when many receivers are used. Our objective, therefore, was to determine if an automated radio‐telemetry system, consisting of just two receivers, could provide an alternative to manual tracking for gathering data on local space use of six individuals of three species of aerial insectivores, including one Cliff Swallow (Petrochelidon pyrrhonota), one Eastern Phoebe (Sayornis phoebe), and four Barn Swallows (Hirundo rustica). We established automated radio‐telemetry systems at three sites near the city of Peterborough in eastern Ontario, Canada, from May to August 2015. We evaluated the location error of our two‐receiver system using data from moving and stationary test transmitters at known locations, and used telemetry data from the aerial insectivores as a test of the system's ability to track rapidly moving birds under field conditions. Median location error was ~250 m for automated telemetry test locations after filtering. More than 90% of estimated locations had large location errors and were removed from analysis, including all locations > 1 km from receiver stations. Our automated telemetry receivers recorded 17,634 detections of the six radio‐tagged birds. However, filtering removed an average of 89% of bird location estimates, leaving only the Cliff Swallow with enough locations for analysis of space use. Our results demonstrate that a minimal automated radio‐telemetry system can be used to assess local space use by small, highly mobile birds, but the resolution of the data collected using only two receiver stations was coarse and had a limited range. To improve both location accuracy and increase the percentage of usable location estimates collected, we suggest that, in future studies, investigators use receivers that simultaneously record signals detected by all antennas, and use of a minimum of three receiver stations with more antennas at each station. 相似文献
104.
Continental‐scale macrofungal assemblage patterns correlate with climate,soil carbon and nitrogen deposition 下载免费PDF全文
105.
106.
107.
108.
109.