全文获取类型
收费全文 | 31324篇 |
免费 | 15923篇 |
国内免费 | 3篇 |
专业分类
47250篇 |
出版年
2023年 | 29篇 |
2022年 | 144篇 |
2021年 | 488篇 |
2020年 | 2229篇 |
2019年 | 3774篇 |
2018年 | 3875篇 |
2017年 | 4153篇 |
2016年 | 4199篇 |
2015年 | 4190篇 |
2014年 | 3857篇 |
2013年 | 4343篇 |
2012年 | 2138篇 |
2011年 | 1853篇 |
2010年 | 3266篇 |
2009年 | 1988篇 |
2008年 | 1002篇 |
2007年 | 568篇 |
2006年 | 534篇 |
2005年 | 624篇 |
2004年 | 560篇 |
2003年 | 567篇 |
2002年 | 549篇 |
2001年 | 340篇 |
2000年 | 238篇 |
1999年 | 226篇 |
1998年 | 118篇 |
1997年 | 77篇 |
1996年 | 64篇 |
1995年 | 67篇 |
1994年 | 71篇 |
1993年 | 68篇 |
1992年 | 92篇 |
1991年 | 57篇 |
1990年 | 49篇 |
1989年 | 48篇 |
1988年 | 55篇 |
1987年 | 48篇 |
1986年 | 35篇 |
1985年 | 54篇 |
1984年 | 56篇 |
1983年 | 34篇 |
1982年 | 45篇 |
1981年 | 52篇 |
1980年 | 45篇 |
1979年 | 30篇 |
1978年 | 34篇 |
1977年 | 33篇 |
1976年 | 23篇 |
1974年 | 25篇 |
1973年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
4.
5.
The RAINFOR database: monitoring forest biomass and dynamics 总被引:1,自引:0,他引:1
Problem: Data from over 100 permanent sample plots which have been studied for 10–20 years need a suitable system for storage which allows simple data manipulation and retrieval for analysis. Methods: A relational database linking tree records, taxonomic nomenclature and corresponding environmental data has been built in MS Access as part of the RAINFOR project. Conclusion: The database allows flexible and long‐term use of a large amount of data: more than 100 tree plots across Amazonia, incorporating over 80 000 records of individual trees and over 300 000 total records of tree diameter measurements from successive censuses. The database is designed to enable linkages to existing soil, floristic or plant‐trait databases. This database will be a useful tool for exploring the impact of environmental factors on forest structure and dynamics at local to continental scales, and long term changes in forest ecology. As an early example of its potential, we explore the impact of different methodological assumptions on estimates of tropical forest biomass and carbon storage. 相似文献
6.
We used self‐reported data from United Methodist clergy to assess the prevalence of obesity and having ever been told certain chronic disease diagnoses. Of all actively serving United Methodist clergy in North Carolina (NC) 95% (n = 1726) completed self‐report height and weight items and diagnosis questions from the Behavioral Risk Factor Surveillance Survey (BRFSS). We calculated BMI categories and diagnosis prevalence rates for the clergy and compared them to the NC population using BRFSS data. The obesity rate among clergy aged 35–64 years was 39.7%, 10.3% (95% CI = 8.5%, 12.1%) higher than their NC counterparts. Clergy also reported significantly higher rates of having ever been given diagnoses of diabetes, arthritis, high blood pressure, angina, and asthma compared to their NC peers. Health interventions that address obesity and chronic disease among clergy are urgently needed. 相似文献
7.
James B Munro Roger B Altman Chang‐Shung Tung Kevin Y Sanbonmatsu Scott C Blanchard 《The EMBO journal》2010,29(4):770-781
A key intermediate in translocation is an ‘unlocked state’ of the pre‐translocation ribosome in which the P‐site tRNA adopts the P/E hybrid state, the L1 stalk domain closes and ribosomal subunits adopt a ratcheted configuration. Here, through two‐ and three‐colour smFRET imaging from multiple structural perspectives, EF‐G is shown to accelerate structural and kinetic pathways in the ribosome, leading to this transition. The EF‐G‐bound ribosome remains highly dynamic in nature, wherein, the unlocked state is transiently and reversibly formed. The P/E hybrid state is energetically favoured, but exchange with the classical P/P configuration persists; the L1 stalk adopts a fast dynamic mode characterized by rapid cycles of closure and opening. These data support a model in which P/E hybrid state formation, L1 stalk closure and subunit ratcheting are loosely coupled, independent processes that must converge to achieve the unlocked state. The highly dynamic nature of these motions, and their sensitivity to conformational and compositional changes in the ribosome, suggests that regulating the formation of this intermediate may present an effective avenue for translational control. 相似文献
8.
The elongation rate, the gradient of the local elongation rate and the surface pH of maize roots were measured over 12 h. A data bank was constituted by storing these values. By sorting these results on the basis of different elongation rates, different classes of root were obtained. Two classes were chosen: the low-growth roots and the high-growth roots. The mean growth of these two root classes was stable with time and differed significantly from one another. The surface pH of the elongation zone was the same for the roots of these two classes, but the roots selected for their higher growth rate had a larger acid efflux in this zone. 相似文献
9.
Thirteen polymorphic microsatellite loci were developed for blowflies for use in studies of genetic differentiation in wild populations of Lucilia illustris, to detect the possible occurrence of bottlenecks and to study changes in genetic variation in laboratory populations of Lucilia sericata following artificial bottlenecks. In this preliminary study it was revealed that heterozygosity was lower than expected in wild populations and genetic variation had been lost in the laboratory population despite being kept at a large size. 相似文献
10.
Francisco Gutirrez‐Corchero M. Victoria Arruga Lourdes Sanz Cristina García M. Angeles Hernndez Francisco Campos 《Molecular ecology resources》2002,2(1):75-77
FTA® cards were used for long‐term storage of avian blood samples. Blood DNA was extracted by a simple method and used in PCR for sex identification of adult and nestling Great Grey Shrikes Lanius excubitor. 相似文献