全文获取类型
收费全文 | 812280篇 |
免费 | 86899篇 |
国内免费 | 5074篇 |
出版年
2021年 | 8146篇 |
2019年 | 7562篇 |
2018年 | 9883篇 |
2017年 | 8791篇 |
2016年 | 12295篇 |
2015年 | 16539篇 |
2014年 | 19370篇 |
2013年 | 26469篇 |
2012年 | 29412篇 |
2011年 | 29097篇 |
2010年 | 19306篇 |
2009年 | 17657篇 |
2008年 | 24206篇 |
2007年 | 24485篇 |
2006年 | 22855篇 |
2005年 | 21584篇 |
2004年 | 20925篇 |
2003年 | 19863篇 |
2002年 | 19001篇 |
2001年 | 36844篇 |
2000年 | 36896篇 |
1999年 | 28963篇 |
1998年 | 9621篇 |
1997年 | 10382篇 |
1996年 | 9680篇 |
1995年 | 9153篇 |
1994年 | 8845篇 |
1993年 | 8719篇 |
1992年 | 23261篇 |
1991年 | 22488篇 |
1990年 | 21612篇 |
1989年 | 21032篇 |
1988年 | 19323篇 |
1987年 | 18127篇 |
1986年 | 16861篇 |
1985年 | 16861篇 |
1984年 | 13771篇 |
1983年 | 11655篇 |
1982年 | 8813篇 |
1981年 | 7989篇 |
1980年 | 7545篇 |
1979年 | 13027篇 |
1978年 | 9975篇 |
1977年 | 9380篇 |
1976年 | 8524篇 |
1975年 | 9419篇 |
1974年 | 10284篇 |
1973年 | 10020篇 |
1972年 | 9126篇 |
1971年 | 8391篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
11.
12.
13.
Altitudinal distribution limits of aquatic macroinvertebrates: an experimental test in a tropical alpine stream 下载免费PDF全文
PHILIP B. MADSEN ANDRÉS MORABOWEN PATRICIO ANDINO RODRIGO ESPINOSA SOPHIE CAUVY‐FRAUNIÉ OLIVIER DANGLES DEAN JACOBSEN 《Ecological Entomology》2015,40(5):629-638
1. Temperature and oxygen are recognised as the main drivers of altitudinal limits of species distributions. However, the two factors are linked, and both decrease with altitude, why their effects are difficult to disentangle. 2. This was experimentally addressed using aquatic macroinvertebrates; larvae of Andesiops (Ephemeroptera), Claudioperla, (Plecoptera), Scirtes (Coleoptera) and Anomalocosmoecus (Trichoptera), and the amphipod Hyalella in an Ecuadorian glacier‐fed stream (4100–4500 m a.s.l.). The following were performed: (i) quantitative benthic sampling at three sites to determine altitudinal patterns in population densities, (ii) transplants of the five taxa upstream of their natural altitudinal limit to test the short‐term (14 days) effect on survival, and (iii) in situ experiments of locomotory activity as a proxy for animal response to relatively small differences in temperature (5 °C vs. 10 °C) and oxygen saturation (55% vs. 62%). 3. The transplant experiment reduced survival to a varying degree among taxa, but Claudioperla survived well at a site where it did not naturally occur. In the in situ experiment, Scirtes and Hyalella decreased their activity at lower oxygen saturation, whereas Andesiops and Anomalocosmoecus did so at a low temperature. The decrease in activity from a high to a low temperature and oxygen for the five taxa was significantly correlated with their mortality in the transplant experiment. 4. Together the present experiments indicate that even relatively small differences in temperature and oxygen may produce effects explaining ecological patterns, and depending on the taxon, either water temperature or oxygen saturation, without clear interacting effects, are important drivers of altitudinal limits. 相似文献
14.
15.
16.
Ronac Mamtani James D. Lewis Frank I. Scott Tariq Ahmad David S. Goldberg Jashodeep Datta Yu-Xiao Yang Ben Boursi 《PLoS medicine》2016,13(4)
BackgroundSeveral prior studies have found an association between statin use and reduced risk of colorectal cancer. We hypothesized that these findings may be due to systematic bias and examined the independent association of colorectal cancer risk with statin use, serum cholesterol, and change in cholesterol concentration.ConclusionsAlthough the risk of colorectal cancer was lower in statin users versus nonusers, no difference was observed among those who continued versus discontinued statin therapy, suggesting the potential for indication bias. The association between decreased serum cholesterol and colorectal cancer risk suggests a cholesterol-lowering effect of undiagnosed malignancy. Clinical judgment should be used when considering causes of cholesterol reduction in patients, including those on statin therapy. 相似文献
17.
I. S. Voroshilova 《Biology Bulletin》2016,43(10):1301-1307
This review summarizes published data on the genetic markers used in studies of dreissenide invasions. Causes of genetic differences between local populations are discussed. It is shown that information about the genetic diversity of populations obtained by marker polymorphism analysis should be compared to study invasion directions. 相似文献
18.
N. Cappetti A. Naddeo G. F. Solitro 《Computer methods in biomechanics and biomedical engineering》2016,19(12):1278-1285
The aim of this work is to show a quick and simple procedure able to identify the geometrical parameters of the intervertebral disc that strongly affect the behavior of the FEM model. First, we allocated a selection criterion for the minimum number of geometrical parameters that describe, with a good degree of approximation, a healthy human vertebra. Next, we carried out a sensitivity analysis using the ‘Taguchi orthogonal array’ to arrive at a quick identification of the parameters that strongly affect the behavior of the Fem model. 相似文献
19.
20.
Joyce S. F. D. de Almeida Teobaldo R. Cuya Guizado Ana P. Guimarães Teodorico C. Ramalho Arlan S. Gonçalves Martijn C. de Koning 《Journal of biomolecular structure & dynamics》2016,34(12):2632-2642
In the present work, we performed docking and molecular dynamics simulations studies on two groups of long-tailored oximes designed as peripheral site binders of acetylcholinesterase (AChE) and potential penetrators on the blood brain barrier. Our studies permitted to determine how the tails anchor in the peripheral site of sarin-inhibited human AChE, and which aminoacids are important to their stabilization. Also the energy values obtained in the docking studies corroborated quite well with the experimental results obtained before for these oximes. 相似文献