全文获取类型
收费全文 | 19406篇 |
免费 | 2052篇 |
国内免费 | 3篇 |
出版年
2022年 | 170篇 |
2021年 | 335篇 |
2020年 | 190篇 |
2019年 | 237篇 |
2018年 | 290篇 |
2017年 | 253篇 |
2016年 | 412篇 |
2015年 | 755篇 |
2014年 | 769篇 |
2013年 | 975篇 |
2012年 | 1302篇 |
2011年 | 1355篇 |
2010年 | 850篇 |
2009年 | 734篇 |
2008年 | 1158篇 |
2007年 | 1145篇 |
2006年 | 1041篇 |
2005年 | 940篇 |
2004年 | 978篇 |
2003年 | 824篇 |
2002年 | 850篇 |
2001年 | 338篇 |
2000年 | 255篇 |
1999年 | 257篇 |
1998年 | 249篇 |
1997年 | 223篇 |
1996年 | 203篇 |
1995年 | 179篇 |
1994年 | 156篇 |
1993年 | 163篇 |
1992年 | 196篇 |
1991年 | 192篇 |
1990年 | 178篇 |
1989年 | 192篇 |
1988年 | 170篇 |
1987年 | 169篇 |
1986年 | 152篇 |
1985年 | 191篇 |
1984年 | 179篇 |
1983年 | 144篇 |
1982年 | 161篇 |
1981年 | 187篇 |
1980年 | 154篇 |
1979年 | 132篇 |
1978年 | 108篇 |
1977年 | 107篇 |
1976年 | 118篇 |
1975年 | 84篇 |
1974年 | 103篇 |
1969年 | 82篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
ABSTRACT The phrase persistent vulnerability reflects the enduring relationship of the rural population in Ceará with a highly variable climate. Persistence underscores the historical and unyielding nature of this vulnerability. Yet contrary to once-catastrophic rates of mortality etched in a public consciousness, no one dies from severe droughts and few people flee them as in the past. Government relief and social transfers have become the institutionalized form of adaptation, giving way to the counterintuitive reality that drought stabilizes the food and income supply for poor people. We analyze how maladaptive risk reduction, which is embedded in clientilistic social relations, undermines resilience, and we examine pathways toward a more sustainable adaptive relationship. 相似文献
3.
Thomas A. Worthington Shannon K. Brewer Nicole Farless Timothy B. Grabowski Mark S. Gregory 《PloS one》2014,9(5)
Habitat fragmentation and flow regulation are significant factors related to the decline and extinction of freshwater biota. Pelagic-broadcast spawning cyprinids require moving water and some length of unfragmented stream to complete their life cycle. However, it is unknown how discharge and habitat features interact at multiple spatial scales to alter the transport of semi-buoyant fish eggs. Our objective was to assess the relationship between downstream drift of semi-buoyant egg surrogates (gellan beads) and discharge and habitat complexity. We quantified transport time of a known quantity of beads using 2–3 sampling devices at each of seven locations on the North Canadian and Canadian rivers. Transport time was assessed based on median capture time (time at which 50% of beads were captured) and sampling period (time period when 2.5% and 97.5% of beads were captured). Habitat complexity was assessed by calculating width∶depth ratios at each site, and several habitat metrics determined using analyses of aerial photographs. Median time of egg capture was negatively correlated to site discharge. The temporal extent of the sampling period at each site was negatively correlated to both site discharge and habitat-patch dispersion. Our results highlight the role of discharge in driving transport times, but also indicate that higher dispersion of habitat patches relates to increased retention of beads within the river. These results could be used to target restoration activities or prioritize water use to create and maintain habitat complexity within large, fragmented river systems. 相似文献
4.
Ian H. Frasher Timothy Coolbear Sailen Mookerjea 《Biochimica et Biophysica Acta (BBA)/General Subjects》1984,799(2):102-105
Induction of inflammation by turpentine injection caused 1.5–2-fold increase of both sialy- and galactosyltransferase activity in liver homogenates. The effect was apparent after 12 h turpentine treatment. Serum sialytransferase activity started to increase in the inflamed rats after 18 h, reaching a maximum of 4-fold at 48 h. In contrast, galactosyltransferase activity in serum showed no significant increase. The coordinated and temporal increase of sialytransferase activity in liver and serum suggest involvement of a specific mechanism for the preferential release of this enzyme into serum. 相似文献
5.
6.
7.
The ultrasonic calls produced by three day old mice when separated from the nest mother and siblings increase in number when naloxone is injected. 相似文献
8.
1. Male Japanese quail were given 2.20 x 10(-4)-14.53 mg uranium/kg intravenously as uranyl ion (235U label). 2. The relationship between dosage and the 18-hr accumulation of U in leg bones, liver, kidneys and testes was linear. 3. Increases in U deposition with increased dosage were approximately 1:1, except for kidneys where 10-fold increases in dosage resulted in 25-fold increases in deposition. 4. Estradiol-17 beta increased U deposition in bones by 15-fold thereby providing some protection for the kidneys as now the ratio of dosage to accumulation was 1:1. 相似文献
9.
R Philipp J O McIntyre B H Robinson H Huth W Trommer S Fleischer 《Biochimica et biophysica acta》1984,790(3):251-258
Calcium efflux and EGTA-induced calcium release from an internal platelet membrane fraction have been studied after the oxalate-supported calcium uptake had reached steady state. Increasing external calcium concentrations stimulate the calcium efflux velocity, with an apparent half-maximal stimulation at about 5 microM outside calcium concentration and a maximal velocity of calcium efflux of 4.66 +/- 2.32 nmol X min-1 X mg-1. Moreover, the ratio of the liberated calcium on the loaded calcium seems to be independent of the increasing external calcium concentration. Increasing the calculated internal calcium concentration by varying the oxalate potassium concentration from 10 mM to 1 mM results in an increase of the liberated calcium from the membrane vesicles from 7.4% to 63%, respectively, without changing the calcium efflux velocity. Similar conclusions can be drawn from the observation of results from the calcium efflux and EGTA-induced calcium release methods. Moreover, calcium pump reversal does not seem to be responsible for the calcium efflux or calcium release. All these different points added to the previously described regulation of calcium efflux by the catalytic subunit of cAMP protein kinase suggest us that the mechanism of calcium liberation by the platelet membranes is different from the calcium uptake. 相似文献
10.