全文获取类型
收费全文 | 15854篇 |
免费 | 1558篇 |
国内免费 | 10篇 |
出版年
2023年 | 105篇 |
2022年 | 74篇 |
2021年 | 610篇 |
2020年 | 292篇 |
2019年 | 352篇 |
2018年 | 417篇 |
2017年 | 356篇 |
2016年 | 629篇 |
2015年 | 1000篇 |
2014年 | 1004篇 |
2013年 | 1140篇 |
2012年 | 1481篇 |
2011年 | 1421篇 |
2010年 | 829篇 |
2009年 | 688篇 |
2008年 | 918篇 |
2007年 | 877篇 |
2006年 | 861篇 |
2005年 | 684篇 |
2004年 | 630篇 |
2003年 | 568篇 |
2002年 | 511篇 |
2001年 | 149篇 |
2000年 | 115篇 |
1999年 | 127篇 |
1998年 | 117篇 |
1997年 | 63篇 |
1996年 | 77篇 |
1995年 | 51篇 |
1994年 | 67篇 |
1993年 | 66篇 |
1992年 | 86篇 |
1991年 | 72篇 |
1990年 | 80篇 |
1989年 | 70篇 |
1988年 | 52篇 |
1987年 | 45篇 |
1986年 | 40篇 |
1985年 | 53篇 |
1984年 | 66篇 |
1983年 | 41篇 |
1982年 | 43篇 |
1981年 | 40篇 |
1980年 | 37篇 |
1979年 | 31篇 |
1978年 | 44篇 |
1977年 | 30篇 |
1975年 | 28篇 |
1974年 | 28篇 |
1973年 | 39篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
Wendy A. Douglass Robert H. Hyland Christopher D. Buckley Aymen Al-Shamkhani Jacqueline M. Shaw Sarah L. Scarth David L. Simmons S.K.Alex Law 《FEBS letters》1998,440(3):125
The cysteine-rich region (CRR) of the β2 integrin subunit was replaced by that of β1 to give the chimera β2NV1. β2NV1 can combine with αL to form a variant leukocyte-function-associated antigen (LFA)-1 on COS cell surface, suggesting that the specificity of the β2 interaction with αL does not lie in the CRR. Unlike those expressing wild-type LFA-1, COS cells expressing αLβ2NV1 are constitutively active in intercellular adhesion molecule (ICAM)-1 adhesion. These results suggest that activation of LFA-1 involves the release of an intramolecular constraint, which is maintained, in part, by the authentic β2 CRR. 相似文献
2.
Abstract Electrofusion between cells of yeast strains with different genetic markers in isotonic sorbitol solutions leads to high yields of hybrids when 0.1 mM Ca2+ and 0.5 mM Mg2+ salts are aded. On average, 1000–2000 hybrids are obtained when electrofusion is performed (in a helical chamber) compared to a yield of about 40–120 in the absence of these bivalent cations. A further increase in yield can be achieved by the addition of 1 mg/ml albumin, which results in up to 4000 hybrids per experimental run. The entire fusion process leads to very reproducible results in the presence of these substances. 相似文献
3.
4.
Diego Sustaita Patty Finfrock Quickert Laura Patterson Laureen Barthman-Thompson Sarah Estrella 《The Journal of wildlife management》2011,75(6):1498-1507
We undertook a 2-year (2002–2004) mark–recapture study to investigate demographic performance and habitat use of salt marsh harvest mice (Reithrodontomys raviventris halicoetes) in the Suisun Marsh. We examined the effects of different wetland types and microhabitats on 3 demographic variables: density, reproductive potential, and persistence. Our results indicate that microhabitats dominated by mixed vegetation or pickleweed (Salicornia spp.) supported similar salt marsh harvest mouse densities, reproductive potential, and persistence throughout much of the year, whereas few salt marsh harvest mice inhabited upland grass-dominated microhabitats. We found that densities were higher in diked wetlands, whereas post-winter persistence was higher in tidal wetlands, and reproductive potential did not differ statistically between wetland types. Our results emphasize the importance of mixed vegetation for providing adequate salt marsh harvest mouse habitat and suggest that, despite their physiognomic and hydrological differences, both diked and tidal wetlands support salt marsh harvest mouse populations by promoting different demographic attributes. We recommend that habitat management, restoration, and enhancement efforts include areas containing mixed vegetation in addition to pickleweed in both diked and tidal wetlands. © 2011 The Wildlife Society. 相似文献
5.
6.
The probability that protected areas will deliver their potential for maintaining or enhancing biodiversity is likely to be
maximised if they are appropriately and effectively managed. As a result, governments and conservation agencies are devoting
much attention to the management of protected areas. In the U.K., the demand for performance accountability has resulted in
Public Service Agreements (PSA) that set out targets for government departments to deliver results in return for investments
being made. One such target for England is to ensure that all nationally important wildlife sites are in favourable condition
by 2010. Here, we tested the hypothesis, of potential strategic importance, that the ecological condition of these sites is
predictable from relationships with a range of physical, environmental and demographic variables. We used binary logistic
regression to investigate these relationships, using the results of English Nature’s 1997–2003 condition assessment exercise.
Generally, sites in unfavourable condition tend to be larger in area, located at higher elevations, but with higher human
population density and are more spatially isolated from units of the same habitat. However, despite the range of different
parameters included in our models, the extent to which the condition of any given site could be predicted was low. Our results
have implications for the delivery of PSA targets, funding allocation, and the location of new protected areas. 相似文献
7.
8.
Katharina Bork Johannes Karstensen Martin Visbeck Andreas Zimmermann 《Ocean Development & International Law》2013,44(3):298-328
This article analyzes the legal status of unmanned instruments (particularly, floats and gliders) for observation purposes in the ocean environment. These new kinds of instruments are being deployed by the thousands into the oceans, not the least as part of the Argo Project of the International Oceanographic Commission. Their uncontrolled drifting has raised legal questions, especially when such instruments enter waters subject to the jurisdiction of foreign states. The authors argue that the current international legal framework is insufficient to address the pertinent issues, and that a new legal regime is needed. 相似文献
9.
10.