全文获取类型
收费全文 | 3555篇 |
免费 | 207篇 |
国内免费 | 1篇 |
专业分类
3763篇 |
出版年
2023年 | 29篇 |
2022年 | 59篇 |
2021年 | 78篇 |
2020年 | 53篇 |
2019年 | 52篇 |
2018年 | 76篇 |
2017年 | 73篇 |
2016年 | 95篇 |
2015年 | 132篇 |
2014年 | 143篇 |
2013年 | 210篇 |
2012年 | 220篇 |
2011年 | 226篇 |
2010年 | 172篇 |
2009年 | 110篇 |
2008年 | 157篇 |
2007年 | 190篇 |
2006年 | 149篇 |
2005年 | 135篇 |
2004年 | 114篇 |
2003年 | 91篇 |
2002年 | 78篇 |
2001年 | 68篇 |
2000年 | 64篇 |
1999年 | 58篇 |
1998年 | 26篇 |
1997年 | 20篇 |
1995年 | 28篇 |
1994年 | 26篇 |
1993年 | 28篇 |
1992年 | 43篇 |
1991年 | 37篇 |
1990年 | 29篇 |
1989年 | 41篇 |
1988年 | 38篇 |
1987年 | 43篇 |
1986年 | 51篇 |
1985年 | 33篇 |
1984年 | 47篇 |
1983年 | 32篇 |
1982年 | 29篇 |
1981年 | 30篇 |
1980年 | 32篇 |
1979年 | 40篇 |
1977年 | 45篇 |
1976年 | 21篇 |
1975年 | 27篇 |
1974年 | 30篇 |
1973年 | 20篇 |
1972年 | 22篇 |
排序方式: 共有3763条查询结果,搜索用时 0 毫秒
961.
Louis R. Iverson Anantha M. Prasad Stephen N. Matthews Matthew P. Peters 《Ecosystems》2011,14(6):1005-1020
We present an approach to modeling potential climate-driven changes in habitat for tree and bird species in the eastern United States. First, we took an empirical-statistical modeling approach, using randomForest, with species abundance data from national inventories combined with soil, climate, and landscape variables, to build abundance-based habitat models for 134 tree and 147 bird species. We produced lists of species for which suitable habitat tends to increase, decrease, or stay the same for any region. Independent assessments of trends of large trees versus seedlings across the eastern U.S. show that 37 of 40 species in common under both studies are currently trending as modeled. We developed a framework, ModFacs, in which we used the literature to assign default modification factor scores for species characteristics that cannot be readily assessed in such models, including 12 disturbance factors (for example, drought, fire, insect pests), nine biological factors (for example, dispersal, shade tolerance), and assessment scores of novel climates, long-distance extrapolations, and output variability by climate model and emission scenario. We also used a spatially explicit cellular model, SHIFT, to calculate colonization potentials for some species, based on their abundance, historic dispersal distances, and the fragmented nature of the landscape. By combining results from the three efforts, we can create projections of potential climate change impacts over the next 100 years or so. Here we emphasize some of the lessons we have learned over 16 years in hopes that they may help guide future experiments, modeling efforts, and management. 相似文献
962.
963.
964.
965.
966.
Experiment was conducted to identify the impacts of the salinity acclimation process on the photosynthetic efficiency, osmotic adjustment, membrane integrity, and yield components in two wheat cultivars differing in their salinity tolerance. The design of the experiment was factorial randomized block, where genotype is factor 1 and acclimation treatments represent factor 2. Genotypes were grown from emergence to 30 days after sowing (DAS) by irrigating with tap water [electrical conductivity (EC) of 0.776 dS m?1]. Thereafter, both the genotypes were divided into two groups and exposed to either irrigation with sublethal level of salinity EC of 2.09 or 3.76 dS m?1 for 21 days. At booting stage (65 DAS), both groups were subjected to lethal level of salinity stress EC of 12 dS m?1 for 21 days, followed by irrigation with tap water till maturity. Non-acclimated plants were irrigated with tap water from emergence to 65 days, then directly irrigated with lethal level of salinity for 21 days, followed by irrigation with tap water till maturity. The control plants were continuously irrigated with tap water from emergence until maturity. The non-acclimated plants had decreased electron transport rates at the donor and acceptor side of PSII and PSI in Giza 168, and decreased electron transport rates at PSII acceptor side in Sakha 8 compared to control plants. In both genotypes, the non-acclimated plants had decreased chlorophyll a, b, carotenoid, proline and total soluble sugar concentration, relative water content, membrane stability index, yield and yield components compared with acclimated plants. While, osmotic potential and lipid peroxidation showed an opposite trend. Overall, acclimation treatment (EC of 2.09 dS m?1) during vegetative stage alleviated the inhibitory effects of lethal level of salinity stress at booting stage through enhanced photosynthetic efficiency and osmotic adjustment, resulting in increased membrane integrity, biomass production and grain yield than in non-acclimated plants. 相似文献
967.
3D-QSAR studies on the derivatives of 1-(3,3-diphenylpropyl)-piperidinyl amide and urea as CCR5 receptor antagonists were
performed by comparative molecular field analysis (CoMFA) and comparative molecular similarity indices (CoMSIA) methods to
rationalize the structural requirements responsible for the inhibitory activity of these compounds. The global minimum energy
conformer of the template molecule, the most active and pharmacokinetically stable molecule of the series, was obtained by
systematic search and used to build structures of the molecules in the dataset. The best predictions for the CCR5-receptor
were obtained with the CoMFA standard model (q
2 = 0.787, r
2 = 0.962) and CoMSIA model combined steric, electrostatic and hydrophobic fields (q
2 = 0.809, r
2 = 0.951). The predictive ability of CoMFA and CoMSIA were determined using a test set of 12 compounds giving predictive correlation
coefficients of 0.855 and 0.83, respectively, indicating good predictive power. Further, the robustness of the model was verified
by bootstrapping analysis. The contour maps produced by the CoMFA and CoMSIA models were used to identify the structural features
relevant to the biological activity in this series. Based on the CoMFA and CoMSIA analysis, we have identified some key features
in the series that are responsible for CCR5 antagonistic activity which may be used to design more potent 1-(3,3-diphenylpropyl)-piperidinyl
derivatives and predict their activity prior to synthesis.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
968.
Structures of rotavirus reassortants demonstrate correlation of altered conformation of the VP4 spike and expression of unexpected VP4-associated phenotypes 总被引:2,自引:0,他引:2 下载免费PDF全文
Numerous prior studies have indicated that viable rotavirus reassortants containing structural proteins of heterologous parental origin may express unexpected phenotypes, such as changes in infectivity and immunogenicity. To provide a structural basis for alterations in phenotypic expression, a three-dimensional structural analysis of these reassortants was conducted. The structures of the reassortants show that while VP4 generally maintains the parental structure when moved to a heterologous protein background, in certain reassortants, there are subtle alterations in the conformation of VP4. The alterations in VP4 conformation correlated with expression of unexpected VP4-associated phenotypes. Interactions between heterologous VP4 and VP7 in reassortants expressing unexpected phenotypes appeared to induce the conformational alterations seen in VP4. 相似文献
969.
Chandrashekar Unnenahally Shivakumar Ramwant Gupta Manjunath Prasad Cholanayakanahalli Thyagaraju Shyamal Kumar Chakrabarty Malavika Dadlani 《Grana》2013,52(2):103-110
The pollen morphology of two species of Drosera has been investigated by means of both light and electron microscopy. The apertures are situated proximally, a state rarely found in the pollen of Angiospermous plants. The pollen morphology of Aldrovanda, Dionea and Drosophyllum is compared. 相似文献
970.
Kshatresh Dutta Dubey Amit Kumar ChaubeyRajendra Prasad Ojha 《Biochimica et Biophysica Acta - Proteins and Proteomics》2013,1834(1):53-64
Change in pH plays a crucial role in the stability and function of the dengue envelope (DENV) protein during conformational transition from dimeric (pre-fusion state) to trimeric form (post-fusion state). In the present study we have performed various molecular dynamics (MD) simulations of the trimeric DENV protein at different pH and ionic concentrations. We have used total binding energy to justify the stability of the complex using the MMPBSA method. We found a remarkable increase in the stability of the complex at neutral pH (pH ~ 7) due to the increment of sodium ions. However, at very low pH (pH ~ 4), the total energy of the complex becomes high enough to destabilize the complex. At a specific pH, almost at a range of 6, the stability of the complex is significantly better than the stability of the trimer at neutral pH, which connotes that the trimer is most stable at this pH (pH ~ 6). 相似文献