全文获取类型
收费全文 | 21392篇 |
免费 | 2340篇 |
国内免费 | 2367篇 |
出版年
2024年 | 61篇 |
2023年 | 481篇 |
2022年 | 510篇 |
2021年 | 706篇 |
2020年 | 804篇 |
2019年 | 930篇 |
2018年 | 783篇 |
2017年 | 905篇 |
2016年 | 880篇 |
2015年 | 880篇 |
2014年 | 1068篇 |
2013年 | 1396篇 |
2012年 | 965篇 |
2011年 | 994篇 |
2010年 | 876篇 |
2009年 | 1107篇 |
2008年 | 1126篇 |
2007年 | 1255篇 |
2006年 | 1152篇 |
2005年 | 1097篇 |
2004年 | 942篇 |
2003年 | 788篇 |
2002年 | 818篇 |
2001年 | 638篇 |
2000年 | 596篇 |
1999年 | 468篇 |
1998年 | 395篇 |
1997年 | 375篇 |
1996年 | 333篇 |
1995年 | 282篇 |
1994年 | 282篇 |
1993年 | 271篇 |
1992年 | 226篇 |
1991年 | 195篇 |
1990年 | 139篇 |
1989年 | 155篇 |
1988年 | 103篇 |
1987年 | 95篇 |
1986年 | 104篇 |
1985年 | 135篇 |
1984年 | 120篇 |
1983年 | 96篇 |
1982年 | 120篇 |
1981年 | 108篇 |
1980年 | 99篇 |
1979年 | 80篇 |
1978年 | 43篇 |
1977年 | 27篇 |
1976年 | 24篇 |
1973年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
951.
Hydrogen isotopic differences between C3 and C4 land plant lipids: consequences of compartmentation in C4 photosynthetic chemistry and C3 photorespiration
下载免费PDF全文
![点击此处可从《Plant, cell & environment》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Youping Zhou Kliti Grice Hilary Stuart‐Williams Charles H. Hocart Arthur Gessler Graham D. Farquhar 《Plant, cell & environment》2016,39(12):2676-2690
The 2H/1H ratio of carbon‐bound H in biolipids holds potential for probing plant lipid biosynthesis and metabolism. The biochemical mechanism underlying the isotopic differences between lipids from C3 and C4 plants is still poorly understood. GC‐pyrolysis‐IRMS (gas chromatography‐pyrolysis‐isotope ratio mass spectrometry) measurement of the 2H/1H ratio of leaf lipids from controlled and field grown plants indicates that the biochemical isotopic fractionation (ε2Hlipid_biochem) differed between C3 and C4 plants in a pathway‐dependent manner: ε2HC4 > ε2HC3 for the acetogenic pathway, ε2HC4 < ε2HC3 for the mevalonic acid pathway and the 1‐deoxy‐D‐xylulose 5‐phosphate pathway across all species examined. It is proposed that compartmentation of photosynthetic CO2 fixation into C4 mesophyll (M) and bundle sheath (BS) cells and suppression of photorespiration in C4 M and BS cells both result in C4 M chloroplastic pyruvate – the precursor for acetogenic pathway – being more depleted in 2H relative to pyruvate in C3 cells. In addition, compartmentation in C4 plants also results in (i) the transferable H of NADPH being enriched in 2H in C4 M chloroplasts compared with that in C3 chloroplasts for the 1‐deoxy‐D‐xylulose 5‐phosphate pathway pathway and (ii) pyruvate relatively 2H‐enriched being used for the mevalonic acid pathway in the cytosol of BS cells in comparison with that in C3 cells. 相似文献
952.
为探究不同地位的克氏原螯虾(Procambarus clakii)胜利者-失败者效应的稳定性。通过视频拍摄优势者和从属者在新的领域中各自与等重量级雄性螯虾连续遭遇战,统计胜负场数、格斗次数、时间、优势指数等多个参数。在与陌生对手的首次交战中,40个优势者中有23只螯虾取得了胜利,17只失败,胜负比例之间差异不显著;而从属者中获胜比例为18/40,与失败者的比例之间也没有差异,说明优势者和从属者之间原有的等级地位并不被陌生对手识别。在与陌生对手的第二次交战中,获胜的优势者中18/23的螯虾再次胜利,极显著高于失败的螯虾比例;获胜的从属者中,仅11/18的螯虾再次胜利,与失败者的比例之间不显著;而失败的优势者中,13/17的螯虾持续失败,失败的从属者中,17/22的螯虾持续失败,均显著高于获胜的比例,说明不同地位的螯虾胜利者-失败者效应稳定性不同,优势者的胜利者-失败者效应均比较稳定,而从属者的失败者效应稳定,胜利者效应并不稳定,一胜之后不能获得稳定、完全的二胜。对格斗次数、时间、优势指数等参数统计分析发现,胜利或失败的优势者以及从属者在格斗策略上有较大的差异。 相似文献
953.
Spatial synchrony can increase extinction risk and undermines metapopulation persistence. Both dispersal and biotic interactions can strongly affect spatial synchrony. Here, we explore the spatial synchrony of a tri-trophic food chain in two patches connected by density-dependent dispersal, namely the strategies of prey evasion (PE) and predator pursuit (PP). The dynamics of the food chain are depicted by both the Hastings–Powell model and the chemostat model, with synchrony measured by the Pearson correlation coefficient. We use the density-independent dispersal in the system as a baseline for comparison. Results show that the density-independent dispersal of a species in the system can promote its dynamic synchrony. Dispersal of intermediate species in the tri-trophic food chain is the strongest synchronizer. In contrast, the density-dependent PP and PE of intermediate species can desynchronize the system. Highly synchronized dynamics emerged when the basal species has a strong PE strategy or when the top species has a moderate PP strategy. Our results reveal the complex relationship between density-dependent dispersal and spatial synchrony in tri-trophic systems. 相似文献
954.
The barnacle, Balanus glandula has recently invaded along the Pacific coast of eastern Hokkaido, Japan. To evaluate the direct and indirect effects of endemic seaweeds, barnacles, and invertebrate predators on the abundance of B. glandula on the rocky intertidal coast of eastern Hokkaido, we conducted a field experiment from June 2011 to October 2012 in which we manipulated the presence or absence of these factors. Seaweeds showed no significant effect on the abundance of B. glandula. The endemic barnacle Chthamalus dalli and the invertebrate predator Nucella lima reduced the abundance of B. glandula. However, the simultaneous influence of N. lima and C. dalli was compensative rather than additive, probably due to keystone predation. These findings suggest that competition by the endemic barnacle C. dalli and predation by the invertebrate predator N. lima decreased the abundance of B. glandula, but that N. lima predation on C. dalli weakened the negative influence of C. dalli on B. glandula. The implications of these findings are twofold: the endemic competitor and invertebrate predator may have played important roles in decreasing the abundance of B. glandula in natural habitats, and conservation of endemic invertebrate predators may be crucial to impede the establishment and survival of introduced barnacles in rocky intertidal habitats. 相似文献
955.
Ecological genetics of Italian peach‐potato aphid (Myzus persicae) populations in relation to geography,dispersal and insecticide resistance as studied using microsatellite and resistance markers
下载免费PDF全文
![点击此处可从《Agricultural and Forest Entomology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Valentina Monti Hugh D. Loxdale Michele Cesari Milo Frattini Michela Panini Emanuele Mazzoni Gian Carlo Manicardi 《Agricultural and Forest Entomology》2016,18(4):376-389
- 相似文献
956.
Midpoint attractors and species richness: Modelling the interaction between environmental drivers and geometric constraints
下载免费PDF全文
![点击此处可从《Ecology letters》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Robert K. Colwell Nicholas J. Gotelli Louise A. Ashton Jan Beck Gunnar Brehm Tom M. Fayle Konrad Fiedler Matthew L. Forister Michael Kessler Roger L. Kitching Petr Klimes Jürgen Kluge John T. Longino Sarah C. Maunsell Christy M. McCain Jimmy Moses Sarah Noben Katerina Sam Legi Sam Arthur M. Shapiro Xiangping Wang Vojtech Novotny 《Ecology letters》2016,19(9):1009-1022
We introduce a novel framework for conceptualising, quantifying and unifying discordant patterns of species richness along geographical gradients. While not itself explicitly mechanistic, this approach offers a path towards understanding mechanisms. In this study, we focused on the diverse patterns of species richness on mountainsides. We conjectured that elevational range midpoints of species may be drawn towards a single midpoint attractor – a unimodal gradient of environmental favourability. The midpoint attractor interacts with geometric constraints imposed by sea level and the mountaintop to produce taxon‐specific patterns of species richness. We developed a Bayesian simulation model to estimate the location and strength of the midpoint attractor from species occurrence data sampled along mountainsides. We also constructed midpoint predictor models to test whether environmental variables could directly account for the observed patterns of species range midpoints. We challenged these models with 16 elevational data sets, comprising 4500 species of insects, vertebrates and plants. The midpoint predictor models generally failed to predict the pattern of species midpoints. In contrast, the midpoint attractor model closely reproduced empirical spatial patterns of species richness and range midpoints. Gradients of environmental favourability, subject to geometric constraints, may parsimoniously account for elevational and other patterns of species richness. 相似文献
957.
Frequency‐dependent selection for rare genotypes promotes genetic diversity of a tropical palm
下载免费PDF全文
![点击此处可从《Ecology letters》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Negative frequency‐dependent selection among species is a key driver of community diversity in natural systems, but the degree to which negative frequency‐dependent selection shapes patterns of survival and genetic diversity within species is poorly understood. In a 5‐year field experiment, we show that seedlings of a tropical palm with rare genotypes had a pronounced survival advantage over seedlings with common genotypes, with effect sizes comparable to that of light availability. This ‘rare genotype advantage’ led to an increase in population‐wide genetic diversity among seedlings compared to null expectations, as predicted by negative frequency‐dependent selection, and increased reproductive success in adult trees with rare genotypes. These results suggest that within‐species negative frequency‐dependent selection of genotypes can shape genetic variation on ecologically relevant timescales in natural systems and may be a key, overlooked source of non‐random mortality for tropical plants. 相似文献
958.
Genes,symptoms, and the “asymptomatic ill”: toward a broader understanding of genetic discrimination
Tino Plümecke 《New genetics and society》2016,35(2):124-148
Since the early 1990s, the term “genetic discrimination” has been used to designate adverse treatment on the grounds of genetic makeup. However, the full spectrum of possible disadvantage associated with genetic information has not been addressed by either the international scientific debate or statutory arrangements on genetic discrimination. Informed by legal contexts, they almost all focus on one specific group: the “asymptomatic ill.” On the basis of the sociological study, “Genetic Discrimination in Germany,” this article proposes to revise the terms of the debate and discusses some limitations of the concept. Drawing on the experiences reported by affected individuals, it advocates a more expansive social understanding which does not require that a person has to be healthy to be at risk of genetic discrimination. 相似文献
959.
The causes of variation in the presence of genetic covariance between sexual traits and preferences
下载免费PDF全文
![点击此处可从《Biological reviews of the Cambridge Philosophical Society》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Kasey D. Fowler‐Finn Rafael L. Rodríguez 《Biological reviews of the Cambridge Philosophical Society》2016,91(2):498-510
Mating traits and mate preferences often show patterns of tight correspondence across populations and species. These patterns of apparent coevolution may result from a genetic association between traits and preferences (i.e. trait–preference genetic covariance). We review the literature on trait–preference covariance to determine its prevalence and potential biological relevance. Of the 43 studies we identified, a surprising 63% detected covariance. We test multiple hypotheses for factors that may influence the likelihood of detecting this covariance. The main predictor was the presence of genetic variation in mate preferences, which is one of the three main conditions required for the establishment of covariance. In fact, 89% of the nine studies where heritability of preference was high detected covariance. Variables pertaining to the experimental methods and type of traits involved in different studies did not greatly influence the detection of trait–preference covariance. Trait–preference genetic covariance appears to be widespread and therefore represents an important and currently underappreciated factor in the coevolution of traits and preferences. 相似文献
960.