全文获取类型
收费全文 | 49707篇 |
免费 | 16622篇 |
国内免费 | 6篇 |
专业分类
66335篇 |
出版年
2024年 | 30篇 |
2023年 | 263篇 |
2022年 | 392篇 |
2021年 | 987篇 |
2020年 | 2612篇 |
2019年 | 4146篇 |
2018年 | 4600篇 |
2017年 | 4659篇 |
2016年 | 4844篇 |
2015年 | 4890篇 |
2014年 | 4648篇 |
2013年 | 5560篇 |
2012年 | 3634篇 |
2011年 | 3481篇 |
2010年 | 4127篇 |
2009年 | 2656篇 |
2008年 | 2135篇 |
2007年 | 1679篇 |
2006年 | 1591篇 |
2005年 | 1335篇 |
2004年 | 1279篇 |
2003年 | 1169篇 |
2002年 | 1057篇 |
2001年 | 842篇 |
2000年 | 845篇 |
1999年 | 536篇 |
1998年 | 219篇 |
1997年 | 158篇 |
1996年 | 146篇 |
1995年 | 139篇 |
1994年 | 125篇 |
1993年 | 117篇 |
1992年 | 155篇 |
1991年 | 143篇 |
1990年 | 107篇 |
1989年 | 89篇 |
1988年 | 85篇 |
1987年 | 72篇 |
1986年 | 65篇 |
1985年 | 71篇 |
1984年 | 63篇 |
1983年 | 49篇 |
1982年 | 58篇 |
1981年 | 49篇 |
1980年 | 42篇 |
1979年 | 51篇 |
1978年 | 43篇 |
1977年 | 28篇 |
1975年 | 37篇 |
1974年 | 29篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Hui Yang Seth M. Munson Chris Huntingford Nuno Carvalhais Alan K. Knapp Xiangyi Li Josep Peñuelas Jakob Zscheischler Anping Chen 《Global Change Biology》2023,29(8):2351-2362
Negative extreme anomalies in vegetation growth (NEGs) usually indicate severely impaired ecosystem services. These NEGs can result from diverse natural and anthropogenic causes, especially climate extremes (CEs). However, the relationship between NEGs and many types of CEs remains largely unknown at regional and global scales. Here, with satellite-derived vegetation index data and supporting tree-ring chronologies, we identify periods of NEGs from 1981 to 2015 across the global land surface. We find 70% of these NEGs are attributable to five types of CEs and their combinations, with compound CEs generally more detrimental than individual ones. More importantly, we find that dominant CEs for NEGs vary by biome and region. Specifically, cold and/or wet extremes dominate NEGs in temperate mountains and high latitudes, whereas soil drought and related compound extremes are primarily responsible for NEGs in wet tropical, arid and semi-arid regions. Key characteristics (e.g., the frequency, intensity and duration of CEs, and the vulnerability of vegetation) that determine the dominance of CEs are also region- and biome-dependent. For example, in the wet tropics, dominant individual CEs have both higher intensity and longer duration than non-dominant ones. However, in the dry tropics and some temperate regions, a longer CE duration is more important than higher intensity. Our work provides the first global accounting of the attribution of NEGs to diverse climatic extremes. Our analysis has important implications for developing climate-specific disaster prevention and mitigation plans among different regions of the globe in a changing climate. 相似文献
102.
Amanda dos Santos Marina Tenório Botelho Willian Reina Joviano Vicente Gomes José Roberto Machado Cunha da Silva Gisela de Aragão Umbuzeiro 《Invertebrate Biology》2023,142(1):e12394
Hemocytes are circulating blood cells that play a crucial function in amphipods and other crustacean immune systems. The hemocytes of the marine tropical amphipod Parhyale hawaiensis have been used for the evaluation of DNA damage and micronuclei, but they have not been characterized in the scientific literature. The aim of this study was to describe the hemolymph cells of P. hawaiensis and study their phagocytotic activity. Basic dyes were used to differentiate the cell types and the presence of lipids. The total hemocyte counts (THCs) and the proportion and sizes of the hemocyte types were determined. Hemolymph was exposed to Escherichia coli for verification of the presence of phagocytosis. Three cell types, all containing lipids, were identified in P. hawaiensis: granulocytes (oval shape, 13.4 × 7.6 μm), semi-granulocytes (oval shape, 14.1 × 7.2 μm), and hyalinocytes (round shape, 9.6 × 7.2 μm). Those three cell types were found in different percentages in males (64.8%, 31.1%, and 4.2%) and females (70.1%, 28.2%, and 1.7%). THCs for males were 9007 ± 3800 cells per individual and 4695 ± 1892 cells per individual for females. The cells of E. coli were phagocytized by the hemocytes. Our findings increased the knowledge of hemocytes in P. hawaiensis and is a step forward in using hemocyte-based immune responses as an endpoint in ecotoxicology. 相似文献
103.
A total of 522 girls and their families from low and middle social strate were examined in the northern part of Merida (Yucatan)
during 1988 and 1989. Marital radius in the parental generation was relatively long (146 km), and it was six times longer
for non-Maya and mixed couples than Maya.
Living and housing conditions were similar for both Maya and non-Maya (mixed couples typically had an intermediate condition),
except for sewage system (sanitation). The Maya income was 64% lower. Maya men and women were short. The girls from Merida
were short, hyper-brachycephalic (short headed), and europrosopic (broad face). Among them, Maya girls were even shorter,
more round-headed, and more broad-faced than non-Maya girls.
Menarche occurred on the average at an age of 12.6 years in mothers and 12.1 years in daughters. As the generation time was
about 25 years, there was a slow acceleration of maturation (0.2 years per decade). Presumably, also stature has increased
in recent years. 相似文献
104.
105.
Paula Tatiana Lopes Seixas Antonio Jacinto Demuner Luiz Claudio Almeida Barbosa Cristiane Isaac Cerceau Daiane Einhardt Blank Marcelo Henrique Dos Santos Elizeu de Sá Farias Marcelo Coutinho Picanço 《Journal of Applied Entomology》2023,147(2):116-125
Aedes aegypti L. (Diptera: Culicidae) is a vector for serious diseases in tropical regions. This pest is mainly controlled by commercial larvicides but the application of such products has led to environmental problems. Essential oils (EO) have been consistently reported as molecules with insecticidal activity and can be used to produce more environmentally friendly larvicides in the control of A. aegypti. In this study, the larvicidal effect of essential oils (EO) from the leaves of three Artemisia species was evaluated against A. aegypti. The oils were obtained from steam distillation and their chemical composition was determined by gas chromatography–mass spectrometry. The EO of Artemisia camphorata was the most active in the screening bioassay and presented LC50 and LC95 of 64.95 and 74.18 μg ml−1, respectively. In addition, we found that germacrene D-4-ol was the constituent responsible for the toxicity of this EO. Artemisia camphorata EO and its major constituent, germacrene D-4-ol, are promising for the development of natural larvicides against A. aegypti. 相似文献
106.
Summary Explants of tomato (Lycopersicon esculentum Mill cv. Ailsa Craig) were co-cultivated with Agrobacterium tumefaciens C58C1Rifr::pGSFR1161 in the presence of 20 (M acetosyringone). Transformed root clones were selected on kanamycin medium and the presence of the nptII gene in the plant DNA confirmed by the polymerase chain reaction. Root clones derived from acetosyringone treatment grew more vigorously in the presence of kanamycin and synthesized a greater amount of NPT-II enzyme. The conclusion is that acetosyringone treatment enhances the transformation process, possibly by stimulating multiple insertions of the T-DNA into the host genome.Abbreviations AS
acetosyringone
- CTAB
hexadecyltrimethylammonium bromide
- EDTA
ethylenediaminetetraacetic acid
- ELISA
enzyme-linked immunosorbant assay
- MS
Murashige and Skoog (1962) medium
- PCR
polymerase chain reaction 相似文献
107.
Lipolytic enzymes were produced using wheat bran as substrate in a solid state fermentation with Penicillium candidum. The best production of lipolytic activity occurred at 29 degrees C. One hundred micromoles of free butyric acid (FBA) was released from tributyrin by 1 mL of cell free supernatant in the absence of control of environmental relative humidity. When a closed chamber saturated with water vapour was used the lipolytic activity increased to 320 micromoles of free butyric acid. The best initial reaction pH was 7.0. The highest activity, 480 micromoles of FBA, was obtained at a moisture content of 67.5 % of saturation. 相似文献
108.
Rosalie J. Harris Callum Bryant Melinda A. Coleman Andrea Leigh Verónica F. Briceño Pieter A. Arnold Adrienne B. Nicotra 《Journal of phycology》2023,59(1):179-192
Foundation seaweed species are experiencing widespread declines and localized extinctions due to increased instability of sea surface temperature. Characterizing temperature thresholds are useful for predicting patterns of change and identifying species most vulnerable to extremes. Existing methods for characterizing seaweed thermal tolerance produce diverse metrics and are often time-consuming, making comparisons between species and techniques difficult, hindering insight into global patterns of change. Using three kelp species, we adapted a high-throughput method – previously used in terrestrial plant thermal biology – for use on kelps. This method employs temperature-dependent fluorescence (T–F0) curves under heating or cooling regimes to determine the critical temperature (Tcrit) of photosystem II (PSII), i.e., the breakpoint between slow and fast rise fluorescence response to changing temperature, enabling rapid assays of photosynthetic thermal tolerance using a standardized metric. This method enables characterization of Tcrit for up to 48 samples per two-hour assay, demonstrating the capacity of T–F0 curves for high-throughput assays of thermal tolerance. Temperature-dependent fluorescence curves and their derived metric, Tcrit, may offer a timely and powerful new method for the field of phycology, enabling characterization and comparison of photosynthetic thermal tolerance of seaweeds across many populations, species, and biomes. 相似文献
109.
Summary An aqueous two-phase system of polyethylene glycol (PEG) and potassium phosphate was developed for extraction of a cutinase from cell debris of a recombinant Escherichia coli strain. Basic studies to identify the primary factors which affect cutinase partition, namely the influence of polymer molecular weight, polymer concentration and pH were carried out using a purified preparation of the cutinase. The enzyme partition coefficient was enhanced with decreasing PEG molecular weight, increasing tie-line length and pH. 相似文献
110.
João Ricardo Sato Claudinei Eduardo Biazoli Ana Paula Arantes Bueno Arthur Caye Pedro Mario Pan Marcos Santoro Jessica Honorato-Mauer Giovanni Abrahão Salum Marcelo Queiroz Hoexter Rodrigo Affonseca Bressan Andrea Parolin Jackowski Euripedes Constantino Miguel Sintia Belangero Luis Augusto Rohde 《Genes, Brain & Behavior》2023,22(2):e12838