全文获取类型
收费全文 | 25346篇 |
免费 | 2271篇 |
国内免费 | 10篇 |
专业分类
27627篇 |
出版年
2024年 | 33篇 |
2023年 | 125篇 |
2022年 | 313篇 |
2021年 | 608篇 |
2020年 | 373篇 |
2019年 | 436篇 |
2018年 | 499篇 |
2017年 | 466篇 |
2016年 | 767篇 |
2015年 | 1246篇 |
2014年 | 1409篇 |
2013年 | 1608篇 |
2012年 | 2138篇 |
2011年 | 2175篇 |
2010年 | 1389篇 |
2009年 | 1132篇 |
2008年 | 1713篇 |
2007年 | 1606篇 |
2006年 | 1446篇 |
2005年 | 1437篇 |
2004年 | 1438篇 |
2003年 | 1230篇 |
2002年 | 1083篇 |
2001年 | 225篇 |
2000年 | 156篇 |
1999年 | 208篇 |
1998年 | 243篇 |
1997年 | 162篇 |
1996年 | 165篇 |
1995年 | 131篇 |
1994年 | 151篇 |
1993年 | 127篇 |
1992年 | 124篇 |
1991年 | 87篇 |
1990年 | 70篇 |
1989年 | 65篇 |
1988年 | 77篇 |
1987年 | 74篇 |
1986年 | 53篇 |
1985年 | 65篇 |
1984年 | 78篇 |
1983年 | 86篇 |
1982年 | 76篇 |
1981年 | 63篇 |
1980年 | 65篇 |
1979年 | 44篇 |
1978年 | 53篇 |
1977年 | 36篇 |
1976年 | 43篇 |
1974年 | 30篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
Christopher N. Anderson Gregory F. Grether 《Proceedings. Biological sciences / The Royal Society》2010,277(1700):3669-3675
Aggression between species is a seldom-considered but potentially widespread mechanism of character displacement in secondary sexual characters. Based on previous research showing that similarity in wing coloration directly influences interspecific territorial aggression in Hetaerina damselflies, we predicted that wing coloration would show a pattern of character displacement (divergence in sympatry). A geographical survey of four Hetaerina damselfly species in Mexico and Texas showed evidence for character displacement in both species pairs that regularly occurs sympatrically. Hetaerina titia, a species that typically has large black wing spots and small red wing spots, shifted to having even larger black spots and smaller red wing spots at sites where a congener with large red wing spots is numerically dominant (Hetaerina americana or Hetaerina occisa). Hetaerina americana showed the reverse pattern, shifting towards larger red wing spots where H. titia is numerically dominant. This pattern is consistent with the process of agonistic character displacement, but the ontogenetic basis of the shift remains to be demonstrated. 相似文献
12.
13.
Christopher A. Fraker Camillo Ricordi Luca Inverardi Juan Domínguez‐Bendala 《Biology of the cell / under the auspices of the European Cell Biology Organization》2009,101(8):431-440
Beyond its role as an electron acceptor in aerobic respiration, oxygen is also a key effector of many developmental events. The oxygen‐sensing machinery and the very fabric of cell identity and function have been shown to be deeply intertwined. Here we take a first look at how oxygen might lie at the crossroads of at least two of the major molecular pathways that shape pancreatic development. Based on recent evidence and a thorough review of the literature, we present a theoretical model whereby evolving oxygen tensions might choreograph to a large extent the sequence of molecular events resulting in the development of the organ. In particular, we propose that lower oxygenation prior to the expansion of the vasculature may favour HIF (hypoxia inducible factor)‐mediated activation of Notch and repression of Wnt/β‐catenin signalling, limiting endocrine cell differentiation. With the development of vasculature and improved oxygen delivery to the developing organ, HIF‐mediated support for Notch signalling may decline while the β‐catenin‐directed Wnt signalling is favoured, which would support endocrine cell differentiation and perhaps exocrine cell proliferation/differentiation. 相似文献
14.
Christelle Hély Kelly K. Caylor Peter Dowty Samuel Alleaume Robert J. Swap Hank H. Shugart Christopher O. Justice 《Ecosystems》2007,10(7):1116-1132
Abstract
We present a regional fuel load model (1 km2 spatial resolution) applied in the southern African savanna region. The model is based on a patch-scale production efficiency
model (PEM) scaled up to the regional level using empirical relationships between patch-scale behavior and multi-source remote
sensing data (spatio-temporal variability of vegetation and climatic variables). The model requires the spatial distribution
of woody vegetation cover, which is used to determine separate respiration rates for tree and grass. Net primary production,
grass and tree leaf death, and herbivory are also taken into account in this mechanistic modeling approach. The fuel load
model has been calibrated and validated from independent measurements taken from savanna vegetation in Africa southward from
the equator. A sensitivity analysis on the effect of climate variables (incoming radiation, air temperature, and precipitation)
has been conducted to demonstrate the strong role that water availability has in determining productivity and subsequent fuel
load over the southern African region. The model performance has been tested in four different areas representative of a regional
increasing rainfall gradient—Etosha National Park, Namibia, Mongu and Kasama, Zambia, as well as in Kruger National Park,
South Africa. Within each area, we analyze model output from three different magnitudes of canopy coverage (<5, 30, and 50%).
We find that fuel load ranges predicted by the model are globally in agreement with field measurements for the same year.
High rainfall sustains green herbaceous production late in the dry season and delays tree leaf litter production. Effect of
water on production varies across the rainfall gradient with delayed start of green material production in more arid regions. 相似文献
15.
16.
17.
The presence of prostaglandins (PGs) was determined in gastric juice obtained from 3 conscious dogs, provided with a chronic gastric fistula. Outputs of acid (mequiv min?1) and PGs (pg min?1) were measured in gastric secretions stimulated by pentagastrin (100 or 200 ng kg?1min?1). Prostaglandin activity was estimated, after extraction and thin layer chromatography, by radioimmuno-assay of the PGB formed by treatment with alkali. Tritiated PGs were added to gastric juice for the purpose of correcting for PGs recovery. Using this method, the minimum mass of PGB which could be satisfactorily distinguished from zero was 25 pg. Prostaglandins A2 and E2 were present in pentagastrin-activated gastric secretions and averaged (mean ± SE, n = 8) 200.7 ± 18.1 and 260.1 ± 18.0 pg min?1 respectively. The identity of PGA2 and PGE2 was confirmed by gas liquid chromatography combined with mass spectrometry. The amount of PGE2 converted to PGA2 during extraction, separation and conversion procedures was estimated from the amount of [3H] PGA2 found when only [3H] PGE2 had been added to a sample of gastric juice and averaged 14.5% ± 2.0. Our preliminary results support the possibility that PGE2 and PGA2 may be of physiological importance in the regulation of canine gastric secretions. 相似文献
18.
Stability and degradation of mRNA 总被引:13,自引:0,他引:13
Christopher F. Higgins 《Current opinion in cell biology》1991,3(6):1013-1018
Differential mRNA stability plays an important role in the regulation of gene expression. Several recent advances have helped to define the general pathways by which mRNA is degraded in prokaryotic cells, although many details remain to be elucidated. Much less is known about the pathways of degradation in eukaryotic cells, but recent studies on specific systems have highlighted both differences from and similarities to prokaryotic pathways. 相似文献
19.
20.
Harder David B.; Capeless Christopher G.; Maggio John C.; Boughter John D. Jr; Gannon Kimberley S.; Whitney Glayde; Azen Edwin A. 《Chemical senses》1992,17(4):391-401
Mice have been characterized as either tasters or non-tastersof the bitter compound sucrose octa-acetate(SOA). However, 11of 17 supposedly non-taster inbred strains were found to avoid1 mM SOA. All 17 strains were indifferent to 0.1 mM SOA. Tasterstrains avoided both concentrations. The intermediate phenotypewas dubbed demitaster. A consistent phenotypic dominance orderwas found in crosses among both inbred and outbred strains (taster> non-taster > demitaster). Demitasters were found (withtasters) in an outbred strain showing monogenic segregationfor SOA avoidance. This, plus monogenic segregation in a back-crossof taster to demitaster inbred strains, suggested a third alleleat the Soa locus (Soac). Demitaster allelism was supported bythe strong associations found in 15 strains between the threeSOA phenotypes and HindIII restriction fragment patterns forthe closely linked Prp (proline rich protein) loci. SOA demitasterstrains were also intermediate in raffinose undeca-acetate (RUA)avoidance. Furthermore, B6.SW-Soa2 congenic mice avoided notonly SOA, but RUA and eight other acetylated sugars. A previouslyproposed separate RUA-sensitivity gene (Rua) thus appeared tobe redundant. 相似文献