全文获取类型
收费全文 | 845篇 |
免费 | 71篇 |
出版年
2023年 | 4篇 |
2022年 | 11篇 |
2021年 | 22篇 |
2020年 | 15篇 |
2019年 | 11篇 |
2018年 | 17篇 |
2017年 | 17篇 |
2016年 | 26篇 |
2015年 | 33篇 |
2014年 | 40篇 |
2013年 | 43篇 |
2012年 | 75篇 |
2011年 | 74篇 |
2010年 | 43篇 |
2009年 | 31篇 |
2008年 | 39篇 |
2007年 | 50篇 |
2006年 | 45篇 |
2005年 | 49篇 |
2004年 | 21篇 |
2003年 | 30篇 |
2002年 | 37篇 |
2001年 | 20篇 |
2000年 | 16篇 |
1999年 | 18篇 |
1998年 | 9篇 |
1997年 | 7篇 |
1996年 | 3篇 |
1995年 | 7篇 |
1994年 | 10篇 |
1993年 | 2篇 |
1992年 | 14篇 |
1991年 | 9篇 |
1990年 | 6篇 |
1989年 | 8篇 |
1988年 | 5篇 |
1987年 | 8篇 |
1986年 | 3篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 6篇 |
1981年 | 2篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1974年 | 3篇 |
1972年 | 2篇 |
1971年 | 2篇 |
1968年 | 1篇 |
1967年 | 1篇 |
1920年 | 1篇 |
排序方式: 共有916条查询结果,搜索用时 173 毫秒
861.
Flavio Quintana Rory Wilson Patricia Dell'Arciprete Emily Shepard Agustina Gómez Laich 《Oikos》2011,120(3):350-358
Colonial seabirds are central place foragers and likely to be subject to substantial competition for resources. Mechanisms proposed for reducing intra‐specific competition include differential inter‐sex area use mediated by adult choice. We used GPS loggers and dive recorders to study area use and dive depth in a total of 27 male and 26 female imperial cormorants Phalacrocorax atriceps breeding at a colony of some 6500 birds at Punta Leon, Chubut, Argentina during 2004 and 2005. Although time spent travelling and distances between the colony and foraging sites were similar for both sexes, males and females travelled away from their colony using routes virtually perpendicular to each other so that their foraging areas were distinctly different; females hunted close to the coast while males foraged offshore in deeper water. Consideration of foraging efficiency underwater, defined as the duration spent on the bottom divided by the dive cycle duration, showed that females were more efficient at depths < 40 m while males more efficient at depths > 40 m. We suggest that the substantial sexual dimorphism in this species may be responsible for the different depth‐linked foraging efficiencies and that selection for appropriate depths could lead to differential habitat use and putative differences in prey selection. 相似文献
862.
Alejandre Alcázar MA Morty RE Lendzian L Vohlen C Oestreicher I Plank C Schneider H Dötsch J 《PloS one》2011,6(10):e26371
Intrauterine growth restriction is associated with impaired lung function in adulthood. It is unknown whether such impairment of lung function is linked to the transforming growth factor (TGF)-β system in the lung. Therefore, we investigated the effects of IUGR on lung function, expression of extracellular matrix (ECM) components and TGF-β signaling in rats. IUGR was induced in rats by isocaloric protein restriction during gestation. Lung function was assessed with direct plethysmography at postnatal day (P) 70. Pulmonary activity of the TGF-β system was determined at P1 and P70. TGF-β signaling was blocked in vitro using adenovirus-delivered Smad7. At P70, respiratory airway compliance was significantly impaired after IUGR. These changes were accompanied by decreased expression of TGF-β1 at P1 and P70 and a consistently dampened phosphorylation of Smad2 and Smad3. Furthermore, the mRNA expression levels of inhibitors of TGF-β signaling (Smad7 and Smurf2) were reduced, and the expression of TGF-β-regulated ECM components (e.g. collagen I) was decreased in the lungs of IUGR animals at P1; whereas elastin and tenascin N expression was significantly upregulated. In vitro inhibition of TGF-β signaling in NIH/3T3, MLE 12 and endothelial cells by adenovirus-delivered Smad7 demonstrated a direct effect on the expression of ECM components. Taken together, these data demonstrate a significant impact of IUGR on lung development and function and suggest that attenuated TGF-β signaling may contribute to the pathological processes of IUGR-associated lung disease. 相似文献
863.
Jetter MC McNally JJ Youngman MA McDonnell ME Dubin AE Nasser N Zhang SP Codd EE Colburn RW Stone DR Brandt MR Flores CM Dax SL 《Bioorganic & medicinal chemistry letters》2008,18(8):2730-2734
High throughput screening of our compound library revealed a series of N-pyridyl-3-benzamides as low micromolar agonists of the human TRPV1 receptor. Synthesis of analogs in this series led to the discovery of a series of N-quinolin-3-yl-benzamides as low nanomolar antagonists of human TRPV1. 相似文献
864.
865.
Nitrogen solutes in an Adirondack forested watershed: Importance of dissolved organic nitrogen 总被引:6,自引:2,他引:6
Michael R. McHale Myron J. Mitchell Jeffrey J. McDonnell Christopher P. Cirmo 《Biogeochemistry》2000,48(2):165-184
Nitrogen (N) dynamics were evaluated from 1 June 1995 through 31 May 1996 within the Arbutus Lake watershed in the Adirondack Mountains of New York State, USA. At the Arbutus Lake outlet dissolved organic nitrogen (DON), NO3
- and NH4
+ contributed 61%, 33%, and 6% respectively, to the total dissolved nitrogen (TDN) flux (259 mol ha-1 yr-1). At the lake inlet DON, NO3
-, and NH4
- constituted 36%, 61%, and 3% respectively, of TDN flux (349 mol ha-1 yr-1). Differences between the factors that control DON, NO3
+, and NH4
+ stream water concentrations were evaluated using two methods for estimating annual N flux at the lake inlet. Using biweekly sampling NO3
- and NH4
+ flux was 10 and 4 mol ha-1 yr-1 respectively, less than flux estimates using biweekly plus storm and snowmelt sampling. DON flux was 18 mol ha-1 yr-1 greater using only biweekly sampling. These differences are probably not of ecological significance relative to the total flux of N from the watershed (349 mol ha-1 yr-1). Dissolved organic N concentrations were positively related to discharge during both the dormant (R2 = 0.31; P < 0.01) and growing season (R2 = 0.09; P < 0.01). There was no significant relationship between NO3
- concentration and discharge during the dormant season, but a significant negative relationship was found during the growing season (R2 = 0.29; P < 0.01). Biotic controls in the growing season appeared to have had a larger impact on stream water NO3
- concentrations than on DON concentrations. Arbutus Lake had a major impact on stream water N concentrations of the four landscape positions sampled, suggesting the need to quantify within lake processes to interpret N solute losses and patterns in watershed-lake systems. 相似文献
866.
The habitat hectares approach to vegetation assessment: An evaluation and suggestions for improvement 总被引:2,自引:0,他引:2
Michael A. McCarthy Kirsten M. Parris Rodney van der Ree Mark J. McDonnell Mark A. Burgman Nicholas S. G. Williams Natasha McLean Michael J. Harper Rachelle Meyer Amy Hahs Terry Coates 《Ecological Management & Restoration》2004,5(1):24-27
Summary The habitat hectares approach is an explicit, quantitative method for assessing the quality of vegetation by adding scores that are assigned to 10 habitat attributes. We believe it will be more repeatable and transparent than other methods that rely on subjective judgement. However, we have four principal criticisms of the method as it is currently proposed: (i) measurement of some of the attributes may be subject to considerable error that varies among assessors; (ii) the comparison of each measure with a single benchmark does not accommodate appropriate disturbance regimes; (iii) the proposed combination of attributes leads to some apparent internal inconsistencies; and (iv) it is not clear how the method will actually be used in practice. We suggest modifications to address these concerns and improve the proposed method. Finally, we make additional suggestions about the method's potential application, including: separate reporting of the extent and quality of different vegetation types to avoid the inappropriate combination of measures of area and quality; valuing appropriate disturbance regimes in natural areas; and considering very carefully the application of compensation or mitigation measures. 相似文献
867.
868.
Biotic homogenization has been predicted to occur in cities across the world. However, the empirical evidence has been less than convincing. Lososová et al. explore the middle ground between these two points of view in this issue of Journal of Vegetation Science. They take a more sophisticated approach, linking homogenization to bigger questions of vegetation assembly in urban environments. 相似文献
869.
Claire Fergus Mashael Al-qasem Michelle Cotter Ciara M McDonnell Emiliano Sorrentino Franciane Chevot Karsten Hokamp Mathias O Senge John
M Southern Stephen J Connon Vincent P Kelly 《Nucleic acids research》2021,49(9):4877
Base-modification can occur throughout a transfer RNA molecule; however, elaboration is particularly prevalent at position 34 of the anticodon loop (the wobble position), where it functions to influence protein translation. Previously, we demonstrated that the queuosine modification at position 34 can be substituted with an artificial analogue via the queuine tRNA ribosyltransferase enzyme to induce disease recovery in an animal model of multiple sclerosis. Here, we demonstrate that the human enzyme can recognize a very broad range of artificial 7-deazaguanine derivatives for transfer RNA incorporation. By contrast, the enzyme displays strict specificity for transfer RNA species decoding the dual synonymous NAU/C codons, determined using a novel enzyme-RNA capture-release method. Our data highlight the broad scope and therapeutic potential of exploiting the queuosine incorporation pathway to intentionally engineer chemical diversity into the transfer RNA anticodon. 相似文献
870.
The atrial wall of Notophthalmus viridescens is 25–75 μm thick and is trabeculated sparsely. Coronary vessels are absent. The endocardial endothelium is continuous and has 50–60 nm-wide fenestrae with diaphragms, rests on a discontinuous basal lamina and lacks occluding junctions. Cells found in the subendothelial connective tissue are xanthophores, melanophores, mast cells, fibroblasts, macrophages, and unmyelinated nerve fibers with Schwann cell investments. Epicardial mesothelial cells contain numerous 6–7 nm filaments and lamellar bodies which resemble myelin figures. Mesothelial cell junctions include maculae adhaerentes diminutae, desmosomes, and interdigitations. The epicardial connective tissue layer is more extensive than that of the endocardium, with xanthophores and melanophores rarely present and nerve fibers never observed. The myocardium consists of a mesh-work of myocytes 3–5 cell layers thick with little intervening connective tissue. Myocytes are 6–10 μm in diameter and have two or three peripheral myofibrillae. Typical A, I, H, Z, and M bands are present with a sarcomere length of 2.5 μm. T tubules are not observed. The sarcoplasmic reticulum has subsarcolemmal dilations. The nuclear pole region contains abundant mitochondria and atrial granules, extensive Golgi, and elements of smooth and rough-surfaced endoplasmic reticulum. Lateral intercellular junctions consisting of dense plaques, frequently continuous with Z-line material, are common. Oblique and transversely oriented junctions consisting of primarily of fascia adhaerentes, are present. It appears that amphibian atrial myocytes more closely resemble those of the amphibian ventricle than those of the mammalian atrium. Structural differences between amphibian atrial and ventricular myocytes seem to be quantitative rather than qualitative in nature. 相似文献