全文获取类型
收费全文 | 9980篇 |
免费 | 1058篇 |
国内免费 | 3篇 |
出版年
2022年 | 65篇 |
2021年 | 134篇 |
2020年 | 77篇 |
2019年 | 87篇 |
2018年 | 105篇 |
2017年 | 118篇 |
2016年 | 206篇 |
2015年 | 362篇 |
2014年 | 339篇 |
2013年 | 477篇 |
2012年 | 575篇 |
2011年 | 532篇 |
2010年 | 367篇 |
2009年 | 314篇 |
2008年 | 502篇 |
2007年 | 517篇 |
2006年 | 473篇 |
2005年 | 461篇 |
2004年 | 486篇 |
2003年 | 484篇 |
2002年 | 438篇 |
2001年 | 194篇 |
2000年 | 145篇 |
1999年 | 167篇 |
1998年 | 170篇 |
1997年 | 112篇 |
1996年 | 101篇 |
1995年 | 107篇 |
1994年 | 73篇 |
1993年 | 99篇 |
1992年 | 108篇 |
1991年 | 115篇 |
1990年 | 114篇 |
1989年 | 94篇 |
1988年 | 84篇 |
1987年 | 115篇 |
1986年 | 120篇 |
1985年 | 101篇 |
1984年 | 102篇 |
1983年 | 71篇 |
1982年 | 89篇 |
1981年 | 97篇 |
1980年 | 83篇 |
1979年 | 71篇 |
1978年 | 79篇 |
1977年 | 64篇 |
1976年 | 74篇 |
1974年 | 81篇 |
1973年 | 78篇 |
1970年 | 57篇 |
排序方式: 共有10000条查询结果,搜索用时 430 毫秒
881.
Selena Humphreys Chris S. Elphick Carina Gjerdrum Margaret Rubega 《Journal of Field Ornithology》2007,78(2):152-158
ABSTRACT. Saltmarsh Sharp-tailed Sparrows ( Ammodramus caudacutus ) build ground nests, often with a closely-woven dome, in marshes that frequently flood during high tides. To test the hypothesis that domed nests help reduce the loss of eggs and chicks due to flooding or predation, we examined the characteristics and fate of 102 nests at sites along the coast of Connecticut. To test whether nest structure was tailored to suit microhabitat conditions, we also measured vegetation characteristics around nests. Finally, we conducted artificial nest-flooding experiments to determine whether removal of domes reduced egg retention during flooding. We found no significant effects of nest structure on breeding success or failure, and few significant correlations between nest structure and microhabitat. The height of nests above the ground, however, increased with vegetation height, supporting the hypothesis that nest construction is influenced by flooding, but not supporting the hypothesis that predation risk is important. Dome removal experiments showed that domes have a highly significant effect on the retention of eggs during flooding, suggesting that domes help eggs survive the regular tidal flooding of marshes. 相似文献
882.
883.
Margaret Swain 《American anthropologist》2003,105(1):200-202
Turbulent Times and Enduring Peoples: Mountain Minorities in the Southeast Asian Massif. Jean Michaud. ed. Richmond, UK: Curazon Press, 2000. 255 pp. 相似文献
884.
Rao CY Riggs MA Chew GL Muilenberg ML Thorne PS Van Sickle D Dunn KH Brown C 《Applied and environmental microbiology》2007,73(5):1630-1634
In August and September 2005, Hurricanes Katrina and Rita caused breeches in the New Orleans, LA, levee system, resulting in catastrophic flooding. The city remained flooded for several weeks, leading to extraordinary mold growth in homes. To characterize the potential risks of mold exposures, we measured airborne molds and markers of molds and bacteria in New Orleans area homes. In October 2005, we collected air samples from 5 mildly water-damaged houses, 15 moderately to heavily water-damaged houses, and 11 outdoor locations. The air filters were analyzed for culturable fungi, spores, (1-->3,1-->6)-beta-D-glucans, and endotoxins. Culturable fungi were significantly higher in the moderately/heavily water-damaged houses (geometric mean=67,000 CFU/m3) than in the mildly water-damaged houses (geometric mean=3,700 CFU/m3) (P=0.02). The predominant molds found were Aspergillus niger, Penicillium spp., Trichoderma, and Paecilomyces. The indoor and outdoor geometric means for endotoxins were 22.3 endotoxin units (EU)/m3 and 10.5 EU/m3, respectively, and for (1-->3,1-->6)-beta-D-glucans were 1.7 microg/m3 and 0.9 microg/m3, respectively. In the moderately/heavily water-damaged houses, the geometric means were 31.3 EU/m3 for endotoxins and 1.8 microg/m3 for (1-->3,1-->6)-beta-D-glucans. Molds, endotoxins, and fungal glucans were detected in the environment after Hurricanes Katrina and Rita in New Orleans at concentrations that have been associated with health effects. The species and concentrations were different from those previously reported for non-water-damaged buildings in the southeastern United States. 相似文献
885.
Eric Cascales Susan K. Buchanan Denis Duch Colin Kleanthous Roland Lloubs Kathleen Postle Margaret Riley Stephen Slatin Danile Cavard 《Microbiological reviews》2007,71(1):158-229
Colicins are proteins produced by and toxic for some strains of Escherichia coli. They are produced by strains of E. coli carrying a colicinogenic plasmid that bears the genetic determinants for colicin synthesis, immunity, and release. Insights gained into each fundamental aspect of their biology are presented: their synthesis, which is under SOS regulation; their release into the extracellular medium, which involves the colicin lysis protein; and their uptake mechanisms and modes of action. Colicins are organized into three domains, each one involved in a different step of the process of killing sensitive bacteria. The structures of some colicins are known at the atomic level and are discussed. Colicins exert their lethal action by first binding to specific receptors, which are outer membrane proteins used for the entry of specific nutrients. They are then translocated through the outer membrane and transit through the periplasm by either the Tol or the TonB system. The components of each system are known, and their implication in the functioning of the system is described. Colicins then reach their lethal target and act either by forming a voltage-dependent channel into the inner membrane or by using their endonuclease activity on DNA, rRNA, or tRNA. The mechanisms of inhibition by specific and cognate immunity proteins are presented. Finally, the use of colicins as laboratory or biotechnological tools and their mode of evolution are discussed. 相似文献
886.
Many herbivores elicit biochemical, physiological, or morphological changes in their host plants that render them more resistant
to co-occurring herbivores. Yet, despite the large number of studies that investigate how induced resistance affects herbivore
preference and performance, very few have simultaneously explored the cascading effects of induction on higher trophic levels
and consequences for prey suppression. In our study system, early-season herbivory by leafhoppers elevated plant resistance
to subsequent attack by chrysomelid beetles sharing the same host plant. Notably, beetles feeding on leafhopper-damaged plants
incurred developmental penalties (e.g., prolonged time in early larval instars) that rendered them more susceptible to predation
by natural enemies. As a result, the combined bottom-up effect of leafhopper-induced resistance and the top-down effect of
enhanced predation resulted in the synergistic suppression of beetle populations. These results emphasize that higher trophic
level dynamics should be considered in conjunction with induced resistance to better understand how plants mediate interspecific
interactions in phytophagous insect communities. 相似文献
887.
Stone KL Bjornson RD Blasko GG Bruce C Cofrancesco R Carriero NJ Colangelo CM Crawford JK Crawford JM daSilva NC Deluca JD Elliott JI Elliott MM Flory PJ Folta-Stogniew EJ Gulcicek E Kong Y Lam TT Lee JY Lin A LoPresti MB Mane SM McMurray WJ Tikhonova IR Westman S Williams NA Wu TL Hongyu Z Williams KR 《The Yale journal of biology and medicine》2007,80(4):195-211
888.
Margaret E. Brosnan Erica E. Edison Robin da Silva John T. Brosnan 《Advances in enzyme regulation》2007,47(1):252-260
The text-book view of the role of the creatine/creatine phosphate system as an energy buffer has been expanded to include functions such as energy shuttling, proton buffering and regulating cytosolic ADP levels. There is continuous need for creatine replacement due to creatinine formation. Replacement involves a combination of diet and de novo synthesis. Creatine synthesis makes very significant demands on amino acid metabolism, in particular that of glycine, arginine and methionine. It uses about 40% of all methyl groups transferred from S-adenosylmethionine. Although the traditional view of the function of the creatine/creatine phosphate system is largely concerned with its role in skeletal and cardiac muscle, recent work obliges us to take a broader view. In particular, its role in the brain is brought into sharp focus by the neurological symptoms displayed by children suffering from inborn errors of creatine synthesis and transport, as well as by suggestions that brain creatine status may play a role in cognitive performance in adults. 相似文献
889.
Foster JM Forster ME 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》2007,177(1):41-47
Plasma catecholamines were measured following surgery under anaesthesia and after exposing hagfish to 90 and 110% sea water
(SW). Plasma noradrenaline (NA) concentration increased from a resting value of 7 to 818 nM l−1 on anaesthesia. Plasma adrenaline (AD) did not change. NA concentrations also increased during volume depletion (110% SW),
but to much lower values (26 nM l−1 at 100 min). AD concentrations were increased at 20 min, then fell. During volume loading (90% SW) NA fell, and AD increased
to a maximum concentration of 511 nM l−1 at 40 min (resting concentration 24 nM l−1). The data are consistent with a vasoconstrictory role for NA on central veins when venous pressures fall and a vasodilatory
role for AD on volume expansion. 相似文献
890.