全文获取类型
收费全文 | 113621篇 |
免费 | 1890篇 |
国内免费 | 888篇 |
专业分类
116399篇 |
出版年
2022年 | 86篇 |
2021年 | 188篇 |
2020年 | 156篇 |
2019年 | 166篇 |
2018年 | 12112篇 |
2017年 | 10918篇 |
2016年 | 7923篇 |
2015年 | 1503篇 |
2014年 | 1136篇 |
2013年 | 1597篇 |
2012年 | 5685篇 |
2011年 | 14185篇 |
2010年 | 12798篇 |
2009年 | 8881篇 |
2008年 | 10991篇 |
2007年 | 12563篇 |
2006年 | 1501篇 |
2005年 | 1682篇 |
2004年 | 1994篇 |
2003年 | 2025篇 |
2002年 | 1732篇 |
2001年 | 692篇 |
2000年 | 610篇 |
1999年 | 425篇 |
1998年 | 200篇 |
1997年 | 169篇 |
1996年 | 152篇 |
1995年 | 151篇 |
1994年 | 145篇 |
1993年 | 135篇 |
1992年 | 258篇 |
1991年 | 267篇 |
1990年 | 221篇 |
1989年 | 176篇 |
1988年 | 199篇 |
1987年 | 160篇 |
1986年 | 147篇 |
1985年 | 130篇 |
1984年 | 127篇 |
1983年 | 113篇 |
1982年 | 90篇 |
1981年 | 112篇 |
1979年 | 112篇 |
1978年 | 107篇 |
1977年 | 89篇 |
1976年 | 98篇 |
1975年 | 88篇 |
1974年 | 96篇 |
1972年 | 300篇 |
1971年 | 355篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
73.
It is widely believed that environmental variability is the main cause for fluctuations in commercially exploited small pelagic
fish populations around the world. Nevertheless, density-dependent factors also can drive population dynamics. In this paper,
we analyzed thirteen years of a relative abundance index of two clupeoids fish populations coexisting in the central-south
area off Chile, namely the common sardine, Strangomera bentincki, and anchovy, Engraulis ringens. We applied the classical diagnostic tools of time series analysis to the observed time-series. Also, the realized per capita
population growth rate was studied with the aim of detecting the feedback structure that is characterizing the population
dynamics of the two species. The analysis suggests that population fluctuations of the two species have an important density-dependent
component, displaying first-order (direct density-dependent) and second-order (delayed density-dependent) simultaneously.
The density-dependent component explained 70.5 and 55.6 % of the realized per capita population growth rate of common sardine
and anchovy, respectively. The deterministic skeleton model showed an asymptotic convergence to equilibrium density. In presence
of a stochastic environment, fluctuations were reproduced for the species showing a component of fluctuation with a period
of 4 year. The intrinsic dynamics of each species is typical of interacting species resulting from trophic interactions. It
is postulated that the second-order dynamics of S. bentincki and E. ringens in central-south Chile, may be the result from interactions with a specialist predator (the fishing fleet), interacting with
exogenous environmental factors. 相似文献
74.
Mutations affecting the catalytic activity of Bacillus cereus 5/B/6 beta-lactamase II 总被引:2,自引:0,他引:2
Random in vitro mutagenesis of a cloned Bacillus cereus 5/B/6 beta-lactamase II gene was used to select defective genes unable to confer ampicillin or cephalosporin C resistance to Escherichia coli. DNA sequencing of mutant genes identified histidine at position 28 as important to beta-lactamase II function. In addition, the isolation of six identical frameshift mutants established that the carboxyl-terminal end of beta-lactamase II is critical for enzyme function. Random mutagenesis also revealed that His88 (implicated previously as one of 4 residues acting as a zinc ligand) is crucial to enzymatic activity and that a glycine to glutamic acid substitution at position 148 produced a defective beta-lactamase. Oligonucleotide mutagenesis directed at Glu37 and Glu212 suggests that these residues are inconsequential to enzyme function but that histidine at position 28 may be involved in substrate binding or recognition. 相似文献
75.
Jacek Kęsy Beata Maciejewska Magdalena Sowa Magdalena Szumilak Krzysztof Kawałowski Maja Borzuchowska Jan Kopcewicz 《Plant Growth Regulation》2008,55(1):43-50
It has been shown that both IAA and ethylene application inhibit flower induction in the short-day plant Pharbitis nil. However application of IAA has elevated ethylene production in this plant, as well. Strong enhancement of ethylene production
is also correlated with the night-break effect, which completely inhibits flowering. In order to determine what the role of
IAA and ethylene is in the photoperiodic flower induction in Pharbitis nil, we measured changes in their levels during inductive and non-inductive photoperiods, and the effects of ethylene biosynthesis
and action inhibitors on inhibition of flowering by IAA. Our results have shown that the inhibitory effect of IAA on Pharbitis nil flowering is not physiological but is connected with its effect on ethylene biosynthesis. 相似文献
76.
77.
M Bernard P Canioni P Cozzone J Berthou P Jollès 《International journal of peptide and protein research》1990,36(1):46-55
Carbon-13 NMR spectroscopy has been used to further document the interaction, at low and high temperatures, of N-acetylglucosamine and its short polymers with hen egg-white lysozyme. The results have been compared with the corresponding X-ray crystallographic data. Two domains, the active site and the hydrophobic box, have been found by NMR to undergo conformational rearrangement while X-ray crystallography only detected changes located in the active site. The extent of the modifications induced by inhibitor binding was proportional to the inhibitor size. The two techniques concurred to show that even in the presence of monosaccharide (N-acetylglucosamine), more than one subsite of the enzyme was occupied at high temperature, the binding at the C-site being the best defined. The thermal transition of lysozyme still occurred in solution when inhibitors were bound. However, in the solid state, crystallographic data showed that the transition was hindered. 相似文献
78.
79.
Four main molecular forms of acetylcholinesterase (AChE) can be solubilized from newborn rat superior cervical ganglia (SCG), homogenized in the presence of a high-ionic-strength, detergent-containing medium. These forms, respectively referred to as 16, 10, 6.5, and 4 S, are characterized by their sedimentation coefficients. Their relative proportions in SCG are notably different in vivo during postnatal maturation, and in culture. The 16-S AChE appears to be mainly neuronal in origin, is maintained in culture independently of original presynaptic in vivo elements, and its cellular pool is not depleted in the presence of tetrodotoxin (TTX). 相似文献
80.
Administration of choline chloride i.p. to rats causes a dose-dependent increase in the brain concentration of the neurotransmitter, acetylcholine (ACh). This increase is maximal (22% after a 60-mg/kg dose) 40 minutes after injection. These observations suggest that precursor availability may influence brain ACh synthesis, just as brain tryptophan and tyrosine levels have previously been shown to control the synthesis of brain serotonin and catecholamines. 相似文献