全文获取类型
收费全文 | 34361篇 |
免费 | 2884篇 |
国内免费 | 10篇 |
专业分类
37255篇 |
出版年
2023年 | 211篇 |
2022年 | 455篇 |
2021年 | 834篇 |
2020年 | 561篇 |
2019年 | 706篇 |
2018年 | 845篇 |
2017年 | 762篇 |
2016年 | 1094篇 |
2015年 | 1507篇 |
2014年 | 1725篇 |
2013年 | 2057篇 |
2012年 | 2348篇 |
2011年 | 2222篇 |
2010年 | 1380篇 |
2009年 | 1200篇 |
2008年 | 1547篇 |
2007年 | 1577篇 |
2006年 | 1364篇 |
2005年 | 1315篇 |
2004年 | 1246篇 |
2003年 | 1095篇 |
2002年 | 1048篇 |
2001年 | 733篇 |
2000年 | 668篇 |
1999年 | 583篇 |
1998年 | 314篇 |
1997年 | 267篇 |
1996年 | 264篇 |
1995年 | 216篇 |
1994年 | 252篇 |
1993年 | 212篇 |
1992年 | 381篇 |
1991年 | 357篇 |
1990年 | 328篇 |
1989年 | 330篇 |
1988年 | 310篇 |
1987年 | 353篇 |
1986年 | 275篇 |
1985年 | 281篇 |
1984年 | 270篇 |
1983年 | 231篇 |
1982年 | 191篇 |
1981年 | 183篇 |
1980年 | 168篇 |
1979年 | 235篇 |
1978年 | 208篇 |
1975年 | 181篇 |
1974年 | 175篇 |
1973年 | 176篇 |
1972年 | 177篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Jacqueline A. Wilson Laurent Vigliola Mark G. Meekan 《Journal of experimental marine biology and ecology》2009,368(1):9-21
Multiple internal and external tagging experiments tested the applicability of five back-calculation models (Biological Intercept, Modified-Fry, Body Proportional Hypothesis, Time-Varying Growth, and an Age-Effects model) as predictors of individual growth trajectories of two marine cleaning gobies, Elacatinus evelynae and E. prochilos, that were raised in aquaria under conditions that resulted in variable growth. Mixed-effect model analyses of longitudinal records of otoliths and somatic growth collected at the individual level revealed that E. evelynae and E. prochilos produced daily increments on their otoliths for up to two months post-settlement and that the Modified-Fry model provided the most accurate size-at-age estimates despite the presence of age, growth and time-varying growth effects in the dataset. Very large errors in predicted size were produced by the Age-Effects model. The four other back-calculation models all provided slightly biased estimates of back-calculated size-at-age, with the Modified-Fry model providing the least biased estimates. Regardless of bias, both experimental and theoretical evidence indicated that back-calculated size was a better proxy of fish length than otoliths radius. Relationships between fish length and otoliths radius were allometric at the level of individuals, which explained why the Modified-Fry model performed better. However, this allometry was undetectable at the population level. This study represents the first attempt to validate modern back-calculation models using longitudinal data collected and analysed at the individual level. Our results suggest the use of the Modified-Fry model as a conservative approach in routine back-calculations of fish size at age from otoliths. 相似文献
992.
Izabela Lesna Fernando R. da Silva Yukie Sato Maurice W. Sabelis Suzanne T. E. Lommen 《Experimental & applied acarology》2014,63(2):189-204
The dry bulb mite, Aceria tulipae, is the most important pest of stored tulip bulbs in The Netherlands. This tiny, eriophyoid mite hides in the narrow space between scales in the interior of the bulb. To achieve biological control of this hidden pest, candidate predators small enough to move in between the bulb scales are required. Earlier experiments have shown this potential for the phytoseiid mite, Neoseiulus cucumeris, but only after the bulbs were exposed to ethylene, a plant hormone that causes a slight increase in the distance between tulip bulb scales, just sufficient to allow this predator to reach the interior part of the bulb. Applying ethylene, however, is not an option in practice because it causes malformation of tulip flowers. In fact, to prevent this cosmetic damage, bulb growers ventilate rooms where tulip bulbs are stored, thereby removing ethylene produced by the bulbs (e.g. in response to mite or fungus infestation). Recently, studies on the role of predatory mites in controlling another eriophyoid mite on coconuts led to the discovery of an exceptionally small phytoseiid mite, Neoseiulus paspalivorus. This predator is able to move under the perianth of coconuts where coconut mites feed on meristematic tissue of the fruit. This discovery prompted us to test N. paspalivorus for its ability to control A. tulipae on tulip bulbs under storage conditions (ventilated rooms with bulbs in open boxes; 23 °C; storage period June–October). Using destructive sampling we monitored predator and prey populations in two series of replicated experiments, one at a high initial level of dry bulb mite infestation, late in the storage period, and another at a low initial dry bulb mite infestation, halfway the storage period. The first and the second series involved treatment with N. paspalivorus and a control experiment, but the second series had an additional treatment in which the predator N. cucumeris was released. Taking the two series of experiments together we found that N. paspalivorus controlled the populations of dry bulb mites both on the outer scale of the bulbs as well as in the interior part of the bulbs, whereas N. cucumeris significantly reduced the population of dry bulb mites on the outer scale, but not in the interior part of the bulb. Moreover, N. paspalivorus was found predominantly inside the bulb, whereas N. cucumeris was only found on the outer scale, thereby confirming our hypothesis that the small size of N. paspalivorus facilitates access to the interior of the bulbs. We argue that N. paspalivorus is a promising candidate for the biological control of dry bulb mites on tulip bulbs under storage conditions in the Netherlands. 相似文献
993.
Cristiane Pilisso Patrícia de Oliveira Carvalho Maria da Graa Nascimento 《Process Biochemistry》2009,44(12):1352-1357
The enzymatic acylation of (RS)-phenylethylamine with different acyl donors catalysed by lipases, was studied in organic solvents with different hydrophobicities and in mixtures with ionic liquids ((ILs); [BMIm][BF4], [BMIm][SCN], [BMIm][Cl] and [BMIm][PF6]). Using lipases from Candida antarctica B (CAL-B) and from Aspergillus niger higher conversion degrees and E-values were obtained with ethyl acetate as the acyl donor. When CAL-B was used as the biocatalyst, in a two-phase system formed by [BMIm][X]/dichloromethane or [BMIm][X]/chloroform, the selectivity was better than that obtained in pure organic solvents. The selectivity was found to be related to individual anions in ILs. In this reaction, the ion effectiveness in enhancing the enzyme selectivity followed the series: Cl− > SCN− > BF4− > PF6− in mixtures with dichloromethane, and PF6− > BF4− > SCN− > Cl− in mixtures with chloroform. 相似文献
994.
It is common to portray conservative and liberal Protestant denominations as “strong” and “weak” on the basis of indices such
as church attendance. Alternatively, they can be regarded as qualitatively different cultural systems that coexist in a multiple-niche
environment. We integrate these two perspectives with a study of American teenagers based on both one-time survey information
and the experience sampling method (ESM), which records individual experience on a moment-by-moment basis. Conservative Protestant
youth were found to be more satisfied, family-oriented, and sociable than liberal Protestant youth, but also more dependent
on their social environment, which is reflected in a deterioration of their mood when they are alone. Liberal Protestant youth
appear to have internalized values that remain constant whether in the presence or absence of others. We relate these results
to the social scientific literature on liberalism and conservatism and to evolutionary theory as a framework for explaining
cultural systems as adaptations to multiple-niche environments.
相似文献
David Sloan WilsonEmail: |
995.
Pereira RP Fachinetto R de Souza Prestes A Puntel RL Santos da Silva GN Heinzmann BM Boschetti TK Athayde ML Bürger ME Morel AF Morsch VM Rocha JB 《Neurochemical research》2009,34(5):973-983
Considering the important role of oxidative stress in the pathogenesis of several neurological diseases, and the growing evidence
of the presence of compounds with antioxidant properties in the plant extracts, the aim of the present study was to investigate
the antioxidant capacity of three plants used in Brazil to treat neurological disorders: Melissa officinalis, Matricaria recutita and Cymbopogon citratus. The antioxidant effect of phenolic compounds commonly found in plant extracts, namely, quercetin, gallic acid, quercitrin
and rutin was also examined for comparative purposes. Cerebral lipid peroxidation (assessed by TBARS) was induced by iron
sulfate (10 μM), sodium nitroprusside (5 μM) or 3-nitropropionic acid (2 mM). Free radical scavenger properties and the chemical
composition of plant extracts were assessed by 1′-1′ Diphenyl-2′ picrylhydrazyl (DPPH) method and by Thin Layer Chromatography
(TLC), respectively. M. officinalis aqueous extract caused the highest decrease in TBARS production induced by all tested pro-oxidants. In the DPPH assay, M. officinalis presented also the best antioxidant effect, but, in this case, the antioxidant potencies were similar for the aqueous, methanolic
and ethanolic extracts. Among the purified compounds, quercetin had the highest antioxidant activity followed by gallic acid,
quercitrin and rutin. In this work, we have demonstrated that the plant extracts could protect against oxidative damage induced
by various pro-oxidant agents that induce lipid peroxidation by different process. Thus, plant extracts could inhibit the
generation of early chemical reactive species that subsequently initiate lipid peroxidation or, alternatively, they could
block a common final pathway in the process of polyunsaturated fatty acids peroxidation. Our study indicates that M. officinalis could be considered an effective agent in the prevention of various neurological diseases associated with oxidative stress. 相似文献
996.
Jose Luis Martinez María Blanca Sánchez Laura Martínez-Solano Alvaro Hernandez Leonor Garmendia Alicia Fajardo & Carolina Alvarez-Ortega 《FEMS microbiology reviews》2009,33(2):430-449
Multidrug efflux pumps have emerged as relevant elements in the intrinsic and acquired antibiotic resistance of bacterial pathogens. In contrast with other antibiotic resistance genes that have been obtained by virulent bacteria through horizontal gene transfer, genes coding for multidrug efflux pumps are present in the chromosomes of all living organisms. In addition, these genes are highly conserved (all members of the same species contain the same efflux pumps) and their expression is tightly regulated. Together, these characteristics suggest that the main function of these systems is not resisting the antibiotics used in therapy and that they should have other roles relevant to the behavior of bacteria in their natural ecosystems. Among the potential roles, it has been demonstrated that efflux pumps are important for processes of detoxification of intracellular metabolites, bacterial virulence in both animal and plant hosts, cell homeostasis and intercellular signal trafficking. 相似文献
997.
Maurecilne Lemes da Silva Daniela Lopes Paim Pinto Miguel Pedro Guerra Eny Iochevet Segal Floh Cláudio Horst Bruckner Wagner Campos Otoni 《Plant Cell, Tissue and Organ Culture》2009,99(1):47-54
The objective of the present work was to induce somatic embryogenesis from zygotic embryos of Passiflora cincinnata Masters. Zygotic embryos formed calli on media with different concentrations of 2,4-dichlorophenoxyacetic acid (2,4-D) and
4.5 μM benzyladenine (BA) after 30 days of in vitro culture. A concentration of 18.1 μM 2,4-D resulted in the largest number
of somatic embryos. Embryogenic calli were yellowish and friable, forming whitish proembryogenic masses. Morphologically,
embryogenic cells were small and had large nuclei and dense cytoplasm, whereas non-embryogenic cells were elongated, with
small nuclei and less dense cytoplasm. Calli cultured under white light on basal Murashige and Skoog’s medium with activated
charcoal produced embryos in all developmental stages. There were differences among the treatments, with some leading to the
production of calli with embryos and some only to callus formation. Some abnormalities were associated with somatic embryos,
including fused axes, fused cotyledons and polycotyledonary embryos. Production of secondary somatic embryos occurred in the
first cycle of primary embryo development. Secondary embryos differentiated from the surface of the protodermal layer of primary
embryos with intense cell proliferation, successive mitotic divisions in the initial phase of embryoid development, and a
vascular system formed with no connection to the parental tissue. This secondary embryogenic system of P. cincinnata is characterized by intense proliferation and maintenance of embryogenic competence after successive subcultures. This reproducible
protocol opens new prospects for massive propagation and is an alternative to the current organogenesis-based transformation
protocol. 相似文献
998.
Fatemeh Rafii Miseon Park Gonçalo Gamboa da Costa Luisa Camacho 《Archives of microbiology》2009,191(12):895-902
The production of short-chain fatty acids, reductive enzymes, and hydrolytic enzymes by four gatifloxacin-selected, fluoroquinolone-resistant,
mutant strains of C. perfringens, with stable mutations either in DNA gyrase or in both DNA gyrase and topoisomerase IV, was compared with that produced by
the wild-type parent strains to investigate the effect of mutations associated with the selection of gatifloxacin resistance
on bacterial metabolic activities. The mutants differed from their respective wild-type parent strains in the enzymatic activities
of azoreductase, nitroreductase, and β-glucosidase and in the ratio of butyric acid to acetic acid production. Microarray
analysis of one wild type and the corresponding mutant revealed different levels of mRNA expression for the enzymes involved
in short-chain fatty acid (SCFA) synthesis and for β-glucosidase and oxidoreductases. In addition to mutations in the target
genes, selection of resistance to gatifloxacin resulted in strain-specific physiological changes in the resistant mutants
of C. perfringens that affected their metabolic activities. 相似文献
999.
Stefano Pluchino Lucia Zanotti Elena Brambilla Patrizia Rovere-Querini Annalisa Capobianco Clara Alfaro-Cervello Giuliana Salani Chiara Cossetti Giovanna Borsellino Luca Battistini Maurilio Ponzoni Claudio Doglioni Jose Manuel Garcia-Verdugo Giancarlo Comi Angelo A. Manfredi Gianvito Martino 《PloS one》2009,4(6)
Background
The systemic injection of neural stem/precursor cells (NPCs) provides remarkable amelioration of the clinico-pathological features of experimental autoimmune encephalomyelitis (EAE). This is dependent on the capacity of transplanted NPCs to engage concurrent mechanisms of action within specific microenvironments in vivo. Among a wide range of therapeutic actions alternative to cell replacement, neuroprotective and immune modulatory capacities of transplanted NPCs have been described. However, lacking is a detailed understanding of the mechanisms by which NPCs exert their therapeutic plasticity. This study was designed to identify the first candidate that exemplifies and sustains the immune modulatory capacity of transplanted NPCs.Methodology/Principal Findings
To achieve the exclusive targeting of the peripheral immune system, SJL mice with PLP-induced EAE were injected subcutaneously with NPCs and the treatment commenced prior to disease onset. NPC-injected EAE mice showed significant clinical improvement, as compared to controls. Exogenous NPCs lacking the expression of major neural antigens were reliably (and for long-term) found at the level of draining lymph nodes, while establishing sophisticated anatomical interactions with lymph node cells. Importantly, injected NPCs were never found in organs other than lymph nodes, including the brain and the spinal cord. Draining lymph nodes from transplanted mice showed focal up-regulation of major developmental stem cell regulators, such as BMP-4, Noggin and Sonic hedgehog. In lymph nodes, injected NPCs hampered the activation of myeloid dendritic cells (DCs) and steadily restrained the expansion of antigen-specific encephalitogenic T cells. Both ex vivo and in vitro experiments identified a novel highly NPC-specific–BMP-4-dependent–mechanism hindering the DC maturation.Conclusion/Significance
The study described herein, identifies the first member of the TGF β/BMP family of stem cell regulators as a novel tolerogenic factor released by NPCs. Full exploitation of this pathway as an efficient tool for vaccination therapy in autoimmune inflammatory conditions is underway. 相似文献1000.