全文获取类型
收费全文 | 18429篇 |
免费 | 1575篇 |
国内免费 | 8篇 |
出版年
2023年 | 77篇 |
2022年 | 172篇 |
2021年 | 400篇 |
2020年 | 232篇 |
2019年 | 284篇 |
2018年 | 363篇 |
2017年 | 320篇 |
2016年 | 506篇 |
2015年 | 849篇 |
2014年 | 933篇 |
2013年 | 1118篇 |
2012年 | 1484篇 |
2011年 | 1354篇 |
2010年 | 899篇 |
2009年 | 850篇 |
2008年 | 1119篇 |
2007年 | 1141篇 |
2006年 | 1012篇 |
2005年 | 1050篇 |
2004年 | 953篇 |
2003年 | 891篇 |
2002年 | 851篇 |
2001年 | 181篇 |
2000年 | 162篇 |
1999年 | 206篇 |
1998年 | 250篇 |
1997年 | 168篇 |
1996年 | 154篇 |
1995年 | 112篇 |
1994年 | 129篇 |
1993年 | 125篇 |
1992年 | 110篇 |
1991年 | 99篇 |
1990年 | 134篇 |
1989年 | 114篇 |
1988年 | 93篇 |
1987年 | 98篇 |
1986年 | 59篇 |
1985年 | 92篇 |
1984年 | 102篇 |
1983年 | 62篇 |
1982年 | 88篇 |
1981年 | 72篇 |
1980年 | 55篇 |
1979年 | 53篇 |
1978年 | 49篇 |
1977年 | 40篇 |
1976年 | 30篇 |
1975年 | 28篇 |
1974年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 578 毫秒
161.
Evaluation of toxic conditions associated with oranging symptoms of rice in a flooded Oxisol in Sumatra,Indonesia 总被引:3,自引:0,他引:3
The toxic conditions of Oxisol soils attributed to oranging symptoms of rice grown in the Sitiung Transmigration area, Sumatra, Indonesia were evaluated in the laboratory. Changes of pH and Eh of flooded soils, and concentrations of nutrients in the soils and in the rice plants were measured. The soils were clayey, kaolinitic, isohyperthermic, Typic Haplorthox. It was found that Eh of the soils sharply decreased from an average value of +460 ± 150 mV to –217 ± 15 mV following 60 days of flooding (DF). During the same period of flooding, soil pH increased from an average value of 5.2 ± 0.6 to 6.6 ± 0.2. Concentrations of NaOAc extractable Fe, Mn, Zn, Cu, Mo, Ca, Mg, P, and K, but not Al, increased markedly whereas their water-soluble form, except Fe, decreased slightly following 60 DF. Leaf tissue analyses indicated that 13, 51 and 58% of the rice plant samples contained potentially toxic level of Mn, Fe and Al, respectively, as their contents were higher than the assumed threshold toxicity levels of 2500, 300, and 300 mg kg–1. Thirteen, 16, 2, and 3% of the leaf tissue also contained potentially deficient levels of P, K, Ca, and Mg, respectively. The oranging symptom in the rice leaf tissue appeared to be due to indirect toxicity of Fe, Mn, and Al, i.e., Fe-induced, Mn-induced, and Al-induced deficiency of P, K, Ca and Mg. As a result of the relatively high concentrations of NaOAc extractable Fe, Mn, and Al in the soil solution, root growth was limited and coated with iron and manganese oxides thereby reducing the root's capacity to absorb nutrients from the soils.The work was supported by USAID Grant No. DPE-5542-G-SS-4055-00 (3.F-10). Contribution from the Wetland Biogeochemistry Institute, Louisiana State University, Baton Rouge, LA 70803-7511, USA. 相似文献
162.
Michel J. Gauthier Gilles N. Flatau René L. Clément Patrick M. Munro 《Microbial ecology》1993,26(1):29-35
Using strains with or without the PhoE porin or different components of the phosphate regulon, we determined that maintenance of the culturability of Escherichia coli in seawater depended significantly on the presence of structures allowing access of phosphate ions to the periplasm, then to the cytoplasm of cells. Cells totally deprived of the two main phosphate transport systems (Pit, Pst) exhibited the highest loss of culturability. Most of this effect resulted from the loss of the high-affinity Pst system, and more specifically that of the periplasmic phosphate-binding protein PhoS. Survival was enhanced in seawater supplemented with phosphate (0.5 mm), whether or not these structures were present. From an ecological point of view, it is assumed that the presence of phosphate ions, even at low concentrations, can influence the behavior of E. coli cells in seawater.
Offprint requests to: M.J. Gauthier 相似文献
163.
Bradley B. Olwin Kirstin Arthur Kevin Hannon Patrick Hein Zhaohui Zhou Michael E. Zuber Arthur J. Kudla Aidan McFall Alan C. Rapraeger Bruce Riley Gyrgyi Szebenyi John F. Fallon 《Molecular reproduction and development》1994,39(1):90-101
Fibroblast growth factors (FGFs) are a family of nine proteins that bind to three distinct types of cell surface molecules: (i) FGF receptor tyrosine kinases (FGFR-1 through FGFR-4); (ii) a cysteine-rich FGF receptor (CFR); and (iii) heparan sulfate proteoglycans (HSPGs). Signaling by FGFs requires participation of at least two of these receptors: the FGFRs and HSPGs form a signaling complex. The length and sulfation pattern of the heparan sulfate chain determines both the activity of the signaling complex and, in part, the ligand specificity for FGFR-1. Thus, the heparan sulfate proteoglycans are likely to play an essential role in signaling. We have recently identified a role for FGF in limb bud development in vivo. In the chick limb bud, ectopic expression of the 18 kDa form of FGF-2 or FGF-2 fused to an artificial signal peptide at its amino terminus causes skeletal duplications. These data, and the observations that FGF-2 is localized to the subjacent mesoderm and the apical ectodermal ridge in the early developing limb, suggest that FGF-2 plays an important role in limb outgrowth. We propose that FGF-2 is an apical ectodermal ridgederived factor that participates in limb outgrowth and patterning. © 1994 Wiley-Liss, Inc. 相似文献
164.
The degree of natal philopatry (the likelihood that individualsbreed at or near their place of origin) can influence the extentof inbreeding in animal populations. Passerine birds have beencited as typically showing high natal philopatry, and natalphilopatry has been proposed as an adaptation to promote optimalinbreeding. A review of published and unpublished studies ofpasserines showed that natal philopatry was typically low, somaintaining a high level of inbreeding appears relatively unimportantfor such birds. Rather, natal philopatry appeared to be morestrongly influenced by ecological factors. Migratory passerineexhibited low natal philopatry compared to resident passerines,as predicted if dispersal costs for young birds are an importantdeterminant of natal philopatry. The erroneous view that natalphilopatry for passerines is generally high has resulted froma reporting bias toward resident species that have sufficientnatal philopatry to study. Natal philopatry was found to beevolutionarily labile; populations of the same species and pairsof closely related species that differed in their degree ofisolation differed considerably in their degree of philopatry.Future studies of natal philopatry should consider both theecological factors that could affect dispersal costs and thereporting biases that influence which data on philopatry tendto be reported. 相似文献
165.
Patrick A. Thorpe Janella Loye Cynthia A. Rote W. J. Dickinson 《Journal of molecular evolution》1993,37(6):590-599
In an effort to understand the forces shaping evolution of regulatory genes and patterns, we have compared data on interspecific differences in enzyme expression patterns among the rapidly evolving Hawaiian picture-winged Drosophila to similar data on the more conservative virilis species group. Divergence of regulatory patterns is significantly more common in the former group, but cause and effect are difficult to discern. Random fixation of regulatory variants in small populations and/or during speciation may be somewhat more likely than divergence driven by selection. Within the picture-winged group, we also have compared enzymes that fulfill different metabolic roles. There are highly significant differences between individual enzymes, but no obvious correlations to functional categories.
Correspondence to: W.J. Dickinson 相似文献
166.
Marc Vandermeeren‡ Marc Mercken‡ Eugeen Vanmechelen‡ Jan Six‡ ré Van de Voorde‡Jean-Jacques Martin† Patrick Cras 《Journal of neurochemistry》1993,61(5):1828-1834
Abstract— Alzheimer's disease is a progressive degenerative dementia characterized by the abundant presence of neurofibrillary tangles in neurons. This study was designed to test whether the microtubule-associated protein, a major component of neurofibrillary tangles, could be detected in CSF. Additionally, we investigated whether CSF levels were abnormal in Alzheimer's disease as compared with a large group of control patients. We developed a sensitive sandwich enzyme-linked immunosorbent assay using AT120, a monoclonal antibody directed to human, as a capturing antibody. With this technique, the detection limit for was less than 5 pg/ml of CSF. Using ATS, which recognizes abnormally phosphorylated ser-ines 199–202 in, the detection limit was below 20 pg/ml of CSF. However, with AT8, we found no immunoreactiv-ity in CSF, suggesting that only a small fraction of CSF contains the abnormally phosphorylated AT8 epitope. Our results indicate that CSF levels are significantly increased in Alzheimer's disease. Also, CSF levels in a large group of patients with a diversity of neurological diseases showed overlap with CSF levels in Alzheimer's disease. 相似文献
167.
Fewer protective cytotoxic T-cell epitopes than T-helper-cell epitopes on vesicular stomatitis virus. 总被引:6,自引:5,他引:1
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
T M Kündig I Castelmur M F Bachmann D Abraham D Binder H Hengartner R M Zinkernagel 《Journal of virology》1993,67(6):3680-3683
Cytotoxic T-lymphocyte (CTL) and T-helper-cell responses in various mouse strains were monitored. Protective CTL responsiveness against three proteins of vesicular stomatitis virus was H-2 linked and inducible only in half of the 15 combinations tested (each of five H-2 haplotypes combined with each of three viral proteins), whereas biologically relevant T-helper-cell responses were inducible in all. This suggests that vesicular stomatitis virus exhibits more T-helper-cell than CTL epitopes. 相似文献
168.
Barium permeability of neuronal nicotinic receptor alpha 7 expressed in Xenopus oocytes. 总被引:2,自引:2,他引:0
下载免费PDF全文
![点击此处可从《Biophysical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The rat alpha 7 neuronal nicotinic acetylcholine receptor was expressed and studied in Xenopus oocytes. The magnitude and reversal potential of instantaneous whole cell currents were examined in solutions containing varying concentrations of either calcium or barium, and in the presence or absence of the intracellular calcium chelator BAPTA. In external barium, application of nicotine elicits an inwardly rectifying response; in calcium the response is larger and has a linear IV relation. Pretreatment of oocytes with BAPTA-AM could not prevent activation of calcium-dependent chloride channels in external Ringer containing calcium. Using an extended GHK equation, the permeability ratio PBa/PNa of the alpha 7 receptor was determined to be about 17. Our results suggest that alpha 7 nicotinic receptors are highly permeable to divalent cations. 相似文献
169.
Patrick C. Phillips 《Evolution; international journal of organic evolution》1993,47(6):1733-1743
The third phase of Wright's shifting-balance theory involves the export of adaptive gene combinations from one subpopulation to another. Previous results have demonstrated that this can occur at very low migration rates, but it has been argued that this simply reflects the ability of migration to overcome selection and fix any (even deleterious) alleles. Here, previous analyses are extended by concentrating on the critical balance between forward and reverse migration rates that still allows phase III to proceed. It is shown that selective advantage, dominance, recombination rate, and the number of loci all affect the ability of a genotype to invade and become fixed in a new subpopulation, but it is unlikely that phase III will occur in the absence of differential migration unless the invading genotype consists of a few dominant loci with a large selection advantage, spreading into a few populations of lower fitness. Therefore, as was envisioned by Wright, differential migration from more to less fit populations will be necessary for phase III to occur under most circumstances. 相似文献
170.
Filamin is a well-characterized actin-associated protein first isolated from chicken smooth muscle. Subsequently, this polypeptide and its nonmuscle homolog actin-binding protein have been shown to be expressed in avian muscle tissue, mammalian smooth muscle, mammalian macrophages and other blood cell types, as well as several cultured cell lines. In this report, the occurrence of this polypeptide in adult mammalian organs has been investigated. Immunoblot analysis using three anti-filamin monoclonal antibodies showed that this protein was largely detected in adult rat organs that possess a substantial smooth muscle component. Furthermore, the limited expression of filamin in smooth muscle tissue was corroborated by immunohistochemical analysis. In contrast to avian systems, filamin was never found in detectable quantities in either mammalian cardiac or skeletal muscle. Quantitative immunoblot analysis demonstrated that filamin amounts roughly correlated with the abundance of the smooth muscle component of a given organ, comprising as much as 16.5% of the total SDS-extractable protein in bovine aorta. Work in avian systems and cells in culture has suggested that filamin is a rather ubiquitous cytoskeletal element. By contrast, this work demonstrates that filamin is highly restricted in its expression in mammalian organ systems, in situ. 相似文献