全文获取类型
收费全文 | 19759篇 |
免费 | 64篇 |
国内免费 | 104篇 |
专业分类
系统科学 | 90篇 |
丛书文集 | 77篇 |
教育与普及 | 48篇 |
理论与方法论 | 56篇 |
现状及发展 | 8587篇 |
研究方法 | 929篇 |
综合类 | 9854篇 |
自然研究 | 286篇 |
出版年
2012年 | 313篇 |
2011年 | 578篇 |
2010年 | 124篇 |
2009年 | 122篇 |
2008年 | 328篇 |
2007年 | 401篇 |
2006年 | 394篇 |
2005年 | 401篇 |
2004年 | 372篇 |
2003年 | 348篇 |
2002年 | 406篇 |
2001年 | 723篇 |
2000年 | 717篇 |
1999年 | 484篇 |
1994年 | 312篇 |
1992年 | 434篇 |
1991年 | 352篇 |
1990年 | 390篇 |
1989年 | 339篇 |
1988年 | 342篇 |
1987年 | 362篇 |
1986年 | 353篇 |
1985年 | 450篇 |
1984年 | 375篇 |
1983年 | 302篇 |
1982年 | 283篇 |
1981年 | 283篇 |
1980年 | 285篇 |
1979年 | 664篇 |
1978年 | 540篇 |
1977年 | 423篇 |
1976年 | 453篇 |
1975年 | 423篇 |
1974年 | 448篇 |
1973年 | 375篇 |
1972年 | 415篇 |
1971年 | 514篇 |
1970年 | 631篇 |
1969年 | 448篇 |
1968年 | 496篇 |
1967年 | 461篇 |
1966年 | 444篇 |
1965年 | 304篇 |
1964年 | 128篇 |
1959年 | 157篇 |
1958年 | 271篇 |
1957年 | 170篇 |
1956年 | 140篇 |
1955年 | 134篇 |
1954年 | 134篇 |
排序方式: 共有10000条查询结果,搜索用时 390 毫秒
981.
The role of barren stalk1 in the architecture of maize 总被引:4,自引:0,他引:4
Gallavotti A Zhao Q Kyozuka J Meeley RB Ritter MK Doebley JF Pè ME Schmidt RJ 《Nature》2004,432(7017):630-635
The architecture of higher plants is established through the activity of lateral meristems--small groups of stem cells formed during vegetative and reproductive development. Lateral meristems generate branches and inflorescence structures, which define the overall form of a plant, and are largely responsible for the evolution of different plant architectures. Here, we report the isolation of the barren stalk1 gene, which encodes a non-canonical basic helix-loop-helix protein required for the initiation of all aerial lateral meristems in maize. barren stalk1 represents one of the earliest genes involved in the patterning of maize inflorescences, and, together with the teosinte branched1 gene, it regulates vegetative lateral meristem development. The architecture of maize has been a major target of selection for early agriculturalists and modern farmers, because it influences harvesting, breeding strategies and mechanization. By sampling nucleotide diversity in the barren stalk1 region, we show that two haplotypes entered the maize gene pool from its wild progenitor, teosinte, and that only one was incorporated throughout modern inbreds, suggesting that barren stalk1 was selected for agronomic purposes. 相似文献
982.
Hydrogenosomes are double-membraned ATP-producing and hydrogen-producing organelles of diverse anaerobic eukaryotes. In some versions of endosymbiotic theory they are suggested to be homologues of mitochondria, but alternative views suggest they arose from an anaerobic bacterium that was distinct from the mitochondrial endosymbiont. Here we show that the 51-kDa and 24-kDa subunits of the NADH dehydrogenase module in complex I, the first step in the mitochondrial respiratory chain, are active in hydrogenosomes of Trichomonas vaginalis. Like mitochondrial NADH dehydrogenase, the purified Trichomonas enzyme can reduce a variety of electron carriers including ubiquinone, but unlike the mitochondrial enzyme it can also reduce ferredoxin, the electron carrier used for hydrogen production. The presence of NADH dehydrogenase solves the long-standing conundrum of how hydrogenosomes regenerate NAD+ after malate oxidation. Phylogenetic analyses show that the Trichomonas 51-kDa homologue shares common ancestry with the mitochondrial enzyme. Recruitment of complex I subunits into a H2-producing pathway provides evidence that mitochondria and hydrogenosomes are aerobic and anaerobic homologues of the same endosymbiotically derived organelle. 相似文献
983.
Mechelli A Crinion JT Noppeney U O'Doherty J Ashburner J Frackowiak RS Price CJ 《Nature》2004,431(7010):757
Humans have a unique ability to learn more than one language--a skill that is thought to be mediated by functional (rather than structural) plastic changes in the brain. Here we show that learning a second language increases the density of grey matter in the left inferior parietal cortex and that the degree of structural reorganization in this region is modulated by the proficiency attained and the age at acquisition. This relation between grey-matter density and performance may represent a general principle of brain organization. 相似文献
984.
Humans differ from other primates in their significantly lengthened growth period. The persistence of a fetal pattern of brain growth after birth is another important feature of human development. Here we present the results of an analysis of the 1.8-million-year-old Mojokerto child (Perning 1, Java), the only well preserved skull of a Homo erectus infant, by computed tomography. Comparison with a large series of extant humans and chimpanzees indicates that this individual was about 1 yr (0-1.5 yr) old at death and had an endocranial capacity at 72-84% of an average adult H. erectus. This pattern of relative brain growth resembles that of living apes, but differs from that seen in extant humans. It implies that major differences in the development of cognitive capabilities existed between H. erectus and anatomically modern humans. 相似文献
985.
Fromhold TM Patanè A Bujkiewicz S Wilkinson PB Fowler D Sherwood D Stapleton SP Krokhin AA Eaves L Henini M Sankeshwar NS Sheard FW 《Nature》2004,428(6984):726-730
Understanding how complex systems respond to change is of fundamental importance in the natural sciences. There is particular interest in systems whose classical newtonian motion becomes chaotic as an applied perturbation grows. The transition to chaos usually occurs by the gradual destruction of stable orbits in parameter space, in accordance with the Kolmogorov-Arnold-Moser (KAM) theorem--a cornerstone of nonlinear dynamics that explains, for example, gaps in the asteroid belt. By contrast, 'non-KAM' chaos switches on and off abruptly at critical values of the perturbation frequency. This type of dynamics has wide-ranging implications in the theory of plasma physics, tokamak fusion, turbulence, ion traps, and quasicrystals. Here we realize non-KAM chaos experimentally by exploiting the quantum properties of electrons in the periodic potential of a semiconductor superlattice with an applied voltage and magnetic field. The onset of chaos at discrete voltages is observed as a large increase in the current flow due to the creation of unbound electron orbits, which propagate through intricate web patterns in phase space. Non-KAM chaos therefore provides a mechanism for controlling the electrical conductivity of a condensed matter device: its extreme sensitivity could find applications in quantum electronics and photonics. 相似文献
986.
Genetic and developmental basis of evolutionary pelvic reduction in threespine sticklebacks 总被引:1,自引:0,他引:1
Shapiro MD Marks ME Peichel CL Blackman BK Nereng KS Jónsson B Schluter D Kingsley DM 《Nature》2004,428(6984):717-723
Hindlimb loss has evolved repeatedly in many different animals by means of molecular mechanisms that are still unknown. To determine the number and type of genetic changes underlying pelvic reduction in natural populations, we carried out genetic crosses between threespine stickleback fish with complete or missing pelvic structures. Genome-wide linkage mapping shows that pelvic reduction is controlled by one major and four minor chromosome regions. Pitx1 maps to the major chromosome region controlling most of the variation in pelvic size. Pelvic-reduced fish show the same left-right asymmetry seen in Pitx1 knockout mice, but do not show changes in Pitx1 protein sequence. Instead, pelvic-reduced sticklebacks show site-specific regulatory changes in Pitx1 expression, with reduced or absent expression in pelvic and caudal fin precursors. Regulatory mutations in major developmental control genes may provide a mechanism for generating rapid skeletal changes in natural populations, while preserving the essential roles of these genes in other processes. 相似文献
987.
988.
van Boekel R Min M Leinert Ch Waters LB Richichi A Chesneau O Dominik C Jaffe W Dutrey A Graser U Henning T de Jong J Köhler R de Koter A Lopez B Malbet F Morel S Paresce F Perrin G Preibisch T Przygodda F Schöller M Wittkowski M 《Nature》2004,432(7016):479-482
Our Solar System was formed from a cloud of gas and dust. Most of the dust mass is contained in amorphous silicates, yet crystalline silicates are abundant throughout the Solar System, reflecting the thermal and chemical alteration of solids during planet formation. (Even primitive bodies such as comets contain crystalline silicates.) Little is known about the evolution of the dust that forms Earth-like planets. Here we report spatially resolved detections and compositional analyses of these building blocks in the innermost two astronomical units of three proto-planetary disks. We find the dust in these regions to be highly crystallized, more so than any other dust observed in young stars until now. In addition, the outer region of one star has equal amounts of pyroxene and olivine, whereas the inner regions are dominated by olivine. The spectral shape of the inner-disk spectra shows surprising similarity with Solar System comets. Radial-mixing models naturally explain this resemblance as well as the gradient in chemical composition. Our observations imply that silicates crystallize before any terrestrial planets are formed, consistent with the composition of meteorites in the Solar System. 相似文献
989.
Tschöp M Castañeda TR Joost HG Thöne-Reineke C Ortmann S Klaus S Hagan MM Chandler PC Oswald KD Benoit SC Seeley RJ Kinzig KP Moran TH Beck-sickinger AG Koglin N Rodgers RJ Blundell JE Ishii Y Beattie AH Holch P Allison DB Raun K Madsen K Wulff BS Stidsen CE Birringer M Kreuzer OJ Schindler M Arndt K Rudolf K Mark M Deng XY Whitcomb DC Halem H Taylor J Dong J Datta R Culler M Craney S Flora D Smiley D Heiman ML Withcomb DC 《Nature》2004,430(6996):1 p following 165; discussion 2 p following 165
Batterham et al. report that the gut peptide hormone PYY3-36 decreases food intake and body-weight gain in rodents, a discovery that has been heralded as potentially offering a new therapy for obesity. However, we have been unable to replicate their results. Although the reasons for this discrepancy remain undetermined, an effective anti-obesity drug ultimately must produce its effects across a range of situations. The fact that the findings of Batterham et al. cannot easily be replicated calls into question the potential value of an anti-obesity approach that is based on administration of PYY3-36. 相似文献
990.
Seymour B O'Doherty JP Dayan P Koltzenburg M Jones AK Dolan RJ Friston KJ Frackowiak RS 《Nature》2004,429(6992):664-667
The ability to use environmental stimuli to predict impending harm is critical for survival. Such predictions should be available as early as they are reliable. In pavlovian conditioning, chains of successively earlier predictors are studied in terms of higher-order relationships, and have inspired computational theories such as temporal difference learning. However, there is at present no adequate neurobiological account of how this learning occurs. Here, in a functional magnetic resonance imaging (fMRI) study of higher-order aversive conditioning, we describe a key computational strategy that humans use to learn predictions about pain. We show that neural activity in the ventral striatum and the anterior insula displays a marked correspondence to the signals for sequential learning predicted by temporal difference models. This result reveals a flexible aversive learning process ideally suited to the changing and uncertain nature of real-world environments. Taken with existing data on reward learning, our results suggest a critical role for the ventral striatum in integrating complex appetitive and aversive predictions to coordinate behaviour. 相似文献