全文获取类型
收费全文 | 31702篇 |
免费 | 129篇 |
国内免费 | 262篇 |
专业分类
系统科学 | 451篇 |
丛书文集 | 624篇 |
教育与普及 | 94篇 |
理论与方法论 | 124篇 |
现状及发展 | 13027篇 |
研究方法 | 1194篇 |
综合类 | 15918篇 |
自然研究 | 661篇 |
出版年
2013年 | 342篇 |
2012年 | 605篇 |
2011年 | 1288篇 |
2010年 | 412篇 |
2009年 | 393篇 |
2008年 | 685篇 |
2007年 | 796篇 |
2006年 | 814篇 |
2005年 | 749篇 |
2004年 | 683篇 |
2003年 | 514篇 |
2002年 | 513篇 |
2001年 | 840篇 |
2000年 | 873篇 |
1999年 | 627篇 |
1992年 | 504篇 |
1991年 | 425篇 |
1990年 | 437篇 |
1989年 | 420篇 |
1988年 | 378篇 |
1987年 | 398篇 |
1986年 | 417篇 |
1985年 | 524篇 |
1984年 | 409篇 |
1983年 | 350篇 |
1982年 | 330篇 |
1981年 | 340篇 |
1980年 | 377篇 |
1979年 | 930篇 |
1978年 | 739篇 |
1977年 | 725篇 |
1976年 | 553篇 |
1975年 | 666篇 |
1974年 | 926篇 |
1973年 | 755篇 |
1972年 | 770篇 |
1971年 | 906篇 |
1970年 | 1093篇 |
1969年 | 891篇 |
1968年 | 864篇 |
1967年 | 899篇 |
1966年 | 747篇 |
1965年 | 613篇 |
1964年 | 200篇 |
1959年 | 325篇 |
1958年 | 522篇 |
1957年 | 392篇 |
1956年 | 312篇 |
1955年 | 325篇 |
1954年 | 303篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
In order to improve the quality of yarn,a research was carried out at ring spinning.A modifiedtraveller has been used to reduce the yarn hairiness vastly.According to the statistical analysis ofthe data,it was found that the geometrical shape of travellers is the key factor affecting the yarnhairiness at ring spinning. 相似文献
952.
The differences of thermal conduction and its temperature-varying track of the heat source body at various structural environments have been studied. Hypothetically , geologic heat source body is cut successively into several segregative bodies with a fixed cubage . With the segmented number increasing , the conductive surface area of heat source body begins to get larger, which separately is similar to the heat geologic body in different tectonic zones that
has a various enclosed coefficient . Finite-element simulation result shows that
the thermal conduction speed of spreading from the heat source body to its wall rock is slow→higher→ highest, when heat source bodies are situated respectively at compressive, shear and tensile deformation zones, corresponding rates of their temperature drop are low→ higher→ highest .
Research indicates that the temperature ' s dropping rate of heat source body has an inverse relationship with enclosed coefficient η for different structural zones . This result can explain the tectono-physicochemical reason of magmatite evolution and its. difference . 相似文献
953.
Sympatric speciation by sexual selection 总被引:14,自引:0,他引:14
There is increasing evidence for the process of sympatric speciation, in which reproductive isolation of species occurs without physical isolation. Theoretical models have focused on disruptive natural selection as the crucial pressure for splitting a species. Here we report the theoretical finding that sympatric speciation may be caused by sexual selection even without disruptive natural selection. Specifically, we show that variation in a male secondary sexual character with two conspicuous extremes and the corresponding variance in female mating preference around no preference may jointly evolve into bimodal distributions with increasing modal divergence of the male and female traits, pulling a population apart into two prezygotically isolated populations. This mode of speciation, driven by two runaway processes in different directions, is promoted by an increase in the efficiency of females in discriminating among males or a decrease in the cost of male conspicuousness, indicating that sympatric speciation may occur more readily if barrier-free or predator-free conditions arise. Although even a slight cost of female preference would cancel the runaway process of sexual selection, it would not cancel the divergent runaway processes of sympatric speciation. 相似文献
954.
A plant regulator controlling development of symbiotic root nodules 总被引:37,自引:0,他引:37
955.
956.
957.
958.
Sequence and analysis of chromosome 2 of the plant Arabidopsis thaliana 总被引:21,自引:0,他引:21
Lin X Kaul S Rounsley S Shea TP Benito MI Town CD Fujii CY Mason T Bowman CL Barnstead M Feldblyum TV Buell CR Ketchum KA Lee J Ronning CM Koo HL Moffat KS Cronin LA Shen M Pai G Van Aken S Umayam L Tallon LJ Gill JE Adams MD Carrera AJ Creasy TH Goodman HM Somerville CR Copenhaver GP Preuss D Nierman WC White O Eisen JA Salzberg SL Fraser CM Venter JC 《Nature》1999,402(6763):761-768
Arabidopsis thaliana (Arabidopsis) is unique among plant model organisms in having a small genome (130-140 Mb), excellent physical and genetic maps, and little repetitive DNA. Here we report the sequence of chromosome 2 from the Columbia ecotype in two gap-free assemblies (contigs) of 3.6 and 16 megabases (Mb). The latter represents the longest published stretch of uninterrupted DNA sequence assembled from any organism to date. Chromosome 2 represents 15% of the genome and encodes 4,037 genes, 49% of which have no predicted function. Roughly 250 tandem gene duplications were found in addition to large-scale duplications of about 0.5 and 4.5 Mb between chromosomes 2 and 1 and between chromosomes 2 and 4, respectively. Sequencing of nearly 2 Mb within the genetically defined centromere revealed a low density of recognizable genes, and a high density and diverse range of vestigial and presumably inactive mobile elements. More unexpected is what appears to be a recent insertion of a continuous stretch of 75% of the mitochondrial genome into chromosome 2. 相似文献
959.
A structural change in the kinesin motor protein that drives motility 总被引:34,自引:0,他引:34
Rice S Lin AW Safer D Hart CL Naber N Carragher BO Cain SM Pechatnikova E Wilson-Kubalek EM Whittaker M Pate E Cooke R Taylor EW Milligan RA Vale RD 《Nature》1999,402(6763):778-784
Kinesin motors power many motile processes by converting ATP energy into unidirectional motion along microtubules. The force-generating and enzymatic properties of conventional kinesin have been extensively studied; however, the structural basis of movement is unknown. Here we have detected and visualized a large conformational change of an approximately 15-amino-acid region (the neck linker) in kinesin using electron paramagnetic resonance, fluorescence resonance energy transfer, pre-steady state kinetics and cryo-electron microscopy. This region becomes immobilized and extended towards the microtubule 'plus' end when kinesin binds microtubules and ATP, and reverts to a more mobile conformation when gamma-phosphate is released after nucleotide hydrolysis. This conformational change explains both the direction of kinesin motion and processive movement by the kinesin dimer. 相似文献
960.
Excavations at the early Neolithic site of Jiahu in Henan Province, China have produced what may be the earliest complete, playable, tightly-dated multinote musical instruments. Jiahu was occupied from 7000 BC to 5700 BC, considerably antedating the well known Peiligang culture. Here we describe six exquisitely made complete flutes which were found in radiocarbon-dated excavation layers, along with fragments of perhaps 30 more. The flutes are made from the ulnae of the red-crowned crane (Grus japonensis Millen) and have 5, 6, 7 and 8 holes. The best preserved flute has been played and tonally analysed. In addition to early musical artefacts, the archaeological record at Jiahu contains important information on the very foundations of Chinese society. We describe the archaeological characteristics of the Jiahu site, details concerning its dating, its place in the prehistory of the Chinese Neolithic, the ethnicity of its population and the results of a tonal analysis of a nearly 9,000-year-old musical instrument found there. 相似文献