全文获取类型
收费全文 | 27441篇 |
免费 | 122篇 |
国内免费 | 254篇 |
专业分类
系统科学 | 447篇 |
丛书文集 | 406篇 |
教育与普及 | 102篇 |
理论与方法论 | 132篇 |
现状及发展 | 9959篇 |
研究方法 | 1100篇 |
综合类 | 14923篇 |
自然研究 | 748篇 |
出版年
2013年 | 355篇 |
2012年 | 601篇 |
2011年 | 1388篇 |
2010年 | 418篇 |
2009年 | 393篇 |
2008年 | 669篇 |
2007年 | 762篇 |
2006年 | 792篇 |
2005年 | 707篇 |
2004年 | 618篇 |
2003年 | 478篇 |
2002年 | 488篇 |
2001年 | 721篇 |
2000年 | 800篇 |
1999年 | 556篇 |
1992年 | 442篇 |
1991年 | 376篇 |
1990年 | 384篇 |
1989年 | 377篇 |
1988年 | 344篇 |
1987年 | 369篇 |
1986年 | 342篇 |
1985年 | 438篇 |
1984年 | 384篇 |
1983年 | 269篇 |
1982年 | 268篇 |
1981年 | 250篇 |
1980年 | 306篇 |
1979年 | 734篇 |
1978年 | 556篇 |
1977年 | 568篇 |
1976年 | 458篇 |
1975年 | 517篇 |
1974年 | 705篇 |
1973年 | 626篇 |
1972年 | 572篇 |
1971年 | 705篇 |
1970年 | 896篇 |
1969年 | 695篇 |
1968年 | 677篇 |
1967年 | 723篇 |
1966年 | 684篇 |
1965年 | 496篇 |
1964年 | 200篇 |
1959年 | 249篇 |
1958年 | 421篇 |
1957年 | 277篇 |
1956年 | 258篇 |
1955年 | 247篇 |
1954年 | 211篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
The murine Elo (eye lens obsolescence) mutation confers a dominant phenotype characterized by malformation of the eye lens. The mutation maps to chromosome 1, in close proximity to the gamma E-crystallin gene which is the 3'-most member of the gamma-crystallin gene cluster. We have analysed the sequence of this gene from the Elo mouse and identified a single nucleotide deletion which destroys the fourth and last "Greek key" motif of the protein. This mutation is tightly associated with the phenotype, as no recombination was detected in 274 meioses. In addition, the mutant mRNA is present in the affected lens, providing further support for our hypothesis that the deletion is responsible for the dominant Elo phenotype. 相似文献
52.
Identification of a new predaceous stink bug pheromone and its attractiveness to the eastern yellowjacket 总被引:1,自引:0,他引:1
J. R. Aldrich W. R. Lusby J. P. Kochansky 《Cellular and molecular life sciences : CMLS》1986,42(5):583-585
Summary Males ofPodisus fretus (Hemiptera: Pentatomidae) release a long-range attractant pheromone containing linalool (49.0%), (E)-2-hexenal (34.5%), benzyl alcohol (12.0%), nerolidol (2.0%),-terpineol (1.1%), and traces of several other compounds. The eastern yellowjacket,Vespula maculifrons (Hymenoptera: Vespidae), is attracted to artificial pheromones forP. fretus and for the sympatric species,Podisus maculiventris.The authors thank Mr T.J. Henry of the USDA Systematic Entomology Laboratory and Dr J.E. McPherson, Southern Illinois University, for examining the pentatomid species and Dr A.S. Menke, USDA-SEL, for determining the yellowjacket species. We also thank S. Wilzer for technical assistance. Mention of a company name does not imply endorsement by the US Department of Agriculture. 相似文献
53.
G. L. Mendz C. R. Middlehurst P. W. Kuchel P. A. Crossie 《Cellular and molecular life sciences : CMLS》1986,42(9):1016-1018
Summary A technique to assay erythrocyte pyrimidine 5-nucleotidase activity in situ using31P nuclear magnetic resonance spectroscopy is presented. The assay is chemically specific, simple and applicable to untreated lysates. A comparison of enzyme levels in normal controls and in multiple sclerosis patients employing the assay yielded no significant differences between both groups. Difficulties encountered in the quantitative analysis of the assay using1H-NMR spectroscopy are briefly discussed. 相似文献
54.
Summary During molt-induced atrophy of the pectoralis muscle of the Giant Canada Goose (Branta canadensis maxima), the zinc content of the muscle was significantly reduced (p0.0139), though the concentration of zinc per unit weight of muscle appeared higher (p0.0232). Zinc lost from the muscle during molt could be utilized for growth of the new flight feathers.Acknowledgments. Funds for this study were obtained from an operating grant awarded to J. C. G. by the Natural Sciences and Engineering Research Council of Canada. 相似文献
55.
New phytotoxic butenolides produced bySeiridium cardinale,the pathogen of cypress canker disease 总被引:1,自引:0,他引:1
L. Sparapano A. Evidente A. Ballio A. Graniti G. Randazzo 《Cellular and molecular life sciences : CMLS》1986,42(6):627-628
Summary Two new butenolides, seiridin andiso-seiridin, were isolated from culture filtrates ofSeiridium cardinale, the pathogen of cypress canker, a destructive disease ofCupressus and relatedConiferae These metabolites were characterized as 3-methyl-4-(2-hydroxyheptyl)-2(5H)-furanone and its 4-(3-hydroxyheptyl) isomer, respectively. Chlorotic, and necrotic symptoms were produced on leaves of either host or non-host test plants by absorption of 0.3 mg/ml solutions of either compound. These also showed antibacterial activity.This study was supported in part by the National Research Council, Rome (special ad hoc project Chimica fine e secondaria) and in part by the Ministry of Education, Rome. 相似文献
56.
57.
Summary Following engorgement, female ixodid ticks secrete a tick salivary gland degeneration factor (TSGDF) into the hemolymph. Here we show that the arthropod ecdysteroid hormones, ecdysome and 20-hydroxyecdysone, induce degeneration of tick salivary glands maintained in organ culture. The effective dose range in vitro is 30–300 ng/ml, a range reported to be physiological for this species following repletion. In addition, infusion of 20-hydroxyecdysome in vivo induces salivary gland degeneration. We therefore propose that TSGDF may be an ecdysteroid.Acknowledgments. Some of the data reported here were presented to the annual meeting of the Canadian Society of Zoologists, 15–18 May 1983; Program of abstracts, page 53. Financial support of the Canadian National Sportsmen's Fund and NSERC Canada to W.R.K. is gratefully acknowledged. 相似文献
58.
J. A. Doebler E. W. Wickersham L. V. Polakovic T. -M. Shih A. Anthony 《Cellular and molecular life sciences : CMLS》1985,41(9):1145-1147
Summary The organophosphate neurotoxin soman produced impairments in adrenocortical RNA and protein metabolism. Fasciculate and reticular cell RNA and protein contents were supporessed with sublethal to acutely lethal dosages (20, 30 and 40 g/kg, s.c.) during the acute excitatory phase of intoxication and at 6–8 h post injection. All three dosages produced ca 90% inactivation of plasma cholinesterase. A transient elevation of plasma corticosterone occurred with 20 g/kg soman whereas there was a protracted increase with 30 g/kg. Corticosterone was not significantly elevated with 40 g/kg, but death occurred at 13±4 min. Thus, the magnitude and/or nature of soman-induced metabolic impairments does not appear to prevent adrenal activation.Supported by US Army Medical Research and Development Command Contract DAMD 17-81-C-1202. 相似文献
59.
Summary InPyrrhocoris apterus, a low temperature, 15°C, prevented the termination of diapause by long days and, unexpectedly, also the induction of diapause by short days. Both responses were enabled at a higher temperature, 26°C. In contrast to current concepts, it was proved that the summation of photoperiodic signals was temperature-dependent, since the morphogenetic development was prevented by starvation. 相似文献
60.
M. W. Browne 《Journal of Classification》1987,4(2):175-190
It is shown that replacement of the zero diagonal elements of the symmetric data matrix of approximate squared distances by certain other quantities in the Young-Householder algorithm will yield a least squares fit to squared distances instead of to scalar products. Iterative algorithms for obtaining these replacement diagonal elements are described and relationships with the ELEGANT algorithm (de Leeuw 1975; Takane 1977) are discussed. In large residual situations a penalty function approach, motivated by the ELEGANT algorithm, is adopted. Empirical comparisons of the algorithms are given.An early version of this paper was presented at the Multidimensional Data Analysis Workshop, Pembroke College, Cambridge, July 1985. I want to thank Jan de Leeuw and Yoshio Takane for bringing the ELEGANT algorithm to my attention and for clarifying its rationale and notation. My thanks go also to Stephen du Toit for help with the ALSCAL computations reported in Section 7. 相似文献