全文获取类型
收费全文 | 42070篇 |
免费 | 89篇 |
国内免费 | 116篇 |
专业分类
系统科学 | 394篇 |
丛书文集 | 1022篇 |
教育与普及 | 108篇 |
理论与方法论 | 237篇 |
现状及发展 | 18832篇 |
研究方法 | 1611篇 |
综合类 | 19583篇 |
自然研究 | 488篇 |
出版年
2013年 | 216篇 |
2012年 | 531篇 |
2011年 | 1086篇 |
2010年 | 241篇 |
2008年 | 709篇 |
2007年 | 757篇 |
2006年 | 753篇 |
2005年 | 773篇 |
2004年 | 694篇 |
2003年 | 784篇 |
2002年 | 716篇 |
2001年 | 1286篇 |
2000年 | 1220篇 |
1999年 | 771篇 |
1992年 | 740篇 |
1991年 | 606篇 |
1990年 | 646篇 |
1989年 | 643篇 |
1988年 | 643篇 |
1987年 | 637篇 |
1986年 | 650篇 |
1985年 | 792篇 |
1984年 | 623篇 |
1983年 | 540篇 |
1982年 | 472篇 |
1981年 | 496篇 |
1980年 | 599篇 |
1979年 | 1302篇 |
1978年 | 1116篇 |
1977年 | 1118篇 |
1976年 | 827篇 |
1975年 | 875篇 |
1974年 | 1305篇 |
1973年 | 1056篇 |
1972年 | 1072篇 |
1971年 | 1326篇 |
1970年 | 1768篇 |
1969年 | 1386篇 |
1968年 | 1269篇 |
1967年 | 1334篇 |
1966年 | 1127篇 |
1965年 | 827篇 |
1964年 | 220篇 |
1959年 | 503篇 |
1958年 | 731篇 |
1957年 | 578篇 |
1956年 | 486篇 |
1955年 | 442篇 |
1954年 | 483篇 |
1948年 | 265篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
Diverse microorganisms ranging from cyanobacteria to eukaryotic algae and fungi live endolithically within ooids, hardgrounds and invertebrate shells on the present-day sea floor. These organisms are involved in the mechanical destruction of carbonates, and are useful ecological indicators of water depth and pollution. The Phanerozoic history of microbial endoliths has been elucidated through the study of microborings (the trace fossils of endolithic microorganisms) and rare cellularly preserved individuals, but nothing was known of the possible Precambrian evolution of comparable microorganisms until Campbell documented the occurrence of microborings in late Proterozoic ooids from central East Greenland. We now report the discovery of large populations of organically preserved endolithic microorganisms in silicified pisolites from 700-800-Myr-old Limestone-Dolomite Series of East Greenland. This fossil assemblage is significant for three reasons: (1) It confirms the prediction that oolites, pisolites and hardgrounds--the substrates for pre-Phanerozoic endoliths--provide a hitherto poorly explored but rewarding set of environments into which the search for early microfossils must be broadened; (2) the assemblage is diverse, containing about 12 taxa of morphologically distinct and previously unknown endolithic cyanobacteria, plus associated epilithic and interstitial populations; and (3) at least six of the fossil populations are indistinguishable in morphology, pattern of development, reproductive biology and inferred ecology from distinctive cyanobacterial species that bore ooids today in the Bahama Banks. 相似文献
132.
Summary By comparing steroid sulphatase levels per se, and also ratios to -galactosidase, in 6 sets of mice — normal females, entire and castrated males both with and without exogenous testosterone administration — we obtained support for the contention that induction of this enzyme is in part controlled by male hormones. 相似文献
133.
134.
Summary The binding of amiodarone to human plasma protein and to bovine serum, albumin was studied by three different methods, ultracentrifugation, equilibrium dialysis and fluorescence spectroscopy. The fraction of amiodarone bound to plasma protein amounted to 96.3%. The changes in the binding properties of 1-anilino-naphthalene-8-sulfonate for bovine serum albumin using warfarin and amiodarone as independent inhibitors were analyzed in terms of binding site specificity. The findings indicated that amiodarone and warfarin have two different binding sites on bovine serum albumin, so a noncompetitive inhibition mechanism was indicated. On the basis of our data we cannot exclude other mechanisms of interaction besides direct displacement of one drug by another; nevertheless, metabolite interference between amiodarone and coagulation cofactors may better explain the enhancement of warfarin's pharmacological action in association with amiodarone.This work was partially funded by the CNR (National Research Council, Rome, Italy), Program on Clinical Pharmacology and Rare Diseases. The authors would like to thanks Drs E. Marzi and E. riva for their help. 相似文献
135.
J. S. Nowak 《Cellular and molecular life sciences : CMLS》1985,41(1):88-89
Summary Somatic cell hybrids between Sp2/O-Ag14 mouse myeloma cells and lymphocytes derived from BALB/c mice hyperimmunized with sheep red blood cells (SRBC) were produced. One hybrid producing IgG1 antibody to SRBC was selected, cloned twice and subsequently transferred to BALB/c mice. After a number of transfers it was found that the antibody titer in ascitec fluid gradually decreased. Cytogenetic analysis revealed gradual chromosome loss in the hybrid clone, which produced progressively less antibody. 相似文献
136.
Antidiuretic effects of oxytocin in the Brattleboro rat 总被引:1,自引:0,他引:1
J. Lyness A. G. Robinson M. N. Sheridan D. M. Gash 《Cellular and molecular life sciences : CMLS》1985,41(11):1444-1446
Summary The antidiuretic activity of oxytocin (OT) was measured in Brattleboro rats with congenital diabetes insipidus. A dose dependent antidiuretic response was found in animals receiving chronic infusions of 0.1 g/h, 1.0 g/h, and 5 g/h of OT. OT infused at the rate of 5 g/h over a 7-day period completely reversed the symptoms of diabetes insipidus. The results support the concept that OT serves as a weak agonist of vasopressin at the level of the kidney and at pharmacological levels exhibits antidiuretic activity. 相似文献
137.
J. R. Aldrich J. P. Kochansky J. D. Sexton 《Cellular and molecular life sciences : CMLS》1985,41(3):420-422
Summary Workers and queens of the eastern yellowjacket,Vespula maculifrons, are attracted to the artificial long-range attractant pheromone of the predaceous pentatomid,Podisus maculiventris. A 11 mixture of linalool or -terpineol and (E)-2-hexenal is as attractive toV. maculifrons workers as the pheromone.We thank A.S. Menke of the Systematic Entomology Laboratory, USDA, for identifying the yellowjackets. Mention of a company name does not imply endorsement by the US Department of Agriculture. 相似文献
138.
J. Lukanov E. Milieva G. Russev B. Trifonov 《Cellular and molecular life sciences : CMLS》1985,41(1):68-70
Summary Follow-up investigation of the blood sera from preparturient women and women with habitual, abortions showed the presence of a factor which has an activating effect on smooth muscle preparations because it causes the release of prostaglandins. Gel-chromatographic counter flow separation and microelectrophoresis of the blood sera have shown that the isolated serum factor is a water soluble glycopeptide with a molecular weight of about 2000. 相似文献
139.
140.
Summary Visual determination of MSH-induced pigment migration in melanophores of small pieces ofAnolis carolinensis skin is standardized by first measuring photoelectrometrically the change in reflection/transmission of the whole dorsal skin in response to different hormone concentrations. This method allows the rapid and precise recording of time-response curves after photoaffinity labeling of MSH receptors or of dose-response curves of large series of synthetic compounds.Acknowledgments. We wish to thank Ms V. Jäggin, Ms C. Schulthess and Ms G. van Hees for excellent technical assistance, Dr. R. Andreatta, Ciba-Geigy AG, Basel, for his generous gift of -MSH and Prof. A Pletscher for his continuous interest. This work was supported by the Swiss National Science Foundation. 相似文献