全文获取类型
收费全文 | 29926篇 |
免费 | 59篇 |
国内免费 | 107篇 |
专业分类
系统科学 | 187篇 |
丛书文集 | 387篇 |
教育与普及 | 65篇 |
理论与方法论 | 111篇 |
现状及发展 | 12189篇 |
研究方法 | 1324篇 |
综合类 | 15280篇 |
自然研究 | 549篇 |
出版年
2013年 | 266篇 |
2012年 | 446篇 |
2011年 | 1010篇 |
2010年 | 180篇 |
2009年 | 132篇 |
2008年 | 498篇 |
2007年 | 615篇 |
2006年 | 623篇 |
2005年 | 592篇 |
2004年 | 592篇 |
2003年 | 534篇 |
2002年 | 536篇 |
2001年 | 1073篇 |
2000年 | 1033篇 |
1999年 | 627篇 |
1992年 | 571篇 |
1991年 | 462篇 |
1990年 | 509篇 |
1989年 | 492篇 |
1988年 | 457篇 |
1987年 | 476篇 |
1986年 | 509篇 |
1985年 | 545篇 |
1984年 | 474篇 |
1983年 | 419篇 |
1982年 | 344篇 |
1981年 | 351篇 |
1980年 | 406篇 |
1979年 | 957篇 |
1978年 | 751篇 |
1977年 | 724篇 |
1976年 | 560篇 |
1975年 | 627篇 |
1974年 | 913篇 |
1973年 | 755篇 |
1972年 | 736篇 |
1971年 | 932篇 |
1970年 | 1188篇 |
1969年 | 817篇 |
1968年 | 788篇 |
1967年 | 799篇 |
1966年 | 725篇 |
1965年 | 508篇 |
1959年 | 278篇 |
1958年 | 464篇 |
1957年 | 349篇 |
1956年 | 275篇 |
1955年 | 242篇 |
1954年 | 247篇 |
1948年 | 204篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
181.
Summary Synthetic 3-methylcyclohex-3-en-1-one, or 3,3-MCH, decreased by 77% the flight aggregation ofDendroctonus pseudotsugae (Scolytidae) to sticky traps containing attractant (frontalin, -pinene, and resin). However, windthrown trees were protected by 3,3-MCH much less than is known to occur with the isomer 3,2-MCH.This research was supported in part by NSF grants BMS 72-02522 and DEB 77-11361 and the O.S.U. School of Forestry. We thank the administration of the Siuslaw National Forest for study sites, and Dr L.M. Libbey for GC/MS of 3,3-MCH. Oregon State University Ag. Exp. Sta. Tech. Paper No. 4313. 相似文献
182.
The levels of membrane alkaline phosphatase have been measured on different lymphocyte fractions from human peripheral blood separated on bovine serum albumin discontinuous gradients. The peak in enzyme activity was observed in a non-T-, non-B-cell fraction, rich in "null" lymphocytes; the lowest values were found in the fraction with the highest proportion of T-cells. 相似文献
183.
Summary Physostigmine in a dose of 0.1 mg/kg i.v. expressly stimulated the oxygen uptake in the rat cerebral cortex. This effect was blocked by propranolol and seems be mediated by catecholamines. Since atropine also antagonized the stimulant effect of physostigmine, it appears that the action of physostigmine is primarily cholinergic and that the adrenergic effect is a secondary phenomenon. The higher dose of physostigmine (0.4 mg/kg i.v.) caused a depression of rat brain oxygen uptake.This work was supported by a grant from Serbian Republic Scientific Fund (ZMNU SR Srbija), Belgrade (Yugoslavia).The authors wish to acknowledge the skilful technical assistance of Lj. Krsti. 相似文献
184.
Field stimulation of isolated, spirally-cut cervix from day-22 pregnant rats produced contractions which could be inhibited by tetrodotoxin or hyoscine and potentiated by propranolol. The rat cervix would appear to receive both cholinergic and noradrenergic innervations whose transmitters activate muscarinic cholinoceptors and beta-adrenoceptors respectively. 相似文献
185.
During the maturation of red blood cells from rats after stress erythropoiesis, adenyl cyclase activity and beta-adrenoceptor density (pmoles/mg protein) decrease at distinctly different rates suggesting a different turnover of these membrane units. 相似文献
186.
(3H)-Ouabain binding to liver and kidney preparations was utilized to estimate the number of Na+, K+-ATPase enzyme units in livers and kidneys from rats fed 2% corn oil supplemented or fat-free diets. The specific (3H)-ouabain binding in liver and kidney preparations from fatty acid deficient rats was increased approximately 40%, but the affinity of the binding sites for ouabain (Kd-value) remained unchanged. The increased concentration of Na+, K+-ATPase enzyme units observed in the essential fatty acid deficient rats may contribute to the reduced body fat accumulation and elevated heat production observed in these animals. 相似文献
187.
Summary An intermediate stage between the elementary and the initial bodies of theRickettsiella genus is defined as the beginning of an intracellular cycle. It is characterized by several structural changes in the dense elementary body: the cytoplasm becomes less electron-dense; thus, the nucleoid and the ribosomes are visible. The inner layer of the cell-wall becomes progressively clearer and the trilamellar structure of the inner and outer membranes appears distinctly. Preinitial body is proposed as name of this stage of development. 相似文献
188.
Summary A rotator culture method is described which provides a continuous flow of oxygenating gas to cultures in rotating bottles. The system maintains constant O2 and CO2 levels in the culture medium throughout the incubation period. It also provides a more stable pH than systems with sealed culture bottles.Acknowledgments. We would like to thank Mr B. Secker, Mr C. Hellon and Mr S. J. Ellis, of the Physiological Laboratory workshop, for constructing the rotator apparatus. We also thank the Medical Research Council, London, for financial support for the project. 相似文献
189.
Summary Viable cerebellar perikarya of mixed cell type obtained from 7–9-day-old rats were maintained in monolayer culture for up to 12 days. During this time extensive neurite formation and outgrowth occurred. The large majority of the cells developing in culture were tentatively identified as granule neurons. This identification was based on the large number isolated from the starting tissue, and the cells general morphological features in culture such as perikaryal and nuclear size, the bipolar nature of neurite extension, and their migratory behaviour.D.N. Currie was supported by a grant from Hoechst Pharmaceuticals, Ld, U.K.J.C. was supported by an S.R.C. Grant to S.P.R. RoseThe authors would like to thank Kathleen Tear for excellent technical assistance and members of the Brain Research Group for helpful discussions. 相似文献
190.
MEASUREMENT of the absolute value of absorption by water vapour at microwave frequencies is difficult because the effect is so small. Far in the wings of the absorption lines, in the so-called 'windows' of the spectrum, it is especially difficult to achieve high accuracy in the free atmosphere. But it is in these windows that the behaviour of the absorption is important from both applied and scientific points of view. Satellite communications, remote sensing of the atmosphere, and radioastronomy, are all influenced by this behaviour. Measurements on an Earth-space path are reported here; the results indicate a nonlinear relationship between absorption and water-vapour content. 相似文献