全文获取类型
收费全文 | 55513篇 |
免费 | 258篇 |
国内免费 | 544篇 |
专业分类
系统科学 | 1230篇 |
丛书文集 | 335篇 |
教育与普及 | 285篇 |
理论与方法论 | 535篇 |
现状及发展 | 32413篇 |
研究方法 | 694篇 |
综合类 | 18632篇 |
自然研究 | 2191篇 |
出版年
2013年 | 802篇 |
2012年 | 539篇 |
2011年 | 2577篇 |
2009年 | 611篇 |
2008年 | 753篇 |
2007年 | 818篇 |
2006年 | 956篇 |
2005年 | 1096篇 |
2004年 | 2340篇 |
2003年 | 1859篇 |
2002年 | 1533篇 |
2001年 | 1336篇 |
2000年 | 842篇 |
1999年 | 936篇 |
1998年 | 644篇 |
1997年 | 774篇 |
1996年 | 531篇 |
1994年 | 687篇 |
1993年 | 690篇 |
1992年 | 876篇 |
1991年 | 741篇 |
1990年 | 828篇 |
1989年 | 642篇 |
1988年 | 594篇 |
1987年 | 632篇 |
1986年 | 730篇 |
1985年 | 875篇 |
1984年 | 743篇 |
1983年 | 651篇 |
1982年 | 797篇 |
1981年 | 840篇 |
1980年 | 904篇 |
1979年 | 1411篇 |
1978年 | 1302篇 |
1977年 | 1264篇 |
1976年 | 1122篇 |
1975年 | 1109篇 |
1974年 | 1029篇 |
1973年 | 1228篇 |
1972年 | 1293篇 |
1971年 | 1334篇 |
1970年 | 1445篇 |
1969年 | 1300篇 |
1968年 | 1255篇 |
1967年 | 1127篇 |
1966年 | 929篇 |
1965年 | 779篇 |
1964年 | 499篇 |
1958年 | 605篇 |
1957年 | 516篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
551.
R. K. Raina D. Njoroge J. N. Ng'ang'a B. V. Telang 《Cellular and molecular life sciences : CMLS》1977,33(11):1499-1500
Summary
Naja mossambica pallida venom administered i.v. (300 /kg) produces an initial brief fall in blood pressure, due to a direct myocardial depressant effect, and a sustained fall due to central depressant effect.This study was supported by University of Nairobi research grants (670–376). We also thank Mr E. Njogu for photographic assistance. 相似文献
552.
H. -J. Hess J. S. Bindra J. W. Constantine W. Elger O. Loge E. Schillinger W. Losert 《Cellular and molecular life sciences : CMLS》1977,33(8):1076-1077
Summary N-methanesulfonyl 16-phenoxy--tetranor PGE2 is a prostaglandin analog which is markedly more tissue selective than PGE2. This compound is 10–30 times more potent than PGE2 in animal models which are considered relevant to antifertility effects in humans. In pharmacological tests which are believed to be predictive for side effects in humans, the compound has potency either equal to or less than that of PGE2. 相似文献
553.
G. Apostol Constanţa Matei-Vladescu Valeria Popescu 《Cellular and molecular life sciences : CMLS》1977,33(3):345-347
Summary Contrary to earlier findings in rats, cerebral lateral ventricular infusions of 1×10–3 M or 2×10–3 M solutions of phloridzin at a rate of 2.5 l/min for 90 min had no significant stimulating effects on food intake and weight gain in hens and cocks. These different responses to intraventricular phloridzin might reflect a difference of sensitivity to the inhibitory action of phloridzin on glucose transport in cerebral cells or certain peculiarities of mechanisms controlling food intake in chickens. 相似文献
554.
555.
An effect which appeared to be pulsating aurora was observed to be associated with explosive releases of barium vapour near 250 km altitude, but only when the explosions were in the path of precipitating electrons associated with the visible aurora. 相似文献
556.
Summary Using the Fell technique of organ culture of cartilage in a chemically defined medium, it has been shown that prostaglandin A1 at a concentration of 25 g/ml caused chondrocyte death in chick embryonic limb rudiments. An equimolar concentration of PGE2 was not toxic to the cells.Acknowledgments. We are grateful for the support of C. J. K. by the Medical Research Council and of D. L. G. by the Arthritis and Rheumatism Council for Research. Our thanks are due to Dame Honor Fell, F. R. S. for invaluable advice and guidance, to Dr J. Pike (Upjohn Co.) for supplies of prostaglandin A1, and to Dr Sylvia Fitton-Jackson for the gift of culture medium. 相似文献
557.
Seed germination in response to diurnal fluctuations of temperature 总被引:20,自引:0,他引:20
Diurnal fluctuations in temperature may initiate or accelerate germination in certain flowering plants, and the effectiveness of the stimulus varies according to the amplitude of fluctuation and the presence or absence of light. Attempts to assess the adaptive significance of the phenomenon, however, have been limited by the scarcity of data for species of contrasted ecology. We report here an investigation of germination responses to fluctuating temperatures, conducted on seeds of herbaceous species collected from native populations near Sheffield. The results suggest that requirements for diurnal fluctuations in temperature are characteristic of the germination of species from particular types of habitat and provide mechanisms which cause seeds to germinate at times and in places favourable for seedling establishment. 相似文献
558.
559.
560.
S. Y. Dike Meera Kamath J. R. Merchant 《Cellular and molecular life sciences : CMLS》1977,33(8):985-985
Summary A one-step synthesis of hexahydrocannabinoid analogs (HHC) is described making use of the condensation of phenolic ketones and aldehydes with citronellal in the presence of pyridine.Grateful thanks are accorded to Drs L. Chopard and S. Selvavinayakam for spectral measurements and Dr I. Fleming for useful discussion.R. Mechoulam and Y. Gaoni, Recent advances in the chemistry of Hashish, in: Progress in the Chemistry of Organic natural products, vol. 25, p. 175. Ed. L. Zechmeister. Springer Verlag, Wien 1967; R. Mechoulam, Marijuana Chemistry and Pharmacology, metabolism and clinical effects. Academic Press, New York 1973; R. Mechoulam, N. K. McCallum and S. Burstein, Chem. Rev.76, 75 (1976). 相似文献