首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   39668篇
  免费   165篇
  国内免费   168篇
系统科学   199篇
丛书文集   546篇
教育与普及   76篇
理论与方法论   126篇
现状及发展   17625篇
研究方法   1630篇
综合类   19174篇
自然研究   625篇
  2013年   315篇
  2012年   562篇
  2011年   1170篇
  2010年   255篇
  2008年   695篇
  2007年   825篇
  2006年   766篇
  2005年   816篇
  2004年   819篇
  2003年   719篇
  2002年   732篇
  2001年   1239篇
  2000年   1186篇
  1999年   802篇
  1994年   388篇
  1992年   773篇
  1991年   551篇
  1990年   618篇
  1989年   601篇
  1988年   566篇
  1987年   613篇
  1986年   655篇
  1985年   818篇
  1984年   595篇
  1983年   519篇
  1982年   479篇
  1981年   487篇
  1980年   515篇
  1979年   1302篇
  1978年   1007篇
  1977年   932篇
  1976年   755篇
  1975年   780篇
  1974年   1089篇
  1973年   942篇
  1972年   952篇
  1971年   1072篇
  1970年   1477篇
  1969年   1148篇
  1968年   1067篇
  1967年   1066篇
  1966年   1000篇
  1965年   747篇
  1959年   376篇
  1958年   720篇
  1957年   477篇
  1956年   407篇
  1955年   366篇
  1954年   414篇
  1948年   285篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
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   
972.
973.
Crosses between two classes of Drosophila melanogaster strains (reactive and inducer) may lead to partially sterile F1 females. This sterility, called S.F. sterility, can be characterized by its physiological features. It appears to be the result of an interaction between two factors: "R" brought into the initial croos by the reactive mother and "I" by chromosomes of the inducer father. Except for chromosome 4, each chromosome of the well known inducer strains may carry the I factor (inducer chromosomes). The present results provide evidence for the presence of an inducer fourth chromosome in the inducer strain Nagasaki.  相似文献   
974.
The use of the stable isotopes 6 and 7 of lithium, and detection with the ionic analyser, allow the measurement of unidirectional transepithelial Li-fluxes, between "external" and "internal" solutions which both contain 80% Na and 20% Li. Efflux was found to be bigger than influx. An important lag-phase on the curve, reveals the interference of a Li-compartment, with a high capacity, in the epithelium itself.  相似文献   
975.
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   
976.
Summary The male antifertility agent 1-amino-3-chloropropan-2-ol (ACP,I) has been shown to be metabolized to-chlorohydrin (III) and metabolites of-chlorohydrin. This accounts for the similar antifertility and renal toxicity effects of both compounds.  相似文献   
977.
Summary Abnormal-8-tetrahydrocannabidiol (ABN-8-THC) failed to elicit central nervous system and cardiovascular effects in laboratory animals. Abnormal-cannabidiol (ABN-CBD) was also devoid of overt behavioral effects but produced marked hypotension with only slight bradycardia in anesthesized dogs.Acknowledgment. This work was supported by NIDA (grant No. DA-00574-01 and DA-00490) and Virginia Heart Society (grant No. RR-05697)  相似文献   
978.
Summary N-(5-Phosphopyridoxyl)-4-aminobutyric acid, a stable adduct of pyridoxal phosphate and 4-aminobutyric acid, has been shown to be a potent inhibitor of rat brain 4-aminobutyric acid aminotransferase (GABA-T) with a Ki of 1.4 M.Acknowledgments. This work was supported in part by the United Parkinson Foundation, l'Association Canadienne l'Ataxie de Friedreich, and the Medical Research Council of Canada.  相似文献   
979.
Summary A convenient one-step procedure, based upon the tyrosinase co-oxidation of dopa and cysteine, is reported for the synthesis of 5-S-cysteinyldopa (I) in 74% yield. Secondary products of the reaction turned out to be 2-S-cysteinyldopa (II, 14%), 2,5-S, S-dicysteinyldopa (IV, 5%), and the hitherto unknown 6-S-cysteinyldopa (III, 1%).The generic term cysteinyldopa is proposed to designate the various cotechol amino-acids arising from addition of cysteine to dopaquinone.This work was supported in part by a grant from Consiglio Nazionale delle Ricerche, Roma.  相似文献   
980.
Summary The purified chitin from the cuticle of a pentastomid was examined by X-ray method. The X-ray photograph discloses that the chitin in question is of -type. Since the arthropod cuticle contains -chitin, it is suggested that Pentastomida may be considered an independent phylum.Thanks are due to Prof. Dr. G. Sundara Rajulu for guidance and encouragement.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号