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   72028篇
  免费   298篇
  国内免费   593篇
系统科学   1285篇
丛书文集   656篇
教育与普及   287篇
理论与方法论   560篇
现状及发展   36586篇
研究方法   1596篇
综合类   29756篇
自然研究   2193篇
  2013年   825篇
  2012年   833篇
  2011年   2980篇
  2009年   627篇
  2008年   1070篇
  2007年   1259篇
  2006年   1296篇
  2005年   1445篇
  2004年   2583篇
  2003年   2173篇
  2002年   1915篇
  2001年   2123篇
  2000年   1612篇
  1999年   1337篇
  1998年   656篇
  1997年   777篇
  1994年   688篇
  1993年   696篇
  1992年   1270篇
  1991年   1020篇
  1990年   1178篇
  1989年   1007篇
  1988年   938篇
  1987年   985篇
  1986年   1007篇
  1985年   1194篇
  1984年   997篇
  1983年   837篇
  1982年   930篇
  1981年   951篇
  1980年   1083篇
  1979年   1888篇
  1978年   1657篇
  1977年   1636篇
  1976年   1381篇
  1975年   1424篇
  1974年   1592篇
  1973年   1626篇
  1972年   1721篇
  1971年   1847篇
  1970年   2148篇
  1969年   1737篇
  1968年   1728篇
  1967年   1578篇
  1966年   1371篇
  1965年   1041篇
  1959年   600篇
  1958年   875篇
  1957年   676篇
  1956年   571篇
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
531.
Summary The activities of monoamine oxidases A and B towards 5-hydroxytryptamine and -phenethylamine, respectively, were compared in the left and right caudatus, hippocampus, parietal cortex, cerebellum and frontal cortex 6 months after gamma-irradiation (single dose of 23 Gy) of either the right hemisphere or of the whole rabbit brain (in which case, a dose of 16 Gy). No difference in monoamine oxidase A or B activities were found in any of the brain regions.Acknowledgment. This study was supported by grants from the Swedish Medical Research Council (12x–5664 and 04x–4145), Hansson's, Lion's, Mångberg's and Sundblad's funds.  相似文献   
532.
Summary The flavoprotein ferredoxin reductase catalyzed the oxidation of styrene to styrene oxide in the presence of NADPH. This reaction was inhibited by the addition of catalase and superoxide dismutase. The addition of the nonheme iron protein ferredoxin partially inhibited styrene oxidation. H2O2 was also able to catalyze this reaction when added to the enzyme in the absence of NADPH.Acknowledgments. This work was supported by C.N.R. (National Research Council), Rome, Italy contract No. 79.03197.04.  相似文献   
533.
Summary Captopril in vitro (50–500 g/ml) increased3H-TdR incorporation in unstimulated and mitogen-stimulated cultures of rat lymphocytes. Unseparated spleen and lymph node cells of rats orally treated with captopril (50 mg/kg/day×4) showed decreased basal and mitogen stimulated3H-TdR incorporation. The removal of macrophages abrogated this inhibitory effect. Leucine aminopeptidase activity of macrophages was reduced — in vivo and in vitro — by captopril.Acknowledgments. The authors thank the Squibb Institute for Medical Research for the gift of Captopril. The excellent technical assistance of Ms B. Hasselriis, Ms B. Rumler and Ms E. Greve Petersen is gratefully acknowledged.  相似文献   
534.
535.
Summary Spectroscopic evidence shows that the flavylium cation (1) undergoes hydroxylation to form the carbinol base (2) and can also deprotonate to form the anhydro (quinonemethane) bases (3a and3b). The anhydro bases hydrate to the carbinol base. The carbinol base above pH 6, or the ionzed carbinol base (2a), undergoes diring fission to produce the cischalcone (4).The editors do not hold themselves responsible for the opinions expressed in the authors' brief reports. — Les auteurs sont seuls responsables des opinions exprimées das ces brèves communications. — Für die Kurzmitteilungen ist ausschliesslich der Autor verantwortlich. — Per le brevi comunicazioni è responsabile solo l'autore. — ë . — Solo los autores son responsables de las opiniones expresadas en estas comunicationes breves.Acknowledgment. This research was supported by a grant from the Welch Food Co., Westfield, NY.  相似文献   
536.
Summary The importance of phytoplankton in stabilizing oil-in-water emulsion and affecting hydrocarbon distribution in the sea was studied by using marine diatoms. All microalgae tested increased emulsion stability and favored the presence of polycyclic aromatics in the sea water.  相似文献   
537.
Summary A new chalcone glucoside has been isolated from the flowers ofAcacia dealbata and shown to be 4,2,4,6-tetrahydroxy-3-methoxychalcone 2-O--D-glucoside (1) by chemical degradations and spectroscopic methods. Cernuoside (4,6,3,4-tetrahydroxyaurone 4-O--D-glucoside), (2) has also been found in this plant material.Acknowledgments. The author thanks Prof. H. Wagner, Institut für pharmazeutische Arzneimittellehre der Universität München, for a sample of homoeriodictyol, and Prof. J.B. Harborne, University of Reading, for a sample of aureusidin.  相似文献   
538.
Summary Leydig cell nuclear area and 5-3-hydroxysteroid dehydrogenase activity were increased in the testis ofBufo melanostictus 2 days after a single short-term cold exposure. Both parameters returned to normal values 4 days later.  相似文献   
539.
Summary The biochemical changes in the hemolymph and tissues were followed during regeneration of the optic tentacles of the snailCryptozona ligulata (Pulmonata-Stylommatophora). There is a remarkable increase in total carbohydrates in hemolymph and tissues and glycogen in tissues at the expense of free amino acids and fatty acids. It is clear that ablation of the optic tentacles stimulates carbohydrate synthesis through glyconeogenic routes. The optic tentacles regenerate completely in 18–21 days.  相似文献   
540.
Summary The defense secretion of soldiers ofReticulitermes lucifugus has been shown to contain, predominantly, (R)-(–)-(E,E)-geranyllinalool together with germacrene A and -farnesene.This report covers part of a collaborative study with J.-L. Clément, Lab. d'Evolution, Université P. et M. Curie, Paris, to whom we are grateful for supplies of material and discussions. We also thank Dr O.T. Jones for collections ofReticulitermes lucifugus.  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