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   73690篇
  免费   1490篇
  国内免费   1689篇
系统科学   2312篇
丛书文集   861篇
教育与普及   1051篇
理论与方法论   627篇
现状及发展   29438篇
研究方法   252篇
综合类   40140篇
自然研究   2188篇
  2021年   629篇
  2016年   678篇
  2015年   930篇
  2014年   1283篇
  2013年   1637篇
  2012年   1356篇
  2011年   3426篇
  2010年   1488篇
  2009年   1688篇
  2008年   1915篇
  2007年   1946篇
  2006年   1797篇
  2005年   2032篇
  2004年   2791篇
  2003年   2283篇
  2002年   1988篇
  2001年   1690篇
  2000年   1325篇
  1999年   2328篇
  1998年   2036篇
  1997年   2233篇
  1996年   2011篇
  1995年   1616篇
  1994年   1798篇
  1993年   1668篇
  1992年   1528篇
  1991年   1363篇
  1990年   1338篇
  1989年   1133篇
  1988年   1003篇
  1987年   818篇
  1986年   769篇
  1985年   746篇
  1984年   638篇
  1982年   723篇
  1981年   757篇
  1980年   821篇
  1979年   1061篇
  1978年   1015篇
  1977年   1017篇
  1976年   924篇
  1975年   891篇
  1974年   613篇
  1973年   960篇
  1972年   1026篇
  1971年   971篇
  1970年   940篇
  1969年   919篇
  1968年   891篇
  1967年   744篇
排序方式: 共有10000条查询结果,搜索用时 502 毫秒
901.
902.
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   
903.
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   
904.
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   
905.
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   
906.
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   
907.
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   
908.
Summary A HPLC assay for L-Dopa, convicine, and vicine in broad-bean extracts has been developed. The distribution of these compounds in the different organs of the plant has been studied, especially in the seeds, where their presence may cause favism if the beans are used as food.Work supported by C.N.R. through Progetto Finalizzato Miglioramento delle produzioni vegetali per fini alimentari ed industriali mediante interventi genetici. Sottogrogetto leguminose da granella. Paper No. 200.  相似文献   
909.
Summary The activity of D-lactate dehydrogenase in the foot of the snail,Helix pomatia, is closely correlated with time of year and mean daily temperature, and increases strikingly after the animals have been exposed to a nitrogen atmosphere for 24 h.I thank U. Platzer for technical assistance and D. Frank for help with the statistics. The investigation was supported by the Fonds zur Förderung der wissenschaftlichen Forschung in Österreich, project No. 1257 and 2919.  相似文献   
910.
Summary In 2 independent samples of low-birth-weight infants the proportion of females and homozygotes for a series of polymorphic systems was higher in light-for-dates than in preterm babies. The observation seems to give support to the hypothesis that homozygosity for normal polymorphisms may decrease in general intrauterine growth rate. Since it is known that survival rate is strongly related to birth weight, a correlation between growth retardation and homozygosity may have a major role in the maintenance of such polimorphisms.We thank Prof. L. Ginzburg for helpful comments.  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