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   61321篇
  免费   321篇
  国内免费   554篇
系统科学   1302篇
丛书文集   569篇
教育与普及   287篇
理论与方法论   540篇
现状及发展   33482篇
研究方法   949篇
综合类   22876篇
自然研究   2191篇
  2013年   809篇
  2012年   655篇
  2011年   2696篇
  2009年   602篇
  2008年   837篇
  2007年   954篇
  2006年   1063篇
  2005年   1227篇
  2004年   2318篇
  2003年   2042篇
  2002年   1651篇
  2001年   1608篇
  2000年   1239篇
  1999年   1094篇
  1998年   647篇
  1997年   779篇
  1996年   535篇
  1994年   686篇
  1993年   696篇
  1992年   993篇
  1991年   874篇
  1990年   968篇
  1989年   759篇
  1988年   751篇
  1987年   739篇
  1986年   793篇
  1985年   1013篇
  1984年   859篇
  1983年   741篇
  1982年   827篇
  1981年   854篇
  1980年   972篇
  1979年   1553篇
  1978年   1388篇
  1977年   1347篇
  1976年   1203篇
  1975年   1217篇
  1974年   1169篇
  1973年   1365篇
  1972年   1436篇
  1971年   1497篇
  1970年   1716篇
  1969年   1497篇
  1968年   1387篇
  1967年   1286篇
  1966年   1122篇
  1965年   882篇
  1959年   519篇
  1958年   679篇
  1957年   575篇
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
31.
It was established that there is little diversity of bryophytes in the derived savanna. Mosses were found in the sampling sites, whereas liverworts were rarely observed. The reproductive methods of four dominant sexually reproducing savanna mosses —Archidium ohioense, Bryum coronatum, Fissidens minutifolius andTrachycarpidium tisserantii were monitored over two consecutive rainy seasons. Protonemal and gametophyte production were noticed in the field in March/April, and capsule dehiscence and spore dispersal occurred in September/October. The sequential stages of development, starting with gametangial production and ending with the falling of the dehisced capsules, occurred within the rainy season. However,A. ohioense andT. tisserantii did not discharge their spores easily (cleistocarpous), unlike the stegocarpous speciesB. coronatum andF. minutifolius. Water availability and possibly high humidity may have contributed to growth. The short period between sex organ formation and dehiscence of capsule seen in these studies, compared with the longer period in some temperate mosses, may be an advantage for bryophytes in a savanna environment.  相似文献   
32.
33.
34.
35.
Summary Hydrochlorothiazide, acutely injected in rats, has a weak analgesic activity per se and potentiates and prolongs the antinociceptive effect of morphine.This work was supported in part by grants from Consiglio Nazionale delle Ricerche, and by Ministero della Pubblica Istruzione, Roma.  相似文献   
36.
37.
1 Results This paper presents the development of platinum nanocomposites structures based on organically modified c.a.2 nm core platinum nanoparticles.The chemical modification of the 4-mercaptoaniline functionalized particles by various in coming molecules is evidenced and precisely quantified.The particles can be dissolved like molecules in various solvents depending on the features of the new crown and X-rays shows that the interparticle distance is affected by the crown modification.These platinum n...  相似文献   
38.
39.
1 Results The development of the concepts of nanotechnology has given an important impact on the design of new polymer based materials which are in most cases characterized by a multiphase morphology. When at least one phase has nanometric dimension(s) the system can be considered as a nanocomposite where the interface is not only determining for the adhesion but also may play a role in some bulk properties. Indeed in nanostructured multiphase solids the interface is significant as a bulk component. The...  相似文献   
40.
1 Results Pyrimidinones (PMs) are a class of important heterocycles which have been well documented throughout the literature due to their biological importance. They exhibit a wide range of pharmaceutical and therapeutic properties[1].A rapid and efficient one-pot method for the synthesis of 4,6-diarylpyrimidin-2(1H)- ones and related heterocycles is described.The condensation of acetophenone derivatives,aldehydes and urea in the presence of sulfamic acid was employed to synthesize a variety of pyrimid...  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