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   44914篇
  免费   237篇
  国内免费   549篇
系统科学   1207篇
丛书文集   90篇
教育与普及   278篇
理论与方法论   510篇
现状及发展   29328篇
研究方法   60篇
综合类   12040篇
自然研究   2187篇
  2013年   802篇
  2012年   411篇
  2011年   2380篇
  2009年   594篇
  2008年   610篇
  2007年   659篇
  2006年   765篇
  2005年   972篇
  2004年   2103篇
  2003年   1686篇
  2002年   1328篇
  2001年   934篇
  2000年   470篇
  1999年   722篇
  1998年   642篇
  1997年   773篇
  1996年   506篇
  1995年   394篇
  1994年   679篇
  1993年   687篇
  1992年   668篇
  1991年   593篇
  1990年   659篇
  1989年   490篇
  1988年   469篇
  1987年   463篇
  1986年   555篇
  1985年   668篇
  1984年   627篇
  1983年   537篇
  1982年   710篇
  1981年   755篇
  1980年   820篇
  1979年   1077篇
  1978年   1012篇
  1977年   1024篇
  1976年   924篇
  1975年   888篇
  1974年   623篇
  1973年   957篇
  1972年   1030篇
  1971年   964篇
  1970年   931篇
  1969年   918篇
  1968年   886篇
  1967年   747篇
  1966年   595篇
  1965年   517篇
  1964年   458篇
  1963年   414篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
71.
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   
72.
This paper discusses the use of the Kast and Rosenzweig systems and contingency model for teaching and practice in organizational analysis and planning, management policy, and organizational development. The need for an organizational model for graduate students and field-based managers and executives is identified. The model is presented beginning with the Kast and Rosenzweig work, with integration of research by Daft, Schein, Trist, Deal and Kennedy, Ackoff, Delbecq, and Mintzberg. The teaching and practice uses of the model in four areas are presented, including organizational analysis and planning, management policy making, and organizational development. In each area case examples of the use of the model in teaching and practice are presented. Needs for future research and implications for the use of the model are discussed.  相似文献   
73.
In this paper two alternative loss criteria for the least squares Procrustes problem are studied. These alternative criteria are based on the Huber function and on the more radical biweight function, which are designed to be resistant to outliers. Using iterative majorization it is shown how a convergent reweighted least squares algorithm can be developed. In asimulation study it turns out that the proposed methods perform well over a specific range of contamination. When a uniform dilation factor is included, mixed results are obtained. The methods also yield a set of weights that can be used for diagnostic purposes.  相似文献   
74.
75.
通过对“洋务运动博物馆”东立面形象的反思 ,讨论了建筑形态设计中隐喻手法的运用 ;就此引申 ,探讨传统问题 ,指出正确理解传统与创新的四个方面。  相似文献   
76.
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   
77.
78.
Summary Traces of nor-adrenaline restore the vascular action of adrenaline altered in epinephrectomized dogs to the reaction of the normal animal. Therefore it is claimed that the adrenals discharge one or several substances into the blood stream, which are necessary for the usual peripheral vascular action of adrenaline. Further investigations are in progress.  相似文献   
79.
The visual field is topographically mapped onto the primary visual cortex (V1), forming a retinotopic map which is far more detailed for the foveal regions than for the periphery. We found that receptive field (RF) size in monkey V1 increases with eccentricity, and that a 1—2 m2 V1 region contains roughly a complete set of machinery necessary to analyze a visual-field area whose size is about that of the RFs of the cells. This allows V1 to be anatomically uniform, and is in sharp contrast with the retina.  相似文献   
80.
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   
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