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   34680篇
  免费   3309篇
  国内免费   13篇
  2022年   206篇
  2021年   494篇
  2020年   280篇
  2019年   392篇
  2018年   458篇
  2017年   416篇
  2016年   684篇
  2015年   1069篇
  2014年   1253篇
  2013年   1648篇
  2012年   1953篇
  2011年   2054篇
  2010年   1274篇
  2009年   1212篇
  2008年   1807篇
  2007年   1865篇
  2006年   1790篇
  2005年   1850篇
  2004年   1757篇
  2003年   1759篇
  2002年   1678篇
  2001年   406篇
  2000年   233篇
  1999年   385篇
  1998年   484篇
  1997年   348篇
  1996年   354篇
  1995年   285篇
  1994年   329篇
  1993年   307篇
  1992年   289篇
  1991年   263篇
  1990年   261篇
  1989年   232篇
  1988年   254篇
  1987年   239篇
  1986年   220篇
  1985年   302篇
  1984年   389篇
  1983年   313篇
  1982年   372篇
  1981年   435篇
  1980年   415篇
  1979年   252篇
  1978年   258篇
  1977年   274篇
  1976年   263篇
  1975年   181篇
  1974年   233篇
  1973年   192篇
排序方式: 共有10000条查询结果,搜索用时 18 毫秒
101.
102.
The ontogeny of large, globular, epidermal cytoplasmic inclusions (ECI) in P. virgatum roots was studied at the ultrastructural level. These ECI were seen to originate in meristematic cells as small electron translucent vesicles. Subsequently, the ECI, which appeared to be temporary storage sites, were seen to enlarge and increase in density by accumulating masses of a granular matrix as well as some small vesicular inclusions. In the zone of elongation, as the epidermal cells matured, the ECI within each cell gradually fused and the contents were lost. The pattern of the ontogeny of the ECI in the growing epidermal cells was consistent with the presence of cells of different physiologies in the zone of cell elongation of these roots.  相似文献   
103.
104.
105.
106.
Women of Principle: Female Networking in Contemporary Mormon Polygyny. Janet Bennion. New York: Oxford University Press. 1998. 178 pp.  相似文献   
107.
108.
109.
Using ethnographic data collected in a second grade classroom over the course of a school year, this paper describes the ways in which one school's discourse of liberalism is deleteriously deployed. We view the school's discipline creed as emblematic of the school's liberal curriculum, and interrogate the effects on four African American boys in the classroom when the school enacts this creed. Despite the agency that these boys obviously had, they were unable to control the ways in which they were placed at a structural disadvantage and manipulated by a system far more powerful than they were. The results were that these four boys suffered. Not only did the intended liberal curriculum fail to be translated fully into the enacted curriculum, the liberal underpinnings of this curriculum precluded teachers and students from taking any critical stance.  相似文献   
110.
The mature male gametophyte of Ginkgo biloba can be divided into two regions: a large saccate structure that is suspended within the fertilization chamber above the archegonia, and a pervasive, highly branched haustorial system that ramifies through the intercellular air spaces of the apex of the nucellus. This morphology appears to differ in many ways from the simpler more typical male gametophytes of most other groups of seed plants. Growth and development of the male gametophyte of Ginkgo biloba were studied using computer reconstruction techniques to generate images of the gametophyte from data derived from serial sections through the ovule. These investigations reveal that morphological development of the male gametophyte of Ginkgo biloba is divided into three distinct phases: 1) Germination, characterized by an initial brief period of diffuse growth. This phenomenon has not been described for any other seed plant male gametophyte; 2) Initiation of tip growth and the formation of a tubular body, as typifies all seed plant male gametophytes. In Ginkgo, this is accompanied by a high degree of branching, giving rise to an extensively branched haustorial system; 3) Late swelling of the proximal unbranched portion of the gametophyte resulting in formation of the saccate structure that is characteristic of the mature gametophyte. This process appears to be very similar to late development in cycad male gametophytes. Thus, despite the seemingly anomalous morphology of the mature male gametophyte of Ginkgo biloba, specific patterns of growth and development are in many ways similar to growth processes expressed by the male gametophytes of some or all major groups of seed plants.  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