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   622391篇
  免费   63126篇
  国内免费   779篇
  2018年   6689篇
  2017年   6332篇
  2016年   8610篇
  2015年   10841篇
  2014年   13136篇
  2013年   18069篇
  2012年   21328篇
  2011年   22167篇
  2010年   14982篇
  2009年   13474篇
  2008年   19540篇
  2007年   20191篇
  2006年   19088篇
  2005年   18054篇
  2004年   18215篇
  2003年   17164篇
  2002年   16802篇
  2001年   25831篇
  2000年   25575篇
  1999年   20209篇
  1998年   7309篇
  1997年   7171篇
  1996年   6810篇
  1995年   6429篇
  1994年   6150篇
  1993年   6107篇
  1992年   16579篇
  1991年   16711篇
  1990年   16434篇
  1989年   15949篇
  1988年   14807篇
  1987年   13982篇
  1986年   13207篇
  1985年   13151篇
  1984年   10778篇
  1983年   9229篇
  1982年   6784篇
  1981年   6024篇
  1980年   5733篇
  1979年   10054篇
  1978年   8051篇
  1977年   7236篇
  1976年   6841篇
  1975年   8021篇
  1974年   8783篇
  1973年   8569篇
  1972年   7697篇
  1971年   7164篇
  1970年   6079篇
  1969年   5994篇
排序方式: 共有10000条查询结果,搜索用时 24 毫秒
111.
112.
Agapophytinae subf.n. is a highly diverse lineage of Australasian Therevidae, comprising eight described and two new genera: Agapophytus Guérin‐Méneville, Acupalpa Kröber, Acraspisa Kröber, Belonalys Kröber, Bonjeania Irwin & Lyneborg, Parapsilocephala Kröber, Acatopygia Kröber, Laxotela Winterton & Irwin, Pipinnipons gen.n. and Patanothrix gen.n. A genus‐level cladistic analysis of the subfamily was undertaken using sixty‐eight adult morphological characters and c. 1000 base pairs of the elongation factor‐1α (EF‐1α) protein coding gene. The morphological data partition produced three most parsimonious cladograms, whereas the molecular data partition gave a single most parsimonious cladogram, which did not match any of the cladograms found in the morphological analysis. The level of congruence between the data partitions was determined using the partition homogeneity test (HTF) and Wilcoxon signed ranks test. Despite being significantly incongruent in at least one of the incongruence tests, the partitions were combined in a simultaneous analysis. The combined data yielded a single cladogram that was better supported than that of the individual partitions analysed separately. The relative contributions of the data partitions to support for individual nodes on the combined cladogram were investigated using Partitioned Bremer Support. The level of support for many nodes on the combined cladogram was non‐additive and often greater than the sum of support for the respective nodes on individual partitions. This synergistic interaction between incongruent data partitions indicates a common phylogenetic signal in both partitions. It also suggests that criteria for partition combination based solely on incongruence may be misleading. The phylogenetic relationships of the genera are discussed using the combined data. A key to genera of Agapophytinae is presented, with genera diagnosed and figured. Two new genera are described: Patanothrix with a new species (Pat. skevingtoni) and Pat. wilsoni (Mann) transferred from Parapsilocephala, and Pipinnipons with a new species (Pip. kroeberi). Pipinnipons fascipennis (Kröber) is transferred from Squamopygia Kröber and Pip. imitans (Mann) is transferred from Agapophytus. Agapophytus bicolor (Kröber) is transferred from Parapsilocephala. Agapophytus varipennis Mann is synonymised with Aga. queenslandi Kröber and Aga. flavicornis Mann is synonymised with Aga. pallidicornis (Kröber).  相似文献   
113.
114.
115.
116.
117.
The cell surface of the parasitic protozoan Leishmania mexicana is coated by glycosylphosphatidylinositol (GPI)-anchored glycoproteins, a GPI-anchored lipophosphoglycan and a class of free GPI glycolipids. To investigate whether the anchor or free GPIs are required for parasite growth we cloned the L.mexicana gene for dolichol-phosphate-mannose synthase (DPMS) and attempted to create DPMS knockout mutants by targeted gene deletion. DPMS catalyzes the formation of dolichol-phosphate mannose, the sugar donor for all mannose additions in the biosynthesis of both the anchor and free GPIs, except for a alpha1-3-linked mannose residue that is added exclusively to the free GPIs and lipophosphoglycan anchor precursors. The requirement for dolichol-phosphate-mannose in other glycosylation pathways in L.mexicana is minimal. Deletion of both alleles of the DPMS gene (lmdpms) consistently resulted in amplification of the lmdpms chromosomal locus unless the promastigotes were first transfected with an episomal copy of lmdpms, indicating that lmdpms, and possibly GPI biosynthesis, is essential for parasite growth. As evidence presented in this and previous studies indicates that neither GPI-anchored glycoproteins nor lipophosphoglycan are required for growth of cultured parasites, it is possible that the abundant and functionally uncharacterized free GPIs are essential membrane components.  相似文献   
118.
Abstract

In Arabidopsis thaliana, cell fate in developing ovules is determined by the action of the homeodomain factor BELL1 (BEL1) and of the MADS-box factors SEEDSTICK (STK), SHATTERPROOF1 (SHP1) and SHP2. The analysis of the bel1 and the stk shp1 shp2 mutants revealed that the functional megaspore is formed, however, it does not proceed into megagametogenesis. In the bel1 stk shp1 shp2, quadruple mutant megasporogenesis does not take place. In this article we describe a detailed morphological analysis of the quadruple mutant, and we discuss the possibility that BELL1, STK, SHP1 and SHP2 not only control integument identity determination and development, but that they might also play a role during megasporogenesis.  相似文献   
119.
120.
International Journal of Primatology - The selection of sleeping sites and sleeping trees in nonhuman primates is related to social and ecological factors. We investigate the role of body...  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