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   28835篇
  免费   3671篇
  国内免费   2篇
  2018年   287篇
  2017年   258篇
  2016年   474篇
  2015年   751篇
  2014年   832篇
  2013年   1064篇
  2012年   1295篇
  2011年   1200篇
  2010年   770篇
  2009年   738篇
  2008年   1100篇
  2007年   1085篇
  2006年   1003篇
  2005年   966篇
  2004年   946篇
  2003年   948篇
  2002年   916篇
  2001年   916篇
  2000年   911篇
  1999年   767篇
  1998年   395篇
  1997年   368篇
  1996年   347篇
  1995年   320篇
  1994年   321篇
  1993年   334篇
  1992年   681篇
  1991年   628篇
  1990年   611篇
  1989年   660篇
  1988年   561篇
  1987年   601篇
  1986年   475篇
  1985年   564篇
  1984年   479篇
  1983年   385篇
  1982年   398篇
  1981年   358篇
  1980年   328篇
  1979年   435篇
  1978年   382篇
  1977年   336篇
  1976年   313篇
  1975年   334篇
  1974年   381篇
  1973年   361篇
  1972年   305篇
  1971年   285篇
  1970年   247篇
  1969年   246篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
4.
5.
The correspondence exchanged between Captain John Blake, John Bradby Blake, and their associates offers a valuable insight into the sociable world of Enlightenment botany. By considering these sources alongside the records of other contemporary scholars of botany, this paper examines the composition of the community of plant collectors and botanical scholars with whom father and son were interacting in the 1760s and 1770s. The documents also reveal the ways in which botanical knowledge circulated among the members of these communities. Amateur botany, it turns out, was deeply linked to gardening and to the emerging trade in horticultural plants, yet the contributions made by gardeners and nurserymen have been largely overlooked. In sum, the Blake collection at Oak Spring helps to answer larger questions about the nature of Enlightenment sociability, about diversity within botanical networks, and about the ways in which that social diversity affected botanical collecting.  相似文献   
6.
7.
A polymorphism was identified in the bovine butyrophilin (BTN) gene by digesting poly-merase chain reaction products with the restriction enzyme Hae III. This polymorphism was segregating in a Holstein-Friesian sire selected as part of an ongoing study directed towards the identification of quantitative trait loci affecting milk composition. Screening of a half-sib family established for the heterozygous sire allowed the localization of BTN to bovine chromosome 23 (BTA23).  相似文献   
8.
9.
As elevated bronchoalveolar lavage (BAL) fluid histamine levels are noted in patients with pulmonary fibrosis (PF), we assayed BAL fluid from 16 patients with PF for the presence of a histamine releasing factor (HRF). HRF activity was assayed by measuring release of the preformed mast cell-derived mediators, histamine, or beta-hexosaminidase (beta-hex) from a purified population of IL-3 dependent mouse bone marrow derived mast cells (MBMMC) or human blood basophils. Mean BAL cell free histamine levels in the patients with PF was 1226 +/- 1349 pg/ml, whereas BAL histamine levels in a comparison group of six non-PF patients was 118 +/- 60 pg/ml. HRF was significantly elevated in BAL fluid of patients with PF (mean beta-hex release 24.5 +/- 12.9%; range 6.8 to 52.4%) compared to the non-PF group of patients (mean beta-hex release 7.9 +/- 7.7%; range 1.8 to 20.7%). The PF HRF not only degranulated MBMMC, but also induced the generation of the arachidonic acid metabolite leukotriene C4 from MBMMC (24.6 +/- 4.2 ng leukotriene C4/10(6) MBMMC). The PF HRF did not appear to be a cytokine previously identified in BAL fluid of patients with PF (i.e., platelet derived growth factor or insulin growth factor-1) or a human cytokine able to degranulate human basophils (i.e., IL-1, or granulocyte-macrophage-CSF) as these recombinant human cytokines did not induce MBMMC beta-hex release. Physicochemical characterization of the HRF revealed that it was relatively heat stable, pronase sensitive and on Sephadex G-75 and G-200 column chromatography had an apparent molecular mass of 30 to 50 kDa. The ability of PF BAL to induce beta-hex release from MBMMC was not dependent on IgE as unsensitized or lactic acid treated MBMMC release similar amounts of beta-hex compared to MBMMC sensitized with IgE. Thus, BAL fluid of patients with PF contains an HRF that induces beta-hex release from MBMMC via an IgE-independent mechanism. The presence of the HRF could explain elevated BAL histamine levels in patients with PF.  相似文献   
10.
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