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   69956篇
  免费   4624篇
  国内免费   6篇
  2023年   491篇
  2022年   519篇
  2021年   1080篇
  2020年   985篇
  2019年   1045篇
  2018年   2101篇
  2017年   1961篇
  2016年   2580篇
  2015年   3449篇
  2014年   3455篇
  2013年   4618篇
  2012年   5481篇
  2011年   4974篇
  2010年   3130篇
  2009年   2536篇
  2008年   3842篇
  2007年   3636篇
  2006年   3454篇
  2005年   2951篇
  2004年   2891篇
  2003年   2565篇
  2002年   2378篇
  2001年   1545篇
  2000年   1508篇
  1999年   1202篇
  1998年   552篇
  1997年   425篇
  1996年   409篇
  1995年   389篇
  1994年   298篇
  1993年   283篇
  1992年   593篇
  1991年   516篇
  1990年   467篇
  1989年   451篇
  1988年   442篇
  1987年   391篇
  1986年   380篇
  1985年   369篇
  1984年   369篇
  1983年   263篇
  1982年   244篇
  1981年   189篇
  1980年   187篇
  1979年   222篇
  1978年   212篇
  1975年   205篇
  1974年   228篇
  1973年   232篇
  1972年   190篇
排序方式: 共有10000条查询结果,搜索用时 375 毫秒
151.
152.
153.
154.
Frequency of infection by Cryptosporidium sp. in 1,039 faecal smears stained by Ziehl-Neelsen, obtained from undernourished patients of a Nutritional Recovery Center and an ambulatory undernourished center from the Fifth Region, Chile, were studied. All underwent a coproparasitological examination by the modified Telemann method. Thirty eight (3.7%) patients infected by the parasite were detected, with an overall frequency of 8.5% among patients of the Nutritional Recovery Center and 1.9% among the ambulatory patients; this difference was statistically significant. The highest percentage of positive results were detected among the younger milk feeding infants. Also the percentage of difference among these and the older milk feeding infants (3.7%) was statistically significant. Association of Cryptosporidium sp. and Giardia lamblia was observed in 6 ambulatory patients (2.3%).  相似文献   
155.
156.
The pmr spectra of fully acetylated 2S: 3R-2 amino-trans-4 octadecene-1, 3 diol (sphingosine) (lb), 2S : 3R-2 aminooctadecane-1, 3 diol (dihydrosphingosine) (2b), 1-O-β-D-galactopyranosyl-2S : 3R-2 tetracosanoylamido-trans-4 octadecene-1, 3 diol (cerasine) (3b), and 1-O-β-D-galactopyranosyl-2S : 3R-2 (2′ hydroxy) tetracosanoylamido-trans-4 octadecene-1, 3 diol (phrenosine) (4b), were determined in chloroform-d, acetone-d6, and benzene-d6 at 220 MHz. The relative chemical shifts of the various protons in the three solvents were different enough as to permit configurational and conformational information to be derived by partial first order analysis.  相似文献   
157.
158.
159.
160.
Seeds of Amaranthus hypochondriacus L. are known to accumulate a trypsin-inhibitor (ATI) member of the potato-I inhibitor family and an α -amylase inhibitor (AAI), possessing a knottin-like fold. They are believed to have a defensive role due to their inhibition of trypsin-like enzymes and α -amylases of insect pests. In this work, both inhibitory activities were found in leaves of young A. hypochondriacus plants. High constitutive levels of foliar inhibitory activity against bovine trypsin and insect α -amylases were detected in in vitro assays. Trypsin inhibitory activity was further increased by exposure to diverse treatments, particularly water stress. Salt stress, insect herbivory and treatment with exogenous methyl jasmonate (MeJA) or abscisic acid (ABA) also induced trypsin inhibitor activity accumulation, although to a lesser degree. In gel and immunoblot analyses showed that foliar trypsin inhibitor activity was constituted by at least three different inhibitors of approximately 29, 8 (including ATI) and 3 kDa, respectively. These inhibitors showed differing patterns of accumulation in response to diverse treatments. On the other hand, significant increases in α -amylase inhibitor activity and AAI levels were detected in leaves of insect-damaged, MeJA- and ABA-treated A. hypochodriacus plantlets, but not in those subjected to water- or salt-stress. A differential induction of trypsin inhibitor activity and α -amylase inhibitor accumulation in response to insect herbivory by two related species of lepidopterous larvae was observed, whereas mechanical wounding failed to induce either inhibitor. The overall results suggest that trypsin and α -amylase inhibitors could protect A. hypochondriacus against multiple types of stress.  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