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   18863篇
  免费   1558篇
  国内免费   1018篇
  2023年   204篇
  2022年   251篇
  2021年   806篇
  2020年   601篇
  2019年   736篇
  2018年   828篇
  2017年   548篇
  2016年   845篇
  2015年   1153篇
  2014年   1309篇
  2013年   1466篇
  2012年   1766篇
  2011年   1555篇
  2010年   964篇
  2009年   848篇
  2008年   1069篇
  2007年   934篇
  2006年   822篇
  2005年   731篇
  2004年   585篇
  2003年   547篇
  2002年   473篇
  2001年   357篇
  2000年   294篇
  1999年   285篇
  1998年   171篇
  1997年   134篇
  1996年   99篇
  1995年   111篇
  1994年   78篇
  1993年   70篇
  1992年   96篇
  1991年   96篇
  1990年   87篇
  1989年   68篇
  1988年   56篇
  1987年   53篇
  1986年   39篇
  1985年   44篇
  1984年   23篇
  1983年   23篇
  1982年   21篇
  1981年   13篇
  1979年   20篇
  1978年   16篇
  1977年   19篇
  1976年   15篇
  1975年   14篇
  1972年   15篇
  1969年   10篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
A haploid callus line from anther cultures of the Asiatic hybrid lily ‘Connecticut King’ was maintained for a long term. The survival and growth of the haploid calluses were affected by auxins of picloram, α-naphthaleneacetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D) and temperatures of 25, 15 and 7 °C during culture. Picloram was more suitable for maintenance of the haploid calluses, whereas NAA and 2,4-D led to root and shoot formation from the haploid calluses. The best temperature for maintenance was 25 °C. About 90% of cells in calluses were maintained in haploid level during 60 weeks of subculture, and about 80% of cells were haploid in the calluses maintained over 2 years with the MS medium containing 4 μM picloram in the dark at 25 °C. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   
22.
23.
RIG-1 signalling is responsible for the detection of cytoplasmic viral RNA molecules. DEXH (Asp-Glu-X-His) box polypeptide 58 (encoded by DHX58) is a negative regulator of the RIG-1 signalling pathway. In human, the DHX58 gene can be upregulated and can inhibit the RIG-1 signalling pathway during viral infection. In this study, porcine DHX58 gene expression patterns were studied. According to our results, the porcine DHX58 gene was upregulated not only by the stimulation of Poly I:C but also by the stimulation of 1ipopolysaccharides (LPS). One polymorphism (g.4919G>C), detected in the ninth intron, was significantly associated with some blood parameters including the red cell distribution width of 1-day-old pigs and white blood cell counts, lymphocyte absolute counts, and platelet distribution width of 17-day-old pigs (P < 0.05). Moreover, the individuals with the genotype GG have a significantly higher mean white blood cell count than individuals with genotype CC or GC (P < 0.05). Our study indicates that DHX58 is an important gene that is associated with the immune response in swine.  相似文献   
24.
B G Han  J Vonck    R M Glaeser 《Biophysical journal》1994,67(3):1179-1186
Changes in protein structure that occur during the formation of the M photointermediate of bacteriorhodopsin can be directly visualized by electron diffraction techniques. A modified preparation technique for glucose-embedded crystals was employed to ensure sufficient hydration of the crystals, which was needed for the formation of the M intermediate at low temperature. Samples containing a high percentage of the M intermediate were trapped by rapidly cooling the crystals with liquid nitrogen after illumination with filtered green light at 240 and 260 K, respectively. Difference Fourier projection maps are presented for the M intermediates formed at these two temperatures. The diffraction data clearly show that statistically significant structural changes occur upon formation of the M intermediate at 240 K and then further upon formation of the second specimen that is produced at 260 K.  相似文献   
25.
26.
Hydrodistillation of the dried leaves of eleven species of the genus Eucalyptus L 'Hér ., i.e., E. astringens Maiden , E. camaldulensis Dehnh ., E. diversifolia Bonpl ., E. falcata Turcz ., E. ficifolia F. Muell ., E. gomphocephala DC., E. lehmannii (Schauer ) Benth ., E. maculata Hook ., E. platypus Hook ., E. polyanthemos Schauer, and E. rudis Endl ., harvested from Korbous arboreta (region of Nabeul, northeast of Tunisia) in April 2006, afforded essential oils in yields varying from 0.1±0.1 to 3.8±0.1%, dependent on the species. E. astringens and E. ficifolia showed the highest and the lowest mean percentage of essential oil amongst all the species examined, respectively. Analysis by GC (RI) and GC/MS allowed the identification of 138 components, representing 74.0 to 99.1% of the total oil. The contents of the different samples varied according to the species. The main components were 1,8‐cineole, followed by trans‐pinocarveol ( 1 ), spathulenol ( 2 ), α‐pinene, p‐cymene, (E,E)‐farnesol, cryptone, globulol ( 3 ), β‐phellandrene, α‐terpineol, viridiflorol, and α‐eudesmol. The principal‐component and the hierarchical‐cluster analyses separated the eleven Eucalyptus leaf essential oils into seven groups, each constituting a chemotype.  相似文献   
27.
28.
Xanthoangelol (XAG), a prenylated chalcone isolated from the Japanese herb Angelica keiskei Koidzumi, has been reported to exhibit antineoplastic properties. However, the specific anti‐tumor activity of XAG in human hepatocellular carcinoma (HCC), and the relevant mechanisms are not known. Herein, we evaluated the effect of XAG against HCC in vitro and in vivo. Although XAG treatment did not significantly reduce the viability of the Hep3B and Huh7 cell lines, it suppressed cell migration, invasion, and EMT. This anti‐metastatic effect of XAG was due to induction of autophagy, because treatment with the autophagy inhibitor 3‐methyadenine (3‐MA) or knockdown of the pro‐autophagy Beclin‐1 effectively abrogated the XAG‐induced suppression of metastasis. Mechanistically, XAG induced autophagy via activation of the AMPK/mTOR signaling pathway, and XAG treatment dramatically increased the expression of p‐AMPK while decreasing p‐mTOR expression. In addition, blocking AMPK/mTOR axis with compound C abrogated the autophagy‐mediated inhibition of metastasis. The murine model of HCC metastasis also showed that XAG effectively reduced the number of metastatic pulmonary nodules. Taken together, our results revealed that autophagy via the activation of AMPK/mTOR pathway is essential for the anti‐metastatic effect of XAG against HCC. These findings not only contribute to our understanding of the anti‐tumor activity of XAG but also provide a basis for its clinical application in HCC. Before this study, evidence of XAG on HCC was purely anecdotal; present study provides the first comprehensive assessments of XAG on HCC metastasis and investigates its underlying mechanism. Results suggest that XAG exerts anti‐metastatic properties against HCC through inducing autophagy which is mediated by the activation of AMPK/mTOR signaling pathway. This research extends our knowledge about the antineoplastic properties of XAG and suggests that induction autophagy may represent future treatment strategies for metastatic HCC.  相似文献   
29.
30.

Background  

A major challenge in evolutionary biology is to understand the typically complex interactions between diverse counter-balancing factors of Darwinian selection for size assortative mating and sexual size dimorphism. It appears that rarely a simple mechanism could provide a major explanation of these phenomena. Mechanics of behaviors can predict animal morphology, such like adaptations to locomotion in animals from various of taxa, but its potential to predict size-assortative mating and its evolutionary consequences has been less explored. Mate-grasping by males, using specialized adaptive morphologies of their forelegs, midlegs or even antennae wrapped around female body at specific locations, is a general mating strategy of many animals, but the contribution of the mechanics of this wide-spread behavior to the evolution of mating behavior and sexual size dimorphism has been largely ignored.  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