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   68614篇
  免费   18434篇
  国内免费   2419篇
  2024年   61篇
  2023年   503篇
  2022年   1233篇
  2021年   2370篇
  2020年   3300篇
  2019年   5038篇
  2018年   5060篇
  2017年   5030篇
  2016年   5604篇
  2015年   6371篇
  2014年   6460篇
  2013年   7155篇
  2012年   5415篇
  2011年   4950篇
  2010年   4939篇
  2009年   3565篇
  2008年   2839篇
  2007年   2217篇
  2006年   1965篇
  2005年   1831篇
  2004年   1576篇
  2003年   1394篇
  2002年   1222篇
  2001年   1176篇
  2000年   999篇
  1999年   906篇
  1998年   478篇
  1997年   501篇
  1996年   470篇
  1995年   425篇
  1994年   410篇
  1993年   337篇
  1992年   505篇
  1991年   425篇
  1990年   373篇
  1989年   291篇
  1988年   266篇
  1987年   227篇
  1986年   174篇
  1985年   222篇
  1984年   142篇
  1983年   135篇
  1982年   94篇
  1981年   75篇
  1980年   68篇
  1979年   94篇
  1978年   76篇
  1977年   64篇
  1976年   60篇
  1973年   56篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
142.
Journal of Plant Growth Regulation - With the increasing popularity of urban landscaping, there is a greater need to address iron deficiency and chlorosis in Cinnamomum camphora. Beneficial...  相似文献   
143.
144.
How morphology changes with size can have profound effects on the life history and ecology of an animal. For apex predators that can impact higher level ecosystem processes, such changes may have consequences for other species. Tiger sharks (Galeocerdo cuvier) are an apex predator in tropical seas, and, as adults, are highly migratory. However, little is known about ontogenetic changes in their body form, especially in relation to two aspects of shape that influence locomotion (caudal fin) and feeding (head shape). We captured digital images of the heads and caudal fins of live tiger sharks from Southern Florida and the Bahamas ranging in body size (hence age), and quantified shape of each using elliptical Fourier analysis. This revealed changes in the shape of the head and caudal fin of tiger sharks across ontogeny. Smaller juvenile tiger sharks show an asymmetrical tail with the dorsal (upper) lobe being substantially larger than the ventral (lower) lobe, and transition to more symmetrical tail in larger adults, although the upper lobe remains relatively larger in adults. The heads of juvenile tiger sharks are more conical, which transition to relatively broader heads over ontogeny. We interpret these changes as a result of two ecological transitions. First, adult tiger sharks can undertake extensive migrations and a more symmetrical tail could be more efficient for swimming longer distances, although we did not test this possibility. Second, adult tiger sharks expand their diet to consume larger and more diverse prey with age (turtles, mammals, and elasmobranchs), which requires substantially greater bite area and force to process. In contrast, juvenile tiger sharks consume smaller prey, such as fishes, crustaceans, and invertebrates. Our data reveal significant morphological shifts in an apex predator, which could have effects for other species that tiger sharks consume and interact with. J. Morphol. 277:556–564, 2016. © 2016 Wiley Periodicals, Inc.  相似文献   
145.
146.
147.
Insulin-induced expression of human heat-shock protein gene hsp70   总被引:1,自引:0,他引:1  
In human hepatoma cell line Hep3B/T2, the human heat-shock-inducible gene hsp70 could be induced by insulin. The dose-dependent insulin effect correlates very well with the dissociation constant of the insulin receptor, indicating that the insulin effect is mediated by the insulin receptor. The expression of hsp70 gene was neither significantly induced by growth factors of epidermal and platelet-derived growth factors, nor by tumor promoter, calcium ionophore, cAMP, and glucocorticoids. These results indicate that the induction of expression of hsp70 gene by insulin is very specific and not cell cycle-related. Furthermore, the insulin-induced expression of hsp70 gene is transient, occurring specifically from 4 to 8 h after insulin addition.  相似文献   
148.
149.
150.
A series of new chiral thiosemicarbazones derived from homochiral amines in both enantiomeric forms were synthesized and evaluated for their in vitro antiproliferative activity against A549 (human alveolar adenocarcinoma), MCF‐7 (human breast adenocarcinoma), HeLa (human cervical adenocarcinoma), and HGC‐27 (human stomach carcinoma) cell lines. Some of compounds showed inhibitory activities on the growth of cancer cell lines. Especially, compound 17b exhibited the most potent activity (IC50 4.6 μM) against HGC‐27 as compared with the reference compound, sindaxel (IC50 10.3 μM), and could be used as a lead compound to search new chiral thiosemicarbazone derivatives as antiproliferative agents. Chirality 27:177–188, 2015. © 2014 Wiley Periodicals, Inc.  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