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   1382784篇
  免费   121052篇
  国内免费   1458篇
  2021年   17852篇
  2019年   16215篇
  2018年   18069篇
  2017年   16756篇
  2016年   28284篇
  2015年   42703篇
  2014年   50671篇
  2013年   77134篇
  2012年   37329篇
  2011年   26027篇
  2010年   44109篇
  2009年   45399篇
  2008年   24627篇
  2007年   22838篇
  2006年   27969篇
  2005年   28987篇
  2004年   28284篇
  2003年   25870篇
  2002年   24022篇
  2001年   33673篇
  2000年   30961篇
  1999年   31392篇
  1998年   25524篇
  1997年   25329篇
  1996年   24924篇
  1995年   23012篇
  1994年   22836篇
  1993年   21903篇
  1992年   27763篇
  1991年   26232篇
  1990年   25240篇
  1989年   26062篇
  1988年   24131篇
  1987年   22706篇
  1986年   21719篇
  1985年   23793篇
  1984年   23453篇
  1983年   20746篇
  1982年   20975篇
  1981年   20157篇
  1980年   18774篇
  1979年   19557篇
  1978年   18242篇
  1977年   17595篇
  1976年   16902篇
  1975年   16373篇
  1974年   17006篇
  1973年   17331篇
  1972年   14857篇
  1971年   13593篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Summary Recombination of an inherited pericentric inversion of chromosome 20 has given rise to a child with partial trisomy 20p. To our knowledge no previous familial inversions of this chromosome have been described in the literature.  相似文献   
992.
993.
A low-pressure microwave-induced helium plasma serves as an excitation source for metal chlorides, nitrates, and sulfates vaporized from a filament, resulting in fractional vaporization and differential sensitivities of detection of the elements depending on the vapor pressures of their salts. The shapes of the single emission peaks, which are simple in the presence of potassium chloride, become complex and may double in number.  相似文献   
994.
995.
It is clear from reviewing the findings of our own studies and those of others that the cerebral cortex has combined two very different strategies of organisation. Firstly it has a strictly defined genetically determined substrate of specific neurons classes, specific rules for which kinds of cells interconnect, a laminar architecture where efferent and afferent relays and interlaminar links are predetermined. But, as well, a second strategy allows great developmental lability in the precise spatial patterns of intralaminar circuits of the excitatory neurons and in the actual weights of excitatory and inhibitory synapses that are contributed to each neuron. This second strategy presumably allows the cortex to be tailor-made to the early experience of each individual and, as well, allow for lability of responses to different conditions of stimulation and adjustment of the system to compensate to some degree for injuries affecting afferents and circuitry in the adult system.  相似文献   
996.
997.
998.
Requirements for seed germination (emergence of radicle) andseedling formation (emergence of both radicle and cotyledons)of a hemi-root parasite Sopubia delphinifolia were studied inthe absence of any host stimulus, in Petri dish and asepticcultures. Both water washing in trickling tap water as wellas cold treatment were effective in inducing/stimulating germinationand seedling formation in the light. Although ethrel, an ethylenereleasing compound, stimulated radicle emergence it inhibitedthe emergence of the cotyledons. Light was found to be essentialfor germination; none of the growth substances could replacethe light requirement. Light responses seem to be mediated throughthe phytochrome system. The results indicate that the emergenceof the radicle, its further growth into the root and the emergenceof cotyledons are controlled by different factors. Sopubia delphinifolia, hemi-root parasite, seed germination requirements, pretreatment, seedling formation  相似文献   
999.
A review of in vitro mutagenesis assessment of metal compounds in mammalian and nonmammalian test systems has been compiled. Prokaryotic assays are ineffective or inconsistent in their detection of most metals as mutagens, with the notable exception of hexavalent chromium. Mammalian assay systems appear to be similarly inappropriate for the screening of metal compounds based upon the limited number of studies that have employed those compounds having known carcinogenic activity. Although of limited value as screening tests for the detection of potentially carcinogenic metal compounds, the well-characterized in vitro mutagenesis systems may prove to be of significant value as a means to elucidate mechanisms of metal genotoxicity.  相似文献   
1000.
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