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   86691篇
  免费   7472篇
  国内免费   42篇
  94205篇
  2022年   546篇
  2021年   1085篇
  2020年   656篇
  2019年   850篇
  2018年   1047篇
  2017年   962篇
  2016年   1604篇
  2015年   2688篇
  2014年   3043篇
  2013年   4189篇
  2012年   5113篇
  2011年   5254篇
  2010年   3453篇
  2009年   3140篇
  2008年   4627篇
  2007年   4689篇
  2006年   4512篇
  2005年   4469篇
  2004年   4538篇
  2003年   4088篇
  2002年   4120篇
  2001年   1069篇
  2000年   766篇
  1999年   1057篇
  1998年   1308篇
  1997年   955篇
  1996年   896篇
  1995年   840篇
  1994年   826篇
  1993年   780篇
  1992年   782篇
  1991年   737篇
  1990年   724篇
  1989年   745篇
  1988年   650篇
  1987年   624篇
  1986年   580篇
  1985年   731篇
  1984年   845篇
  1983年   752篇
  1982年   860篇
  1981年   889篇
  1980年   820篇
  1979年   574篇
  1978年   635篇
  1977年   615篇
  1976年   597篇
  1975年   459篇
  1974年   583篇
  1973年   529篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
Vaccination with DNA and recombinant vaccinia viruses (rec.VV) has been studied with the coxsackievirus B3 (CVB3) model system. Plasmids encoding all structural proteins of CVB3, when injected intramuscularly, induced only low levels of virus-specific antibodies. However, DNA vaccination with the major structural protein VP1 protected 72.2% of mice from lethal challenge, whereas VP1 expressed by rec.VV was much less efficient.  相似文献   
62.
63.
64.
65.
66.
67.
This study investigated the ability of pretreatment variables from three different domains (social-demographic, psychological, and psychophysiological) to predict posttreatment mean arterial pressure (MAP) for 59 unmedicated males with mild hypertension who were participating in a cross-cultural (USA-USSR) comparison of autogenic training and thermal biofeedback to a self-relaxation control. The overall multiple regression equation consisted of two variables and indicated that higher diastolic blood pressures during a cold pressor task were predictive of greater MAP reductions while higher scores on the Irritability subscale of the Buss-Durkee Hostility Scale were predictive of less MAP reductions. Suggestions for future research in this area are provided.  相似文献   
68.
A significant proportion of enzymes display cooperativity in binding ligand molecules, and such effects have an important impact on metabolic regulation. This is easiest to understand in the case of positive cooperativity. Sharp responses to changes in metabolite concentrations can allow organisms to better respond to environmental changes and maintain metabolic homeostasis. However, despite the fact that negative cooperativity is almost as common as positive, it has been harder to imagine what advantages it provides. Here we use computational models to explore the utility of negative cooperativity in one particular context: that of an inhibitor binding to an enzyme. We identify several factors which may contribute, and show that acting together they can make negative cooperativity advantageous.  相似文献   
69.
Summary Spontaneous cell-to-cell transformation between naturally competent bacteria on selective media resulted in an overestimation of the transferability of genetic information. EDTA effectively prevented transformation on selective media whereas DNaseI did not reliably inhibit cell-to-cell transformation. An improved method to estimate gene transfer frequencies is described.  相似文献   
70.
An ecosystem service is a benefit derived by humanity that can be traced back to an ecological process. Although ecosystem services related to surface water have been thoroughly described, the relationship between atmospheric water and ecosystem services has been mostly neglected, and perhaps misunderstood. Recent advances in land-atmosphere modeling have revealed the importance of terrestrial ecosystems for moisture recycling. In this paper, we analyze the extent to which vegetation sustains the supply of atmospheric moisture and precipitation for downwind beneficiaries, globally. We simulate land-surface evaporation with a global hydrology model and track changes to moisture recycling using an atmospheric moisture budget model, and we define vegetation-regulated moisture recycling as the difference in moisture recycling between current vegetation and a hypothetical desert world. Our results show that nearly a fifth of annual average precipitation falling on land is from vegetation-regulated moisture recycling, but the global variability is large, with many places receiving nearly half their precipitation from this ecosystem service. The largest potential impacts for changes to this ecosystem service are land-use changes across temperate regions in North America and Russia. Likewise, in semi-arid regions reliant on rainfed agricultural production, land-use change that even modestly reduces evaporation and subsequent precipitation, could significantly affect human well-being. We also present a regional case study in the Mato Grosso region of Brazil, where we identify the specific moisture recycling ecosystem services associated with the vegetation in Mato Grosso. We find that Mato Grosso vegetation regulates some internal precipitation, with a diffuse region of benefit downwind, primarily to the south and east, including the La Plata River basin and the megacities of Sao Paulo and Rio de Janeiro. We synthesize our global and regional results into a generalized framework for describing moisture recycling as an ecosystem service. We conclude that future work ought to disentangle whether and how this vegetation-regulated moisture recycling interacts with other ecosystem services, so that trade-offs can be assessed in a comprehensive and sustainable manner.  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