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   525745篇
  免费   47348篇
  国内免费   1181篇
  2018年   16569篇
  2017年   15458篇
  2016年   12719篇
  2015年   6888篇
  2014年   7432篇
  2013年   10524篇
  2012年   16564篇
  2011年   26638篇
  2010年   21819篇
  2009年   17588篇
  2008年   21872篇
  2007年   24500篇
  2006年   10187篇
  2005年   11170篇
  2004年   10748篇
  2003年   10494篇
  2002年   10122篇
  2001年   18657篇
  2000年   19138篇
  1999年   15075篇
  1998年   5064篇
  1997年   5299篇
  1996年   4765篇
  1995年   4697篇
  1994年   4646篇
  1993年   4673篇
  1992年   12179篇
  1991年   11789篇
  1990年   11389篇
  1989年   11034篇
  1988年   10433篇
  1987年   9937篇
  1986年   9416篇
  1985年   9527篇
  1984年   7902篇
  1983年   6785篇
  1982年   5435篇
  1981年   5032篇
  1980年   4569篇
  1979年   7681篇
  1978年   6187篇
  1977年   5700篇
  1976年   5390篇
  1975年   6057篇
  1974年   6658篇
  1973年   6614篇
  1972年   6723篇
  1971年   6126篇
  1970年   4788篇
  1969年   4753篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Plasma choline concentrations were measured in humans, rats, and rabbits within the first few hours of life, before any food was ingested. Neonatal animals and humans had markedly elevated plasma choline levels compared to adult animals or humans. Possible mechanisms responsible for this elevation are discussed and possible consequences for brain function, lung function, and growth are presented.  相似文献   
992.
993.
994.
Astrocytes have long been considered as just providing trophic support for neurons in the central nervous system, but recently several studies have highlighted their importance in many functions such as neurotransmission, metabolite and electrolyte homeostasis, cell signaling, inflammation, and synapse modulation. Astrocytes are, in fact, part of a bidirectional crosstalk with neurons. Moreover, increasing evidence is stressing the emerging role of astrocyte dysfunction in the pathophysiology of neurological disorders, including neurodegenerative disease, stroke, epilepsy, migraine, and neuroinflammatory diseases.  相似文献   
995.
Comparative mineral and hormonal analyses were made on tissue culture derived truncated leaf syndrome and wild type oil palm seedlings. Mineral analysis confirmed that Boron, Zinc and chlorophyll levels were significantly lower in truncated leaf syndrome leaves than those of wild type. Hormonal analysis also revealed various cytokinin derivatives such as trans-zeatin, trans-zeatin riboside, trans-zeatin O-glucoside and trans-zeatin riboside 5??mono phosphate were significantly higher in truncated leaf syndrome leaves compared to wild type leaves. Brassinolide level was also significantly higher in truncated leaf syndrome leaves than those of the wild type. These observations suggest that the truncated leaf syndrome abnormality could be associated to high cytokinin and brassinosteroid production which affects the uptake of Boron and Zinc.  相似文献   
996.
997.
998.
We described the bacterial diversity of walnut grove soils under organic and conventional farming. The bacterial communities of rhizospheric and nonrhizospheric soils of pecan tree (Carya illinoensis K. Koch) were compared considering two phenological stages (sprouting and ripening). Sixteen operational taxonomic units (OTUs) were identified significantly more abundant according to the plant development, only one according to the farming condition, and none according to the soil origin. The OTUs specificaly abundant according to plant development included Actinobateria (2) and Betaproteobacteria (1) related OTUs more abundant at the sprouting stage, while at the fruit ripening (FR) stage the more abundant OTUs were related to Actinobacteria (6), Alphaproteobacteria (6), and unclassified Bacteria (1). The Gaiellaceae OTU18 (Actinobacteria) was more abundant under conventional farming. Thus, our study revealed that the plant development stage was the main factor shaping the bacterial community structure, while less influence was noticed for the farming condition. The bacterial communities exhibited specific metabolic capacities, a large range of carbon sources being used at the FR stage. The identified OTUs specifically more abundant represent indicators providing useful information on soil condition, potential tools for the management of soil bacterial communities.  相似文献   
999.
1000.
The quality of milk products is threatened by the formation of biofilms of thermophilicstreptococci on the internal surfaces of plate heat exchangers used in milk processing. Althoughattachment to stainless steel surfaces is one of the first stages in the development of a biofilm, themechanisms involved in attachment have not been reported. The cell surface properties of 12strains of thermophilic streptococci were examined to determine their importance in attachment tostainless steel surfaces. Hydrophobicity, extracellular polysaccharide production and cell surfacecharge varied between the different strains but could not be related to numbers attaching. Treatingthe cells with sodium metaperiodate, lysozyme or trichloroacetic acid to disrupt cell surfacepolysaccharide had no effect on attachment. Treatment with trypsin or sodium dodecyl sulphate toremove cell surface proteins resulted in a 100-fold reduction in the number of bacteria attaching.This result suggests that the surface proteins of the thermophilic streptococci are important intheir attachment to stainless steel.  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