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   15059篇
  免费   384篇
  国内免费   27篇
林业   659篇
农学   810篇
基础科学   139篇
  2031篇
综合类   1682篇
农作物   503篇
水产渔业   1084篇
畜牧兽医   7141篇
园艺   232篇
植物保护   1189篇
  2023年   98篇
  2021年   96篇
  2020年   134篇
  2019年   209篇
  2018年   378篇
  2017年   352篇
  2016年   341篇
  2015年   311篇
  2014年   330篇
  2013年   586篇
  2012年   487篇
  2011年   576篇
  2010年   469篇
  2009年   432篇
  2008年   565篇
  2007年   590篇
  2006年   486篇
  2005年   533篇
  2004年   416篇
  2003年   455篇
  2002年   367篇
  2001年   769篇
  2000年   729篇
  1999年   593篇
  1998年   172篇
  1997年   164篇
  1996年   127篇
  1995年   150篇
  1994年   156篇
  1993年   120篇
  1992年   323篇
  1991年   349篇
  1990年   292篇
  1989年   268篇
  1988年   274篇
  1987年   321篇
  1986年   246篇
  1985年   195篇
  1984年   171篇
  1983年   147篇
  1982年   97篇
  1981年   83篇
  1980年   79篇
  1979年   141篇
  1978年   95篇
  1977年   73篇
  1975年   78篇
  1974年   82篇
  1973年   70篇
  1969年   70篇
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
941.
942.
Primary porcine endothelial cells have a limited life span in culture. After four to five passages, they tend to de-differentiate and eventually reach senescence. The aim of this work was to establish immortalized porcine aortic endothelial cell lines (AOCs) to facilitate in vitro studies of different pathological process involving the endothelium. Primary porcine aortic endothelial cells (PAECs) were transfected with a plasmid containing the SV40 genome and selected on the basis of morphological and phenotypical features. Flow cytometry analysis demonstrated uptake of acetylated low density lipoproteins (Ac-LDL) and constitutive expression of SLA class I, CD29, CD31, CD41/61, CD80/86, CD46, SWC3, and LAMP-1 antigens by all analyzed lines and showed little differences to primary cells. The functional similarity between primary and immortalized endothelial cells was demonstrated in a cytotoxicity assay using a human natural killer cell line (NKL) as effector. The AOCs cell lines should be valuable tools for in vitro study of the human immune response against pig endothelial cells. In addition, they would be very useful to gain insight in the pathogenesis of some viral haemorrhagic diseases of pig such as African swine fever (ASF) or classical swine fever (CSF).  相似文献   
943.
  总被引:2,自引:0,他引:2  
The semi-arid Loess Plateau is a central zone of Chinese rain-fed farming. Drought and soil erosion are the two major obstacles to restrict economic development in this semi-arid Loess Plateau. The traditional conservation agriculture and runoff agriculture have been developed for several decades, which have reduced water and soil loss and improved rainwater utilization efficiency to a certain extent. However, its regulative role in rainwater is very much limited, and problems of agriculture production, such as severe seasonal drought and water shortage, low agriculture productivity, fragile ecological environment, and low yield-invest ratio, are still going on. Supplemental irrigation of harvested rainwater is an important form of rainfall regulation and utilization on the basis of sophisticated technologies and theories of conservation agriculture and runoff agriculture, which not only promotes fulfilling rainfall harvesting in spatial location like the conservation agriculture and runoff agriculture measure but also gets at true rainfall harvesting in time distribution by principle of superposition and also decreases invalid evaporation for rainfall runoff. Agriculture productivity will be highly increased by using rainwater harvesting for supplemental irrigation. It not only brings about the integration of ecological reconstruction with economic development, but also provides a new approach to the agriculture sustainable development and overcomes two of the biggest obstacles (drought and soil erosion). It is suggested that this technology will become a strategic measure and comprehensive development model in semi-arid Loess Plateau of China.  相似文献   
944.
945.
Neospora caninum was isolated from the brain of a 2-year-old dairy cow that had aborted confirmed N. caninum-infected fetuses on two occasions. The cow had an indirect fluorescent antibody titer of 1:1600 to N. caninum. The cow was killed 24 days after its second abortion and the brain was bioassayed for N. caninum in nude mice. Multifocal areas of perivascular cuffing and glial nodules were observed in the cerebrum and mesencephalon of the cow, but N. caninum was not identified in histological sections of the brain. All three nude mice inoculated with brain homogenate of the cow, developed emaciation and paralysis. Microscopical examination of the nude mice revealed systemic N. caninum infection with demonstrable tachyzoites in various organs. The parasites isolated from fresh mouse brain were transferred successfully into Vero cell cultures. PCR procedure on the purified tachyzoites obtained from the Vero cell cultures amplified the specific DNA sequence for N. caninum.  相似文献   
946.
The migration of vinyl chloride monomer (VCM) from unplasticizedpolyvinyl chloride (uPVC) pipes was investigated using locallymanufactured pipes. Specimens of 33 cm long were used throughoutthe research. The investigation was carried out under differentconditions of water temperature, pH and total dissolved solidsconcentration and at different durations of exposure. The VCMconcentration in the water was evaluated using the gaschromotography (GC)/head-space technique. A VCM concentration ofmore than 2.5 ppb was detected after 30 days of exposure at45 °C. The initial VCM concentration in the uPVC pipewas predicted using equations derived from Fick's first law ofdiffusion. Water tenperature did not affect the migration ofVCM, unless it was raised to high values (i.e. 45 °C).Total dissolved solids (TDS) and pH of water were found toaffect the release of VCM from uPVC pipes. Diffusion rate of VCMwas predicted as a function of pH or TDS values.  相似文献   
947.
China is the world’s leading producer of potatoes, growing 22% of all potatoes. Production continues to rise, owing to increases in both land devoted to potato production and yield per hectare. Most potato production occurs in the northern and southwest regions of the country. The processing of coarse starch is the most important component of the potato processing industry in China, but other processing industries, such as crisps and French fries, are expanding. Major production constraints include inadequate germplasm resources for cultivar development, the lack of high quality seed potatoes, and limited access to equipment for mechanized cultivation, planting, fertilizing, spraying, and harvesting. Additional weaknesses in storage and transportation technologies must be addressed, as they are the major constraints for the healthy development of the potato industry. The introduction and improvement of these technologies will ensure the sustainable development of the potato industry in China.  相似文献   
948.
    
  相似文献   
949.
    
Japanese oak wilt causes widespread oak mortality in Japan. Possible differences in susceptibility to the causal fungus, Raffaelea quercivora, may be due to vessel arrangements in host trees. To clarify whether constitutive defence mechanisms including vessel arrangements or induced defence mechanisms are the main determinants of host susceptibility, we inoculated the fungus into living seedlings or sterilized stem segments of four Japanese fagaceous species. In seedlings, water conductance was assessed with dye. In both seedlings and stem segments, hyphal growth was examined by fluorescence microscopy. In seedlings, the area of non‐conductive sapwood in stem cross sections and hyphal growth differed significantly among species. In the susceptible species Quercus crispula and Quercus serrata, hyphal growth was significantly and positively correlated with the proportion of non‐conductive sapwood. In stem segments, hyphal growth was not significantly different among species or between vessel arrangements and was similar to or greater than that in seedlings. These results suggest that the extent of sapwood colonization by R. quercivora could be used as a marker for susceptibility and that susceptibility is determined mainly by induced defence responses.  相似文献   
950.
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