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   320943篇
  免费   17432篇
  国内免费   678篇
林业   24496篇
农学   14202篇
基础科学   2995篇
  46506篇
综合类   45310篇
农作物   17354篇
水产渔业   18276篇
畜牧兽医   137628篇
园艺   6533篇
植物保护   25753篇
  2019年   3721篇
  2018年   4297篇
  2017年   4656篇
  2016年   5014篇
  2015年   4495篇
  2014年   5878篇
  2013年   16570篇
  2012年   7819篇
  2011年   10119篇
  2010年   7932篇
  2009年   8361篇
  2008年   9446篇
  2007年   8655篇
  2006年   9171篇
  2005年   8349篇
  2004年   8183篇
  2003年   7949篇
  2002年   7378篇
  2001年   8479篇
  2000年   8149篇
  1999年   7215篇
  1998年   4402篇
  1997年   4451篇
  1996年   4139篇
  1995年   4807篇
  1994年   4273篇
  1993年   3952篇
  1992年   5912篇
  1991年   6203篇
  1990年   6041篇
  1989年   6010篇
  1988年   5482篇
  1987年   5554篇
  1986年   5585篇
  1985年   5657篇
  1984年   5024篇
  1983年   4620篇
  1982年   3569篇
  1981年   3380篇
  1980年   3316篇
  1979年   4512篇
  1978年   3878篇
  1977年   3502篇
  1976年   3193篇
  1975年   3199篇
  1974年   3905篇
  1973年   3940篇
  1972年   3718篇
  1971年   3518篇
  1970年   3341篇
排序方式: 共有10000条查询结果,搜索用时 515 毫秒
71.
A systematic immunohistochemical study of the ultimobranchial tubule (UBT) has been carried out in 45 Wistar rats of different ages (0, 5, 10, 15, 20, 25, 30, 60 and 120 days). The existence of calcitonin immunoreactive cells in the UBT wall has been demonstrated in a 5-days old rat. In addition, immunohistochemical studies for thyroglobulin revealed positive staining in follicular cells connected to the UBT and, occasionally, in isolated cells lying within solid clusters from the UBT. These last results together with the continued and repeated existence of numerous mitosis and PAS (+) microfollicles, apparently rising from the UBT, support the hypothesis that the ultimobranchial body (UBB) may contribute partially to the formation of a part of the follicular component.  相似文献   
72.
73.
74.
A research program to study tropical pastures used by domestic livestock was implemented in Senegal. During field studies, numerous information was collected together with many samples with three main aims: pasture composition, diet composition and relationship between each other. The so-called method of epiderms, more often used in temperate areas, was chosen for analyzing and comparing the botanical composition of 105 samples, e.g., shepherd's hand-picking, alimentary bolus and faeces collected during a twelve-month period. In view of the results, some methodological problems are raised and discussed.  相似文献   
75.
Abstract. The use of an indirect enzyme-linked immunosorbent assay ( elisa ) for the detection of channel catfish antibody to Edwardsiella ictaluri is described. Changes in agglutination titre in fish immunized with Edwardsiella ictaluri heat killed whole bacterins or lipopolysaccharides were reflected by corresponding changes in elisa readings. Relatively high correlations were observed among elisa OD readings, computed elisa titres and corresponding agglutination titres.  相似文献   
76.
A sensitive and precise immunoassay for equine serum amyloid A protein (SAA) was established and used to determine, for the first time, the circulating concentration of this protein in health and disease. As in other species, equine SAA was present only at trace levels in healthy animals but behaved as an extremely sensitive and rapidly responding acute phase reactant following most forms of tissue injury, infection and inflammation, objectively reflecting the extent and activity of disease. Measurements of SAA should make a significant contribution to diagnosis and management of viral and bacterial infection in horses, and routine serial assays could provide an objective criterion for monitoring prospectively the general health of horses in training and racing.  相似文献   
77.
1. The incidence of microscopically detectable degenerative characteristics in 5 skeletal muscles (m. pectoralis thoracicus, m. supracoracoideus, m. biceps femoris, m. semitendinosus, m. femorotibialis medius) of turkeys was investigated. 2. Samples were obtained from 30 Large White turkey males 14, 16 and 18 weeks old. Hyaline degeneration, infiltration of mononuclear cells and necrotic fibres were observed. 3. Individual fibres varied greatly in size and muscle fibre nuclei were often shrunken and pyknotic. 4. Weak and/or uniform reaction for Ca++-ATPase and SDH in all types of muscle fibres and loss of alkaline phosphatase activity in cell membranes were noted. A positive reaction for acid phosphatase occurred in regions of perivascular infiltration and in necrotic muscle fibres. The majority of muscle fibres possessed high activity for phosphorylase a and b. 5. Based on the use of fluorescein alpha-bungarotoxin conjugate, motor end-plates appeared to be morphologically intact. Direct immunofluorescence with anti-chicken IgG showed positive reaction in muscle fibres undergoing necrosis and in the involved connective tissue. 6. Degenerative changes varied with age and were most marked in the oldest birds. 7. Because gross degenerative symptoms were absent from both the birds and the meat from them, the condition appears to be either different from or a precursor to the degenerative myopathy characterised by other authors.  相似文献   
78.
Purified populations of bovine antigen presenting cells (APCs) and T cells have been isolated from peripheral blood and characterised using various monoclonal antibodies (mAbs) for cell surface markers. Bovine APCs were found in an adherent cell fraction and were non-specific esterase positive, phagocytic and expressed bovine major histocompatibility complex (MHC) class II determinants, all of which are typical macrophage characteristics. T cells were rigorously depleted of accessory cell function before being used in an antigen presenting cell assay. The generation of T helper cells in response to the soluble antigen, ovalbumin, was entirely dependent upon a critical number of APCs. Further the proliferative response was inhibited by several mAbs to bovine MHC class II molecules. Thus the interaction between bovine APCs and helper/inducer T lymphocytes (TH/I) appears to be similar to that in other species.  相似文献   
79.
80.
Comprehensive studies into domestic pigs and wild boars together with literature data provided a basis for an assessment of aerobic metabolic capacity (VO2max) of swine. The values quoted, from 20 to 94 ml/min-1/kg, had been due to several factors of methodological approach, growth, training, and nutrition as well as to pathophysiological aspects. For full capacity utilisation of VO2max, untrained pigs can be challenged at belt velocities between 0.8 and 1.8 m/s-1 and trained animals at 5 m/s-1.  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