全文获取类型
收费全文 | 245210篇 |
免费 | 15032篇 |
国内免费 | 650篇 |
专业分类
林业 | 23520篇 |
农学 | 13968篇 |
基础科学 | 2952篇 |
42351篇 | |
综合类 | 21160篇 |
农作物 | 17147篇 |
水产渔业 | 16451篇 |
畜牧兽医 | 93592篇 |
园艺 | 6264篇 |
植物保护 | 23487篇 |
出版年
2021年 | 2460篇 |
2020年 | 2859篇 |
2019年 | 3643篇 |
2018年 | 3914篇 |
2017年 | 4314篇 |
2016年 | 4757篇 |
2015年 | 4281篇 |
2014年 | 5624篇 |
2013年 | 16230篇 |
2012年 | 5884篇 |
2011年 | 7707篇 |
2010年 | 7169篇 |
2009年 | 7803篇 |
2008年 | 7169篇 |
2007年 | 6157篇 |
2006年 | 6799篇 |
2005年 | 6113篇 |
2004年 | 5935篇 |
2003年 | 5794篇 |
2002年 | 5099篇 |
2001年 | 5569篇 |
2000年 | 5211篇 |
1999年 | 5003篇 |
1998年 | 4093篇 |
1997年 | 4178篇 |
1996年 | 3875篇 |
1995年 | 4470篇 |
1994年 | 3857篇 |
1993年 | 3555篇 |
1992年 | 4087篇 |
1991年 | 4300篇 |
1990年 | 4021篇 |
1989年 | 3992篇 |
1988年 | 3535篇 |
1987年 | 3568篇 |
1986年 | 3534篇 |
1985年 | 3838篇 |
1984年 | 3621篇 |
1983年 | 3371篇 |
1982年 | 2834篇 |
1981年 | 2719篇 |
1980年 | 2706篇 |
1979年 | 3173篇 |
1978年 | 2825篇 |
1977年 | 2687篇 |
1976年 | 2502篇 |
1975年 | 2346篇 |
1974年 | 2510篇 |
1973年 | 2512篇 |
1972年 | 2226篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
E. Conde A. M. Moreno I. Martin-Lacave A. Fernandez y H. Galera 《Anatomia, histologia, embryologia》1992,21(1):94-100
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
52.
A good correlation exists between specific neutralising antibody titre and protection against challenge with foot-and-mouth disease virus (FMDV) in infected or virus-vaccinated cattle, but not in the case of animals immunised with synthetic FMDV peptides. Therefore, mechanisms other than simple neutralisation are likely to be important in vivo. Antibody affinity may influence the protective capacity of sera from immunised animals and experiments were carried out to measure the functional affinity for synthetic FMDV peptide of sera from guinea pigs and cattle given various synthetic vaccines. In guinea pigs given a single dose of synthetic vaccine, antibody affinity increased with time after immunisation. In cattle, however, administration of a second dose of peptide 21 days after the first markedly retarded the process of affinity maturation. For guinea pig sera of equivalent neutralising activity, those of higher functional affinity had higher protective indices than those of lower functional affinity. Knowledge of the importance of antibody affinity in protection against FMD is important for an improved understanding of the mechanisms of protection and for the design of novel vaccines. 相似文献
53.
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
54.
Serum amyloid A protein (SAA) in horses: objective measurement of the acute phase response 总被引:1,自引:0,他引:1
M B Pepys M L Baltz G A Tennent J Kent J Ousey P D Rossdale 《Equine veterinary journal》1989,21(2):106-109
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
55.
A Sosnicki R G Cassens D R McIntyre R J Vimini M L Greaser 《British poultry science》1989,30(1):69-80
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
56.
57.
Kevin R. Stuart Arnold G. Eversole David E. Brune 《Journal of the World Aquaculture Society》2001,32(1):105-111
Abstract.— The freshwater mussel Elliptio complanata was provided water containing green algae and cyanobacteria delivered from the Partitioned Aquaculture System (PAS) at eight flow rates to determine algal filtration rates as mg of particulate organic carbon (POC)/kg wet tissue weight per h. The dominant taxon in cyanobacterial waters was Microcystis while the dominant taxa in green algal waters were Scenedesmus and Ankistrodesmus . The cell counts of Scenedesmus and Ankistrodesmus were the only algal taxa that were significantly different between the incoming water and water filtered by mussels. Filtration rates of POC obtained from green algal water were significantly greater than from cyanobacteria-dominated waters at all flow rates. A significant increase in mean filtration rate was observed as flow rates increased. The filtration rate of green algae increased as POC concentration increased, peaking at 28 mg C/L. A maximum filtration rate was not observed with cyanobacterial waters. 相似文献
58.
The effect of heat stress on the components of grain weight was analysed in wheat ( Triticum aestivum L.) varieties differing in grain weight stability. The wheat varieties PBW154, Sonalika and Hindi62 were raised in the field (New Delhi, India; 77°12' E, 28°40' N, 228.6 m a.s.l.) at three dates of sowing: 19 November 1992 (DOS I), 14 December 1992 (DOS II) and 18 January 1993 (DOS III). The late-sown crop (DOS III) experienced 6–8 °C warmer temperatures during grain development than the crop sown at the normal time (DOS I). The heat susceptibility index (S) revealed that grain weight was less susceptible to heat in Sonalika and PBW154 than in Hindi62. Heat stress reduced both the grain growth duration (GGD) and the grain growth rate (GGR). The grain weight reduction in PBW154 and Sonalika was mainly due to a reduction in GGR, while that of Hindi62 was due to a reduction in GGD. In vivo studies on starch and protein synthesis in excised endosperm at 15, 25 and 35 °C revealed that both processes were more thermotolerant in Hindi62 than in PBW154. The grain starch content was stable in Hindi62 while that in PBW154 was significantly reduced under heat stress. The grain nitrogen content at maturity increased in both varieties under heat stress. It was concluded that the heat susceptibility of grain weight in Hindi62 was mainly due to a reduction in GGD, although GGR, starch and protein synthesis were more thermotolerant in developing grains of Hindi62 than in those of PBW154. 相似文献
59.
60.