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   73955篇
  免费   18592篇
  国内免费   28814篇
林业   8389篇
农学   7905篇
基础科学   8667篇
  34677篇
综合类   31494篇
农作物   6453篇
水产渔业   6141篇
畜牧兽医   8535篇
园艺   2607篇
植物保护   6493篇
  2024年   973篇
  2023年   1092篇
  2022年   1461篇
  2021年   1415篇
  2020年   2996篇
  2019年   6411篇
  2018年   6140篇
  2017年   6974篇
  2016年   7739篇
  2015年   7816篇
  2014年   6871篇
  2013年   7240篇
  2012年   7156篇
  2011年   6061篇
  2010年   5052篇
  2009年   4583篇
  2008年   3471篇
  2007年   3637篇
  2006年   3296篇
  2005年   2944篇
  2004年   2739篇
  2003年   2484篇
  2002年   2270篇
  2001年   1920篇
  2000年   1595篇
  1999年   1558篇
  1998年   1396篇
  1997年   1379篇
  1996年   1228篇
  1995年   1295篇
  1994年   1256篇
  1993年   1012篇
  1992年   1005篇
  1991年   846篇
  1990年   650篇
  1989年   631篇
  1988年   564篇
  1987年   406篇
  1986年   406篇
  1985年   215篇
  1984年   199篇
  1983年   194篇
  1982年   193篇
  1981年   233篇
  1980年   193篇
  1979年   150篇
  1978年   130篇
  1975年   109篇
  1965年   137篇
  1964年   140篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
21.
22.
提出了环泵泡沫比例混合器和泡沫压力比例混合器在相应灭火系统中的应用、安装和操作中应注意的问题,对两种泡沫比例混合器组成的系统进行了对比,认为环泵比例混合泡沫灭火系统采用正压进水,可以克服负压进水方式存在的缺陷,比较适合油库火灾的扑救;压力比例混合泡沫灭火系统灵活性、匹配性较差,适用于泡沫混合液流量相对不变的火灾扑救。  相似文献   
23.
The purpose of this study was to determine the frequency of selected pathogens in the tissues of a group of feedlot cattle with chronic disease (most often respiratory disease and/or arthritis). Samples of lung and joint tissues from 49 feedlot animals that had failed to respond to antibiotic therapy were tested by immunohistochemical staining for the antigens of Mycoplasma bovis, Haemophilus somnus, Pasteurella (Mannheimia) hemolytica, and bovine viral diarrhea virus (BVDV). Mycoplasma bovis was demonstrated in over 80% of cases, including in 45% of joints and 71% of lungs tested. Mycoplasma bovis was the only bacterial pathogen identified in the joints. Haemophilus somnus and Pasteurella (Mannheimia) haemolytica were found in 14% and 23% of cases, respectively, and were confined to the lungs in all instances. Infection with BVDV was demonstrated in over 40% of cases. Mycoplasma bovis and BVDV were the most common pathogens persisting in the tissues of these animals that had failed to respond to antibiotic therapy.  相似文献   
24.
我国海水网箱养鱼发展现状是以个体联合体生产经营为主体的低技术水平的数量规模化,众多传统港湾已形成了大范围的严重超负荷养殖状态,其对环境巨大的污染力引发鱼病猖獗和频受赤潮侵袭,已面临严峻的养殖环境危机。本文综合分析了我国海水网箱养鱼的发展现状与负面效应及其现状危机,并初步进行了危机根源和可持续发展的讨论分析。  相似文献   
25.
SUMMARY: Aoshio is hypoxic milky blue-green seawater observed in some eutrophic bays. Previous studies have shown that colloidal sulfur causes the coloration and that the source of aoshio water is attributed to coastal anoxic bottom water. Occurrences of aoshio have been reported in limited areas of coastal seawater, although hypoxic transparent water seems rather universal. Promotion in auto-oxidation of sulfide by metal ions in seawater was investigated to explain the occurrences of aoshio. Artificial seawater containing 10 μM metal ion was assayed for the sulfide oxidation rate. The velocity constant which represents the oxidation rate within the initial 30 min and the amount of reacted sulfide in 2 h were determined by oxygen monitoring and sulfide quantification, respectively. Fe2+ and Cu2+ enhanced the initial 30 min reaction. Fe2+, Fe3+ and Ni2+ increased the amount of reacted sulfide in 2 h, forming whitish turbid water. Seawater from a suspected source of aoshio water was also assayed for the auto-oxidation rate of sulfide. The oxidation rate in water from 12 m depth was 13–19 times higher than the artificial seawater without an addition of heavy metal ions. More than 15% of the oxidation rate in 12 m deep seawater was explained by dissolved iron in the seawater.  相似文献   
26.
27.
AIM: To evaluate the genotype , muscle histopathology and ultrastructure in dko mice. METHODS: Dystrophin/Utrophin-deficient double knockouts (dko) mice were obtained from university of Oxford, UK. Genotype of filial generation of heterozygote was evaluated by PCR-SSP. HE staining and fluorescent immunohistochemistry by SABC-Cy3 were used to detect striated muscle of dko mouse, and the muscle ultrastructure was observed by transmission electron microscope(TEM). RESULTS: In 112 filial generation mice, there were 28 mdx (25.0%), 26 dko (23.2%) and 58 heterozygote (51.8%), which coincided with the law of Mendelian inheritance. HE staining showed that the myocytes were not very uniform, there were phenomenon of round outline, centrally nucleated fibers, widening interspace, inflammatory cell infiltration and connective tissue proliferation in dko mice. There were no any immunofluorescent expression of dystrophin and utrophin in sarcolemma in dko mice. TEM showed sarcolemma breakage, separation and edema, and loose myofibril texture, inflammatory cell infiltration and connective tissue proliferation in dko mice. CONCLUSION: PCR-SSP is a very quick and accurate way for genotype evaluation of filial generation. The pathophysiology of dko mouse was very similar to Duchenne muscular dystrophy (DMD), and dko mouse is an ideal animal model for study of DMD clinical therapy.  相似文献   
28.
槟榔红色素的抗氧化活性   总被引:2,自引:1,他引:1  
测定槟榔红色素(areca red pigment,AP)对 DPPH·和·OH的清除能力、对脂质体过氧化的抑制能力、对Fe2+的络合能力,并测定其还原能力.结果表明:槟榔色素对DPPH·、·OH的清除能力较强:对脂质体过氧化的抑制能力很强,抑制率高达79.84%;与VE、没食子酸(gallic acid,GA)、BHT相比,槟榔色素的还原力、对Fe2+的络合能力较弱.说明槟榔红色素是较好的抗氧化剂.  相似文献   
29.
鼠李糖制备新工艺研究   总被引:2,自引:0,他引:2  
采用正交设计方法探讨发酵法从芦丁水解液里提取制备鼠李糖的最佳制备工艺,优选出鼠李糖纯度达到99%以上的最佳制备工艺为:温度30℃,pH值5.0,酵母量为母液的2%。  相似文献   
30.
钙在橄榄树器官中的定量研究   总被引:3,自引:0,他引:3  
本研究用原子吸收光谱法分析了果园土壤及橄榄树体各器官中钙量和不同组分钙的含量。结果表明:0~30cm土层中钙含量最高为0.65mg/g。橄榄器官中叶的钙含量最高为4.523mg/g,其次为根、果、结果枝。地上部器官钙含量由主干一主枝一结果枝一叶片逐渐增加。在各器官中钙主要以醋酸溶钙和盐酸溶钙的形式存在。  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