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   41616篇
  免费   1908篇
  国内免费   4366篇
林业   5104篇
农学   6250篇
基础科学   3269篇
  6942篇
综合类   11977篇
农作物   2664篇
水产渔业   1710篇
畜牧兽医   5531篇
园艺   1458篇
植物保护   2985篇
  2024年   118篇
  2023年   413篇
  2022年   1143篇
  2021年   1634篇
  2020年   1451篇
  2019年   1481篇
  2018年   1011篇
  2017年   1322篇
  2016年   1275篇
  2015年   1745篇
  2014年   1602篇
  2013年   2111篇
  2012年   2356篇
  2011年   2591篇
  2010年   2399篇
  2009年   2395篇
  2008年   2260篇
  2007年   2348篇
  2006年   2108篇
  2005年   2158篇
  2004年   1013篇
  2003年   889篇
  2002年   749篇
  2001年   807篇
  2000年   926篇
  1999年   1303篇
  1998年   1091篇
  1997年   933篇
  1996年   899篇
  1995年   799篇
  1994年   745篇
  1993年   730篇
  1992年   592篇
  1991年   531篇
  1990年   440篇
  1989年   365篇
  1988年   284篇
  1987年   177篇
  1986年   147篇
  1985年   87篇
  1984年   58篇
  1983年   44篇
  1982年   42篇
  1981年   54篇
  1980年   36篇
  1979年   32篇
  1978年   16篇
  1965年   12篇
  1964年   14篇
  1963年   12篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
在乡村旅游发展过程中,以生态休闲为特色的绿道应当作为乡村旅游规划的元素之一。文中简述目前绿道建设存在的误区,提出适宜城市居民游憩的绿道选择,并对城乡绿道的游憩功能实现途径进行探讨,对基于绿道建设的旅游开发进行展望。  相似文献   
52.
In order to optimize design and operation, it is necessary to calculate accurately hydraulic characteristics large central air-conditioning systems. Simplified Model can not be applied to hydraulic calculation of large pipe network topological structure for its large calculation error. Taking into account the regulating characteristics of terminal branch temperature regulating valve, a direct-return chilled water system is studied, an accurate mathematical model of pipe network hydraulic characteristics has been established and a computer logic algorithm with virtual flow has been given. A direct-return pipe network with ten AHU branches is used as simulation calculation object, the pipe network supply-return water pressure difference is calculated, and also the temperature regulating valve opening and actual flow of each branch are calculated in the condition of different supply-return water pressure difference. The calculated results accord with the natural characteristics of high pressure difference unbalanced for direct-return pipe network. The calculation method can ensure calculation convergence.  相似文献   
53.
摘要:运用nested-PCR技术对广西黄皮和九里香进行柑桔黄龙病(Citrus Huanglongbing, HLB)检测,结果表明:从部分黄皮和九里香样品抽提的DNA中,可扩增出一条特异性的400 bp条带,说明广西种植的黄皮和九里香均可感染黄龙病;在柑桔黄龙病疫区,黄皮和九里香黄龙病感染率分别为35.0%和40.0%。  相似文献   
54.
提出了基于数学模型的幼苗外观特征自动检测方法,检测项目包括生长状态、子叶参数和胚轴参数。首先经过图像预处理提取幼苗二值图,利用行像素统计图确定特征参数基准点位置。然后以标定胚轴最小矩形倾斜度和宽度判定弯曲状态;子叶跨度通过两子叶端点距离确定,子叶展开角通过两子叶底端平展位置拟合线夹角判定;胚轴弯曲度通过胚轴中心线上曲率最大的位置为分界点分别判断两段斜度而求得,胚轴长、轴径结合斜度补偿求得。与手工测量数据对比,轴长、轴径和子叶跨度的相关系数分别为0.935 1、0.899 9和0.903 4,相对误差分别小于7%、5%和7%,绝对误差分别小于4 mm、0.2 mm和6 mm。  相似文献   
55.
High-resolution electron microscope observations of shock-loaded boron carbide have revealed the formation of nanoscale intragranular amorphous bands that occur parallel to specific crystallographic planes and contiguously with apparent cleaved fracture surfaces. This damage mechanism explains the measured, but not previously understood, decrease in the ballistic performance of boron carbide at high impact rates and pressures. The formation of these amorphous bands is also an example of how shock loading can result in the synthesis of novel structures and materials with substantially altered properties.  相似文献   
56.
棉花群体叶面积载荷量与产量关系及对源的调节效应研究   总被引:16,自引:6,他引:16  
通过对棉花单位叶面积负载的生殖器官量的研究结果表明:较高的叶面积载荷量[以单位叶面积负载的果节数(个/m2叶)、结铃数(个/m2叶)及生殖器官干重(kg/m2叶))能够促进棉花群体叶片光合强度提高和养分向棉铃输送增多,因而群体成铃数增多,产量提高。同时由剪叶、疏蕾试验揭示了,当群体叶面积系数控制在适宜范围内,可通过提高叶面积载荷量使产量进一步上升,群体叶面积系数过大时,通过减少叶面积,提高叶面积载铃量可以使产量增加。而当叶面积过小时,虽叶面积载荷量增大,但终因光合面积太小,产量下降,因此叶面积载荷量可作为反映棉花群体源库是否协调及改进栽培技术的依据。  相似文献   
57.
2008年9月14日上午9时许,广东省高州市宝光街道朝培桐村发生一起凶杀案,茂名市茂南区村民沈某在中秋节时前往朝培桐村岳母莫某家,劝说因争吵而赌气回娘家的妻子程某回家,但遭到妻子和岳母的反对。一气之下,沈某持菜刀将77岁的岳母莫某杀死,将妻子程某及出租车司机砍致重伤,  相似文献   
58.
Biosecurity, biocontainment, and disease risk management on dairy replacement operations are time- and labor-intensive, planned programs. Oftentimes the value of these programs is realized only after disease is introduced to a facility or a disease outbreak occurs. There is no "one-plan-fits-all;" each plan must be tailored to meet the needs of management's goals and expectations and problems specific to a production enterprise or geographic region. A standard framework applicable to biosecurity programs includes: (1) hazard identification, (2) exposure assessment, (3) risk characterization, and (4) risk management. The discussion presented here helps lay the framework for development and implementation of biosecurity and risk-management programs within dairy replacement facilities.  相似文献   
59.
Porcine haptoglobin (Hp) is an acute phase protein. Its plasma level increases significantly during inflammation and infection. One of the main functions of Hp is to bind free hemoglobin (Hb) and inhibit its oxidative activity. In the present report, we studied the Hp phenotype of Taiwanese Lanyu miniature pigs (TLY minipigs; n=43) and found their Hp structure to be a homodimer (beta-alpha-alpha-beta) similar to human Hp 1-1. Interestingly, Western blot and high performance liquid chromatographic (HPLC) analysis showed that 25% of the TLY minipigs possessed low or no plasma Hp level (<0.05 mg/ml). The Hp cDNA of these TLY minipigs was then cloned, and the translated amino acid sequence was analyzed. No sequences were found to be deficient; they showed a 99.7% identity with domestic pigs (NP_999165). The mean overall Hp level of the TLY minipigs (0.21 +/- 0.25 mg/ml; n=43) determined by enzyme-linked immunosorbent assay (ELISA) was markedly lower than that of domestic pigs (0.78 +/- 0.45 mg/ml; p<0.001), while 25% of the TLY minipigs had an Hp level that was extremely low (<0.05 mg/ml). In addition, the initial recovery rate (first 40 min) in the circulation of infused fluorescein isothiocyanate (FITC)-Hb was significantly higher in the TLY minipigs with extremely low Hp levels than those with high levels. This data suggests that the low concentration of Hp-Hb complex is responsible for the higher recovery rate of Hb in the circulation. TLY minipigs have been used as an experimental model for cardiovascular diseases; whether they can be used as a model for inflammatory diseases, with Hp as a marker, remains a topic of interest. However, since the Hp level varies significantly among individual TLY minipigs, it is necessary to prescreen the Hp levels of the animals to minimize variation in the experimental baseline. The present study may provide a reference value for future use of the TLY minipig as an animal model for inflammation-associated diseases.  相似文献   
60.
近年来,蜜蜂失踪现象多发引起全球蜂业从业者关注,其归因众说纷纭。有认为是农作物滥施农药或频繁在蜂群中使用杀螨药物导致;有解释为移动、电信基站电磁干扰蜜蜂飞行所致;也有说是气候异常引起;等等。笔者以几十年亲身观察本地蜂群的群势变化与实践证明,推测蜜蜂失踪现象另有其因。在此呈现,供同行们从另一个角度探究愈演愈烈的蜜蜂失踪之谜。  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