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   1607篇
  免费   68篇
  国内免费   92篇
林业   146篇
农学   248篇
基础科学   54篇
  364篇
综合类   277篇
农作物   93篇
水产渔业   69篇
畜牧兽医   375篇
园艺   51篇
植物保护   90篇
  2023年   11篇
  2022年   49篇
  2021年   49篇
  2020年   41篇
  2019年   33篇
  2018年   27篇
  2017年   41篇
  2016年   38篇
  2015年   43篇
  2014年   27篇
  2013年   97篇
  2012年   73篇
  2011年   76篇
  2010年   79篇
  2009年   69篇
  2008年   76篇
  2007年   66篇
  2006年   70篇
  2005年   73篇
  2004年   30篇
  2003年   34篇
  2002年   36篇
  2001年   40篇
  2000年   39篇
  1999年   42篇
  1998年   27篇
  1997年   39篇
  1996年   32篇
  1995年   17篇
  1994年   18篇
  1993年   14篇
  1992年   16篇
  1991年   18篇
  1990年   21篇
  1989年   16篇
  1988年   18篇
  1987年   19篇
  1985年   13篇
  1983年   12篇
  1979年   23篇
  1977年   9篇
  1976年   11篇
  1975年   8篇
  1971年   21篇
  1970年   14篇
  1969年   14篇
  1968年   8篇
  1967年   20篇
  1966年   21篇
  1965年   10篇
排序方式: 共有1767条查询结果,搜索用时 15 毫秒
81.
Rao  M.R.  Palada  M.C.  Becker  B.N. 《Agroforestry Systems》2004,61(1-3):107-122
Agroforestry Systems - A large number of people in developing countries have traditionally depended on products derived from plants, especially from forests, for curing human and livestock...  相似文献   
82.
Ficus species are multipurpose trees well known by rural populations in Sahelian and Sudanian zones of Africa. Their uses are well documented but their amenability for vegetative propagation has not been extensively studied. This paper compares the rooting ability of stem and aerial root cuttings from thirteen Ficus species found in West Africa. It highlights the differences between species belonging to the sub-genera Sycomorus and Urostigma. The former show no capacity to propagate from cuttings whereas the latter, with epiphytic development, can be propagated by cuttings, although this capacity varies among species. Thus, F. thonningii, F. leprieurii and F. ovata are easily propagated, while F. platyphylla and F. elasticoides are propagated with difficulty. The rooting capacity also varies depending on the cutting material used. It decreases in the following order: long leafless hardwood cuttings (pole) > nodal cuttings > apical cuttings. Rooting potential increases when the cuttings are harvested towards the end of the dry season(March to May). Aerial root can be used for cuttings in all species of the sub-genus Urostigma. The capacity of root cuttings to regenerate is greatest when cuttings are collected at the beginning of the dry season (November). In this case, wound-induced adventitious roots arise at the basal end of the cutting while de novo buds are developed from the cambium at the distal end. The subsequent morphological development is identical to that of a stem cutting. These results clarify and allow the optimal use of the knowledge and methods developed by the indigenous people of the Sahel and could assist and promote fig tree (Ficus sp.) domestication in the dry tropics.This revised version was published online in November 2005 with corrections to the Cover Date.  相似文献   
83.
厚朴伐桩有很强的萌蘖能力,在生长季节采伐也能得到满意的萌芽更新效果,每个伐桩可萌发1-7个萌芽条,7年生厚朴矮林的树干与树枝鲜重达60.2t.hm^-2,平均净生产量为8.6t.hm^-2.a^-1,表明厚朴完全适宜矮林经营,伐桩高度对萌芽条数量,矮林生长影响不大;伐桩萌芽条数量越多,矮林生长越好。  相似文献   
84.
A new phorbol diester, 13-O-myristyl-20-O-acetyl-12-deoxyphorbol (1), has been isolated from the benzene extract of the heartwood of Aleurites moluccana. In addition, hentriacontane, 6,7-dimethoxycoumarin, 5,6,7-trimethoxycoumarin and beta-sitostenone are being reported for the first time from this species.  相似文献   
85.
86.
Bovine plasma proteins are used as high-quality protein supplements in animal feed and as binders or colorants in food for human consumption. Religious observance, as well as recent fears of epidemic bovine spongiform encephalopathy, highlights the need for methods to detect bovine blood in processed food and animal feed for regulatory purposes, as the currently available methods are neither species-specific, blood-specific, nor valid for excessively heat-processed samples. This paper reports the development of monoclonal antibodies (MAbs) raised against bovine thermostable plasma proteins that display a unique species specificity pattern for plasma proteins. Immunoblotting revealed several thermostable antigenic proteins (10, 25, 40, and 60 kDa) in bovine plasma sterilized at 121 degrees C for 15 min. These MAbs can be employed individually or combined in immunoassays for analytical purposes and investigations of the chemical and biological properties of the thermostable plasma proteins identified here.  相似文献   
87.
通过对国内外线性遗产系列战略措施和文献的梳理,发现在国际层面,线性遗产研究以欧洲文化线路和美国遗产廊道为主,同时涉及绿道、风景道、历史路径、文化廊道等部分内容。文化线路与遗产廊道侧重点不同,前者以历史文化挖掘与保护为核心,后者则注重打造景观和游憩功能。整体上欧美国家以国家战略措施和项目应用为主,学术研究中会议论文、研究报告居多,文献数量较少,理论研究相对欠缺。国内线性遗产研究可划分为两个部分:一是对文化线路、遗产廊道、绿道、风景道等国外概念的借鉴及本土化研究;二是围绕线性文化遗产、国家文化公园等国内创新概念进行多方探索。其中文化线路、遗产廊道和线性文化遗产的国内研究主要围绕遗产价值评估、廊道构建和旅游开发三个方面展开,相关研究较为完善,但整体方法技术创新不够。国家文化公园作为线性遗产研究新兴领域,尚无明确的概念定义和完整的理论框架体系,具有较大的研究空间与潜力。  相似文献   
88.
Transgenic cotton carrying the Cry1Ac gene has revolutionized insect pest control since its adoption, although the development of resistance in insect pests has reduced its efficacy.  After 10 years of cultivating Bacillus thuringiensis (Bt) cotton with a single Cry1Ac gene, growers are on the verge of adopting Bt cotton that carries the double gene (Cry1Ac+Cry2A) due to its better effectiveness against insect pests.  Thus, the current study was designed to evaluate the role of each gene in the effectiveness of Bt cotton carrying the double gene.  The expression levels of the Cry1Ac and Cry2A genes were evaluated in the leaves of 10 genotypes (2 parents and 8 F1 hybrids) at 30 days after sowing (DAS), while samples of leaves, bolls and flowers were taken from the upper and lower canopies at 70 and 110 DAS.  The F1 hybrids were developed through reciprocal crosses between two Bt (CKC-1, CKC-2) and two non-Bt (MNH-786, FH-942) parents.  The differential expression of transgenes was evaluated through Enzyme Linked Immuno-Sorbent Assay (ELISA).  The results showed that the MNH786×CKC-1 hybrid had the highest concentrations of Cry1Ac gene at 30 DAS (3.08 µg g–1) and 110 DAS (1.01 µg g–1) in leaves.  In contrast, the CKC-2×MNH-786 hybrid showed the lowest concentrations of Cry1Ac gene at 30 DAS (2.30 µg g–1) and 110 DAS (0.86 µg g–1).  The F1 hybrid FH-942×CKC-2 showed the highest concentrations of Cry2A gene at 30 DAS (8.39 µg g–1) and 110 DAS (7.74 µg g–1) in leaves, while the CKC-1×MNH-786 hybrid expressed the lowest concentrations of Cry2A gene at 30 DAS (7.10 µg g–1) and 110 DAS (8.31 µg g–1).  A comparison between the two stages of plant growth showed that leaves had the highest concentrations at 30 DAS, whereas the lowest concentrations were observed at 110 DAS for both genes in leaves.  When the expression pattern was compared between various plant parts in genotype CKC-2, it was found that leaves had higher concentrations of Cry1Ac (3.12 µg g–1) and Cry2A (8.31 µg g–1) at 70 DAS, followed by bolls (Cry1Ac (1.66 µg g–1) and Cry2A (8.15 µg g–1)) and flowers (Cry1Ac (1.07 µg g–1) and Cry2A (7.99 µg g–1)).  The genotype CKC-2 had higher concentrations of Cry1Ac (3.12 µg g–1) and Cry2A (8.31 µg g1) in the upper canopy but less accumulation (2.66 µg g–1 of Cry1Ac, 8.09 µg g–1 of Cry2A) in the lower canopy at 70 DAS.  Similarly, at 110 DAS, the expression levels of Cry1Ac and Cry2A in upper and lower canopy leaves were 1.52 and 7.92 µg g–1, and 0.99 and 7.54 µg g–1, respectively.  Hence, the current study demonstrates that different genotypes showed variable expression for both of the Cry1Ac and Cry2A genes during plant growth due to different genetic backgrounds.  The Cry2A gene had three-fold higher expression than Cry1Ac with significant differences in expression in different plant parts.  The findings of this study will be helpful for breeding insect-resistant double-gene genotypes with better gene expression levels of Cry1Ac and Cry2A for sustainable cotton production worldwide.  相似文献   
89.
PHANTOM 4 RTK+大疆像控处理技术在燕麦长势模拟中的应用   总被引:1,自引:1,他引:0  
【目的】 利用小型消费级无人机航拍获取地物影像,通过地物阴影、高度差、色差快速提取地物,进而获取地物结构信息。【方法】 文章选取云南省曲靖市会泽县的大桥乡为研究区域,针对冬闲田闲置土地资源、种植结构相对单一的区域展开试验,利用高分辨率无人机遥感影像对燕麦进行识别,同时结合超声波传感器数据估算地物高度,并与实际高度和无人机生成的传统测高方法得到的高度进行相关性分析,获取高精度、可靠性强的数据。【结果】 基于可见光燕麦的总体分类精度为91.46%,Kappa系数为0.857,在增加DSM数据后的分类总体精度为98.91%,Kappa系数为0.982。研究表明由无人机获取的代表燕麦冠层高度信息的DSM数据能够显著提升燕麦的识别效果。相对于传统无人机测高方法生成数字表面模型提取地物高度的方法,依赖于光谱和高程信息识别地物信息的方法在计算地物高度时,精度更高,识别结果更可靠。【结论】 该文提出的小型消费级无人机利用地物阴影计算燕麦高度的方法,改进了相机镜头光心地位和RTK天线中心点地位补偿作用,打通了RTK模块、飞控模块及相机云台模块之间的通讯,能够应用于实际准确获取影像地位信息,为无人机遥感快速、准确地获取地物高度信息提供了一种新的思路。  相似文献   
90.
以生态文明理念为桥梁,梳理国家公园与人的全面发展的内在逻辑,以武夷山国家公园为实证,根据对居民和游客的调查数据分析,探讨国家公园促进“人的全面发展”的内在机理。研究发现,国家公园能够从提供“生态物质基础”、促进“生态精神富有”、提高“生态道德素质”、构建“生态社会关系”4个方面推动人的全面发展,具体包含生存环境、生活环境、发展环境、文化自信、生态自信、精神享受、生态责任、生态行为、人与人的关系、人与自然的关系等内容。  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