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   255篇
  免费   25篇
  国内免费   17篇
林业   10篇
农学   61篇
基础科学   32篇
  21篇
综合类   117篇
农作物   15篇
水产渔业   16篇
畜牧兽医   9篇
园艺   2篇
植物保护   14篇
  2023年   1篇
  2022年   4篇
  2021年   2篇
  2020年   12篇
  2019年   12篇
  2018年   8篇
  2017年   13篇
  2016年   19篇
  2015年   14篇
  2014年   14篇
  2013年   10篇
  2012年   17篇
  2011年   5篇
  2010年   18篇
  2009年   11篇
  2008年   9篇
  2007年   5篇
  2006年   5篇
  2005年   8篇
  2004年   3篇
  2003年   10篇
  2002年   24篇
  2001年   20篇
  2000年   9篇
  1999年   5篇
  1998年   10篇
  1997年   5篇
  1996年   4篇
  1995年   3篇
  1994年   4篇
  1993年   3篇
  1992年   3篇
  1991年   2篇
  1989年   2篇
  1985年   1篇
  1982年   1篇
  1980年   1篇
排序方式: 共有297条查询结果,搜索用时 15 毫秒
81.
为选育非洲菊新品种,开展了国外引进品种间的人工杂交和自交结籽试验.2002年杂交结籽率达96%,每朵花平均的结籽数达62粒,千粒重达3.89 g,自交则分别为46.80%,26粒,3.20g,均有明显下降.随着花朵重瓣程度的上升,杂交和自交的结籽数均有下降的趋势.杂交的结籽数与自交的结籽数出现表现为正相关,因此建议选择杂交结籽容易的品种作为自交的亲本.  相似文献   
82.
常规玉米自交系及其杂交种幼胚培养反应的研究   总被引:5,自引:0,他引:5  
以生产上常规的19个玉米自交系和10个杂交种为实验材料,在8114培养基上进行培养,按幼胚培养的特点进行观察分类。结果发现,29种基因型均可诱导出愈伤组织,基因型间差异较大,同时在8114培养基里附加氯化钠和氯化钾各1g/L,不同基因型反应不同。  相似文献   
83.
基于要素聚集,区域自我发展能力指数可分为要素数量指数、要素质量指数、要素配置指数,根据东、西部地区共22个省市区2006~2011年的数据计算结果可知,东、西部地区自我发展能力差距悬殊,且无缩小趋势。为减小二者之间的差距,西部地区应提升资本集聚能力,大力发展资本市场;加强教育力度,培育高素质人才;完善东西部地区的对口支援机制;以政府制度激励为契机,促:进西部地区制度创新。  相似文献   
84.
通过研究P2P网络流量数据,证实了在Maze应用中少量的流和主机产生了大部分的网络流量,但同时也发现同WWW应用和整个链路流量的情况相比较这一现象并不突出.针对流的自相似性分析表明,Maze应用的流到达间隔时间和流到达速率具有明显的自相似特征,但是自相似系数并没有达到很高程度.  相似文献   
85.
A study was conducted at Msekera Regional Agricultural Research Station in eastern Zambia to (1) describe canopy branching properties of Acacia angustissima, Gliricidia sepium and Leucaena collinsii in short rotation forests, (2) test the existence of self similarity from repeated iteration of a structural unit in tree canopies, (3) examined intra-specific relationships between functional branching characteristics, and (4) determine whether allometric equations for relating aboveground tree biomass to fractal properties could accurately predict aboveground biomass. Measurements of basal diameter (D10) at 10cm aboveground and total height (H), and aboveground biomass of 27 trees were taken, but only nine trees representative of variability of the stand and the three species were processed for functional branching analyses (FBA) of the shoot systems. For each species, fractal properties of three trees, including fractal dimension (Dfract), bifurcation ratios (p) and proportionality ratios (q) of branching points were assessed. The slope of the linear regression of p on proximal diameter was not significantly different (P < 0.01) from zero and hence the assumption that p is independent of scale, a pre-requisite for use of fractal branching rules to describe a fractal tree canopy, was fulfilled at branching orders with link diameters >1.5 cm. The proportionality ration q for branching patterns of all tree species was constant at all scales. The proportion of q values >0.9 (fq) was 0.8 for all species. Mean fractal dimension (Dfract) values (1.5?1.7) for all species showed that branching patterns had an increasing magnitude of intricacy. Since Dfract values were ≥1.5, branching patterns within species were self similar. Basal diameter (D10), proximal diameter and Dfract described most of variations in aboveground biomass, suggesting that allometric equations for relating aboveground tree biomass to fractal properties could accurately predict aboveground biomass. Thus, assessed Acacia, Gliricidia and Leucaena trees were fractals and their branching properties could be used to describe variability in size and aboveground biomass.  相似文献   
86.
本研究通过磨损试验台研究了铧面角与刃口磨损形式之间的关系,试验结果表明:适当降低硬度,增大铧面角可以减小背棱宽度和刃口圆弧半径,这些是单金属犁铧形成自磨刃的必要条件。  相似文献   
87.
4MZ—2(3)自走式采棉机静液压驱动应用分析   总被引:2,自引:0,他引:2  
通过对4MZ-2(3)自走式采棉机方案特点和变量泵-定量马达的调节性能的分析表明,采用静液压无级变速与机械有级变速组配的驱动方案在满足采棉机采棉作业、运输的要求下,实现了不同范围内的无级变速度,且发动机与静液压驱动装置组配较经济。  相似文献   
88.
The aim of this work was to determine whether juvenile and adult European seabass (Dicentrarchus labrax L.), fed ad libitum manually and with self‐feeding demand feeders, respectively, were able to discriminate between a fish oil‐based diet and a series of diets made with vegetable oils (soybean, linseed, rapeseed and sunflower oil), as well as a fat‐free diet. Adult D. labrax (443.6 ± 108.8 g) selected diets made from soybean (SYO) and rapeseed (RO) in proportions similar to fish oil (FO). Demand levels for the other two diets, sunflower (SUO) and linseed (LO) oils, were significantly lower than FO, whilst juveniles (56.4 ± 11.2 g) displayed a preference for SYO and FO. The lowest demand levels were associated with a fat‐free diet (FF). Selection percentages of the diets for adult and juvenile fish were, respectively, FF—22.96% and 39.76%, SYO—43.02% and 53.26%, LO—36.48% and 36.19%, SUO—39.87% and 37.32%, and RO—46.38% and 40.94%. The animals’ size and feeding method did not significantly affect dietary selection. However, a statistical difference in specific growth rate (1.37% ± 0.26% day?1) for juveniles was only observed for FO+FF combination. For juveniles, no differences were observed in the conversion index. The juvenile group (FO + FF) had the highest food intake (2.00 ± 0.35 g 100 g?1 body weight). Moreover, the groups fed FO + SUO and FO + RO consumed the least. In conclusion, this study shows D. labrax can discriminate between diets where the only difference is the lipid source. These findings should be used to understand the acceptance or rejection of different source oils, which are increasingly being used in aquaculture such as aquafeeds for European seabass.  相似文献   
89.
Nile tilapia (Oreochromis niloticus) is one of the most important cultured species worldwide. The research aims to clarify the feeding characteristics, such as daily feeding pattern of Nile tilapia using a self‐feeding technique. The feeding pattern was conducted under two rearing conditions, indoor treatments under a controlled light regime (LD 12:12) and constant water temperature (25°C), and outdoor treatments under natural conditions which consisted of duplicate trials with two periods each. The outdoor treatment was carried out from early summer through late autumn in Mie, Central Japan. Daily self‐feeding activity of Nile tilapia in indoor treatments was nearly daytime feeding pattern, synchronizing with the given photoperiod (24 hr). However, the self‐feeding activity in the outdoor experiments from early summer to early autumn was almost daytime feeding pattern, but it became less clear and shifted to a nighttime feeding profile in late autumn. The results revealed that Nile tilapia has a dualistic capacity for demand‐feeding both in light and dark phases. These results might have been caused by the seasonal change in light intensity and/or water temperature. Information obtained from the self‐feeding experiments enables us to identify the influence of environmental changes on the physiological condition of farmed fish through their expression of appetite.  相似文献   
90.
利用配体交换反应将巯基修饰的“8-17”脱氧核酶(“8-17”DNAzyme/“8-17”Dz)底物链(17S)通过Au S共价键自组装在纳米金胶(AuNPs)表面,制得一种新型纳米复合物17S AuNPs。利用紫外 可见吸收光谱和动态光散射(dynamic light scattering, DLS)对其进行了表征。紫外 可见吸收光谱表明17S AuNPs在523 nm波长处有表面等离子共振峰,较AuNPs红移了3 nm;经DLS测定的17S AuNPs的流体动力学平均直径为38.6 nm,较 AuNPs的21.8 nm增大了16.8 nm。另外,利用紫外 可见光谱和目视比色法对AuNPs和17S AuNPs的耐盐稳定性分别进行了比较研究,结果表明:随着水溶液中Na+浓度的逐渐提高,AuNPs在600 nm处的吸收峰不断增强,溶液颜色逐渐由红变成蓝紫色;而17S AuNPs在600 nm处并未出现吸收峰,且溶液颜色一直保持为红色;表明17S AuNPs纳米复合物的稳定性较AuNPs显著提高。  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