全文获取类型
收费全文 | 1365篇 |
免费 | 67篇 |
国内免费 | 86篇 |
专业分类
林业 | 144篇 |
农学 | 116篇 |
基础科学 | 232篇 |
149篇 | |
综合类 | 631篇 |
农作物 | 58篇 |
水产渔业 | 24篇 |
畜牧兽医 | 90篇 |
园艺 | 24篇 |
植物保护 | 50篇 |
出版年
2024年 | 4篇 |
2023年 | 19篇 |
2022年 | 31篇 |
2021年 | 26篇 |
2020年 | 30篇 |
2019年 | 40篇 |
2018年 | 15篇 |
2017年 | 52篇 |
2016年 | 60篇 |
2015年 | 50篇 |
2014年 | 54篇 |
2013年 | 89篇 |
2012年 | 89篇 |
2011年 | 88篇 |
2010年 | 81篇 |
2009年 | 87篇 |
2008年 | 62篇 |
2007年 | 83篇 |
2006年 | 87篇 |
2005年 | 56篇 |
2004年 | 60篇 |
2003年 | 52篇 |
2002年 | 42篇 |
2001年 | 65篇 |
2000年 | 33篇 |
1999年 | 28篇 |
1998年 | 17篇 |
1997年 | 24篇 |
1996年 | 15篇 |
1995年 | 15篇 |
1994年 | 19篇 |
1993年 | 12篇 |
1992年 | 5篇 |
1991年 | 10篇 |
1990年 | 6篇 |
1989年 | 7篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1962年 | 1篇 |
排序方式: 共有1518条查询结果,搜索用时 218 毫秒
71.
介绍了一种求解旅行商问题的新算法“最小调整法”,给出了该算法求解旅行商问题的具体步骤以及有效性证明,对算法的复杂性及近似程度进行了分析.最后通过典型算例进行了检验说明.与经典算法相比,新算法体现了简单易行的特点,对求解旅行商问题具有一定的启发意义. 相似文献
72.
产业结构优化理论综述及新进展 总被引:1,自引:0,他引:1
产业结构优化在经济学界是一个长久不衰的热点问题。经济发展到一定阶段,必然伴随着产业结构的调整和优化。在简要回顾了有关产业结构优化理论文献的同时,对近年来国内外在产业结构优化研究方面的若干新进展加以梳理和评述。 相似文献
73.
河北滨海平原区种植业结构调整探析 总被引:2,自引:1,他引:1
以1985-2010年统计数据为基础,运用定量与定性相结合的分析方法,对河北滨海平原区1985-2010年种植结构调整过程进行了初步研究,明确了该区不同时期粮食作物、油料作物、棉花、蔬菜、水果等作物的结构和变化趋势,分析了该区种植业结构调整的内在原因及动力,并对河北滨海平原区未来种植结构调整的方向和重点进行了讨论. 相似文献
74.
梨树在自然水分亏缺下的生理反应 总被引:11,自引:0,他引:11
1994~1996年对梨树在自然水分亏缺和水分充足条件下的主要水分生理参数、气孔相对开张度(RSOD)、蒸腾速率(Tr)和光合速率(Pn)进行了比较研究。结果表明,梨树属于低水势耐旱类型,在自然水分亏缺下,其叶水势(ψw)显著降低,膨压(ψp)明显增高,而RSOD、Tr和Pn则无显著变化。梨树具有明显的渗透调节和一定的弹性调节作用,在中度水分亏缺下,其渗透调节能力△π100或△π0分别为0.44MPa和0.77MPa,弹性调节能力△|β|和△εmax分别为2.78和3.19。另外,在自然水分亏缺下,梨树叶片还具有较强的保水能力,反映在基本无变化的膨压消失点的相对含水量(R0)、叶表面水分散失率(WL0)和明显高于CK的一定ψw下的相对含水量(R)值以及无灌溉园较高的εmax或|β|等方面。 相似文献
75.
加入WTO与中国农业政策调整 总被引:7,自引:0,他引:7
李岳云 《南京农业大学学报》2001,24(1):99-102
首先揭示了中国现行农业政策的主要特征,然后以WTO《农业协议》为参照,指出中国农业政策在市场准入,国内支持及卫生检疫措施等方面的相悖之处,最后就新时期中国农业政策调理 提出如下主要建议:1.农业政策的首要目标应从保证供应转向提高农民收入;2.农业政策的主要手段应从边境措施转向国内支持;3.国内支持应从价格支持力向收入支持;4.政策应加大对农业的支持力度。 相似文献
76.
农业结构调整是我国当前农村工作的一项重要任务,分析了农业结构调整中资金投入存在的问题、原因,并从金融体制改革、政府宏观调控、农户与乡镇企业积累方面,提出了解决农业结构调整资金投入的对策。 相似文献
77.
78.
Summary To get an improved impression of the genotypic value of individual plants the literature suggests to adjust their phenotypic values according to a moving mean correction or by means of an analysis of covariance, where the mean phenotypic value calculated across neighbours is used as the concomitant variable. In the present paper it is shown that the theoretical merits of these adjustments are promising: they eliminate the contribution due to the trend in soil fertility from the phenotypic value. In an experimental verification actual merits did, however, not show up.It is concluded that interplant competition is the main cause for the failure of the studied adjustments. These may be effective in virtual absence of intergenotypic competition. Thus, in case of single plant selection an extremely low plant density would be required, whereas selection among lines or families would require evaluation by means of multi-row plots. 相似文献
79.
Summary Diverse landraces of wheat, collected from the semi-arid (150 to 250 mm of total annual rainfall) Northern Negev desert in Israel were considered as a potential genetic resource of drought resistance for wheat breeding. These materials were therefore evaluated for their reponses to drought stress in agronomical and physiological terms. Up to 68 landraces, comprising of Triticum durum, T. aestivum, and T. compactum were tested in two field drought environments, in one favourable field environment, under post-anthesis chemical plant desiccation which revealed the capacity for grain filling from mobilized stem reserves, under a controlled drought stress in a rainout shelter and in the growth chamber under polyethylene glycol (PEG)-induced water stress. Biomass, grain yield and its components, harvest index, plant phenology, canopy temperatures, kernel weight loss by chemical plant desiccation, growth reduction by PEG-induced drought stress and osmotic adjustment were evaluated in the various experiments.Landraces varied significantly for all parameters of drought response as measured in the different experiments, which was in accordance to their documented large morphological diversity. Variation in grain yield among landraces under an increasing drought stress after tillering was largely affected by spike number per unit area. Kernel weight contributed very little to yield variation among landraces under stress, probably because these tall (average of 131 cm) landraces generally excelled in their capacity to support kernel growth by stem reserve mobilization under stress. Yield under stress was reduced with a longer growth duration of landraces only under early planting but not under late planting. Landraces were generally late flowering but they were still considered well adapted phenologically to their native region where they were always planted late.Landraces differed significantly in canopy temperature under drought stress. Canopy temperature under stress in the rainout shelter was negatively correlated across landraces with grain yield (r=0.67**) and biomass (r=0.64**) under stress. Canopy temperature under stress in the rainout shelter was also positively correlated across landraces (r=0.50**) with canopy temperature in one stress field environment. Osmotic adjustment in PEG-stressed plants was negatively correlated (r=–0.60**) with percent growth reduction by PEG-induced water stress. It was not correlated with yield under stress in any of the experiments. In terms of yield under stress, canopy temperatures and stem reserve utilization for grain filling, the most drought resistant landrace was the Juljuli population of T.durum. 相似文献
80.