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   406篇
  免费   18篇
  国内免费   11篇
林业   174篇
农学   16篇
基础科学   2篇
  51篇
综合类   142篇
农作物   2篇
水产渔业   2篇
畜牧兽医   20篇
园艺   3篇
植物保护   23篇
  2023年   3篇
  2022年   11篇
  2021年   12篇
  2020年   20篇
  2019年   12篇
  2018年   6篇
  2017年   18篇
  2016年   17篇
  2015年   22篇
  2014年   24篇
  2013年   27篇
  2012年   34篇
  2011年   25篇
  2010年   31篇
  2009年   18篇
  2008年   27篇
  2007年   25篇
  2006年   17篇
  2005年   8篇
  2004年   9篇
  2003年   8篇
  2002年   8篇
  2001年   11篇
  2000年   2篇
  1999年   9篇
  1998年   7篇
  1997年   4篇
  1996年   1篇
  1995年   5篇
  1994年   5篇
  1993年   2篇
  1992年   1篇
  1991年   1篇
  1990年   4篇
  1989年   1篇
排序方式: 共有435条查询结果,搜索用时 15 毫秒
101.
红叶腺柳引种驯化研究   总被引:1,自引:0,他引:1  
对红叶腺柳在11个引种试验地苗期生长量、苗木越冬、苗木顶端叶片呈色、物候特征及病虫害进行了研究,结果表明:辽宁沈阳、山西临汾、河南郑州、河南鄢陵、河南淅川、甘肃白银、新疆库车、宁夏灵武为红叶腺柳适宜引种栽培区,甘肃山丹马场、山西忻州、辽宁朝阳为不适宜引种地区。随着纬度的降低和海拔的降低,即平均气温越高、无霜期越长,芽开始膨胀时间、展叶期就越早,而高生长停止、主梢封顶期、落叶期到来的越晚;红叶腺柳1a生苗期在一些试验点有病虫害发生。  相似文献   
102.
Survival, growth, plant structure and insect damage of in vitro‐derived cuttings from two Salix clones (5. schwerinii and 5. viminalis) were compared in a field trial with performance of conventional cuttings of the same clones. No obvious morphological abnormalities were induced by in vitro treatment. The survival rate of in vitro‐derived cuttings of the S. schwerinii clone was much higher than that of conventional cuttings. In the 5. viminalis clone, height growth was slightly but significantly reduced for in vitro‐derived cuttings compared with conventional cuttings although stem basal area and estimated plant dry weight were unaffected. The number of stems per cutting, the number of side shoots per stem and gall midge damage did not appear to be affected by the type of cutting. Electrophoretic analysis of 12 enzyme systems revealed identical isozyme patterns for m vitro‐derived and conventionally propagated plants.  相似文献   
103.
杨柳科植物的分类与分布   总被引:14,自引:5,他引:9  
杨柳科包含2个亚科、5个属、约500种;杨亚科由杨属和胡杨属组成,柳亚科由原柳属、钻天柳属和柳属组成;该科广泛自然分布天大约从北纬82度至南纬52度的非洲、欧洲、亚洲、南美洲和北美洲。杨属约50种,分布于欧洲、亚洲、北美洲和非洲北缘;胡杨属约3种,间断分布于热带非洲、古地中海地区和墨西哥;原柳属约50种,广泛分布于非洲、欧洲、亚洲、南美洲和北美洲;钻天柳属约4种,局限分布于鄂霍次克海和日本海四周地  相似文献   
104.
2009年在若尔盖县针对川西北地区恢复治理难度最大的高寒流动沙地进行配套试验,通过成活率、出露地面高度、萌生枝条长度等多项试验指标的测定和统计分析,提出沙障+营养土法+网格状播种披碱草的治理方案,有助于推动高寒流动沙地恢复治理的科研及实践相关工作。  相似文献   
105.
Several studies in human subjects have demonstrated greater diuresis with constant rate infusion (CRI) furosemide than intermittent bolus (IB) furosemide. This study was conducted to compare the diuretic efficacy of the same total dose of IB furosemide and CRI furosemide in 6 healthy, adult Greyhound dogs in a randomized crossover design with a 2-week washout period between treatments. For IB administration, dogs received 3 mg/kg at 0 and 4 hours. For CRI administration, dogs received a 0.66 mg/kg loading dose followed by 0.66 mg/kg/h over 8 hours. The same volume of fluid was administered for both methods. Urine output was quantified hourly. Urine electrolyte concentrations, urine specific gravity (USG), packed cell volume (PCV), total protein (TP), serum electrolyte concentrations, total carbon dioxide (TCO2), serum creatinine (sCr), and blood urea nitrogen (BUN) were determined every 2 hours. Urine production and water intake were greater (P < or = 0.05) for CRI than IB. Urine sodium and calcium losses were greater (P < 0.05) and urine potassium loss was less (P = 0.03) for CRI than IB, but there was no evidence of a difference between methods for urine magnesium and chloride losses. Serum chloride concentration was less (P < 0.001), sCr concentration greater (P = 0.04). TP greater (P = 0.01), and PCV greater (P = 0.003) for CRI than IB. No differences in USG, TCO2, BUN, or serum potassium, sodium, and magnesium concentrations were detected between methods. The same total dose of CRI furosemide resulted in more diuresis, natriuresis, and calciuresis and less kaliuresis than IB furosemide in these normal Greyhound dogs over 8 hours, suggesting that furosemide is a more effective diuretic when administered by CRI than by IB.  相似文献   
106.
The effects of temperature on the growth characteristics of a Japanese willow (Salix gilgiana Seemen) were studied with respect to dry matter production, wood anatomy and the net photosynthetic rates of leaves. Clonal materials were grown in natural-light rooms (phytotrons) at 30°C/25°C, at 25°C/20°C or at 20°C/15°C (day/night: 12 h/12 h), respectively. The elongation rate of shoots from the beginning of June to the end of July was approximately 22 cm per week under the three different sets of conditions. The shoots reached approximately 3 m in height after 4 months of growth. Total dry matter production and dry weights of stems did not differ very much among the three sets of temperature conditions after 9 months of growth. The dry weight of leaves increased at high temperatures while that of roots decreased. The number of vessels increased significantly at 30°C/25°C and 25°C/20°C, whereas the diameters of vessels and fibers, and the double wall thickness of fibers did not vary markedly under all three temperature regimes. The net photosynthetic rate and the respiration rate in mature leaves were highest at 30°C/25°C, and both activities increased with increases in growth temperature. The increase in the photosynthetic rate was much smaller than that in the respiration rate. Individual plants had been completely adapted to the respective growth temperatures, as judged from the decrease in the photosynthetic rate after the transfer to different temperatures. Part of this work was reported at the 102nd Annual Meeting of the Japanese Forestry Society (Nagoya, Japan, 1991). This work was supported by Grants-in-Aid (Integrated Research Program for Effective Use of Biological Activities to Create New Demand) from the Ministry of Agriculture, forestry and Fisheries of Japan (BRP 97-IV-B-10).  相似文献   
107.
垂柳瘿叶蜂Pontaniasp.在沈阳严重为害柳树。该虫在沈阳1年发生1代,以老熟幼虫于表土中结茧越冬。成虫于5月上旬开始出现;幼虫生活于无毛、闪现着红色近卵形或近豌豆形虫瘿中;虫瘿表面具瘤状颗粒,着生于叶背中脉上。已知其寄生天敌2种,即啮小蜂Tetrostichussp.和沈阳宽唇姬蜂LathrostizusshenyangensisXuetSheng。以2.5%溴氰菊酯(敌杀死)乳油3000倍液于5月上旬至中旬喷洒成虫效果很好。  相似文献   
108.
徐莹梅  田胜尼  周疆丽 《安徽农业科学》2012,40(4):2225-2227,2277
对安庆月山铜尾矿废弃地定居的旱柳群落结构进行了野外调查和分析。调查发现,该调查区域内自然定居的高等植物24种,隶属于13科24属。木本植物中以旱柳为主,草本植物中以白茅、芦苇和水蜡烛为主;群落中木本植物中旱柳的重要值为2.34,远高于其他定居的木本植物,其为铜尾矿木本植物群落的建群种和优势种,该群落为旱柳群落。该群落的多样性较低,其Simpson指数为0.615 3,Shannon-Wiener指数为2.557 0,均匀度E为0.193 6。结果表明,铜尾矿废弃地定居的旱柳群落结构简单,群落多样性指数低,属于草本植物群落向木本植物群落演替的初期阶段。  相似文献   
109.
测定人工气候室中培养的旱柳当年生枝条和完全展开叶片的光合色素含量,并应用蔗糖密度梯度离心法提取枝条皮层和完全展开叶片叶绿体,测定其完整性,比较分析二者光响应特性和叶绿素荧光特性的差异。结果表明:旱柳皮层叶绿素含量约是叶片的10.0%,类胡萝卜素含量约是叶片的13.7%,叶绿素 a/b 比值小于叶片,叶绿素/类胡萝卜素比值大于叶片;应用蔗糖密度梯度离心法可获得被膜完整率超过80%的离体叶绿体,能够满足叶绿体光合生理特性研究的需要;100,200,300,500,800μmol·m -2 s -1光强下,旱柳皮层叶绿体的光合放氧速率均低于叶片,光饱和点出现在300μmol·m -2 s -1,最大光合放氧速率为35.58μmolO2·mg -1 Chl h -1,而叶片的光饱和点在500μmol·m -2 s -1,最大光合放氧速率为42.78μmolO2·mg -1 Chl h -1;皮层叶绿体的 PSⅡ最大光化学效率( Fv/Fm )平均高出叶片30.2%,PSⅡ潜在光化学活性(Fv/Fo)平均高出叶片36.1%。旱柳皮层叶绿体的光合速率和光饱和点低于叶片,皮层叶绿体表现出较高的光能转化效率、较低的叶绿素 a/b比值等适应枝条内光环境的特点。  相似文献   
110.
基于应力波和阻抗仪技术的旱柳内部腐朽定量检测   总被引:1,自引:0,他引:1  
利用应力波和阻抗仪两种仪器对旱柳行道树内部腐朽情况进行检测,应力波法每个断面采用10~12个传感器,阻抗仪法每个断面测试2~4个方向。结果表明:由应力波断层扫描得到的二维图像和由阻抗仪得到的阻力曲线均能较准确地反映活立木内部腐朽的位置、程度等状况,两者对应关系较好;阻抗仪每个测试方向的平均阻力与相应方向应力波传播速度之间呈现一定相关性,相关系数 r ﹥0.5;每个测试断面上的整体平均阻力与应力波平均传播速度之间有显著的相关性,相关系数 r =0.817。这为联合两种仪器的检测结果制定一个表征立木腐朽程度的量化指标奠定了基础。  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