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   11178篇
  免费   458篇
  国内免费   1412篇
林业   821篇
农学   1133篇
基础科学   700篇
  1464篇
综合类   4938篇
农作物   717篇
水产渔业   510篇
畜牧兽医   1501篇
园艺   690篇
植物保护   574篇
  2024年   44篇
  2023年   125篇
  2022年   264篇
  2021年   308篇
  2020年   368篇
  2019年   502篇
  2018年   422篇
  2017年   495篇
  2016年   612篇
  2015年   483篇
  2014年   534篇
  2013年   599篇
  2012年   680篇
  2011年   688篇
  2010年   655篇
  2009年   633篇
  2008年   569篇
  2007年   535篇
  2006年   518篇
  2005年   380篇
  2004年   304篇
  2003年   268篇
  2002年   225篇
  2001年   182篇
  2000年   202篇
  1999年   184篇
  1998年   162篇
  1997年   239篇
  1996年   223篇
  1995年   206篇
  1994年   229篇
  1993年   225篇
  1992年   270篇
  1991年   272篇
  1990年   226篇
  1989年   86篇
  1988年   43篇
  1987年   25篇
  1986年   13篇
  1985年   7篇
  1984年   8篇
  1982年   4篇
  1981年   4篇
  1980年   2篇
  1979年   5篇
  1978年   4篇
  1977年   2篇
  1956年   5篇
  1955年   5篇
  1944年   1篇
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
991.
Summary

Carbon (C) and nitrogen (N) metabolisms are two major metabolic pathways which have been intensively studied in plants. Both N and C metabolisms are tightly linked in numerous plant biochemical pathways. Therefore the C to N ratio has to be carefully regulated to ensure proper functioning of the huge metabolic network. In order to maintain a viable C/N status under a large range of growth conditions plants have evolved complex mechanisms to regulate the delicate network of these two major assimilatory pathways. C and N metabolisms are both highly regulated. We will present the current knowledge on the regulation of N and C metabolisms by sugars and N metabolites. Players involved in these regulatory processes are just starting to be uncovered and possible signaling molecules involved in these regulations as well as known or potential candidate regulatory genes willbe discussed.  相似文献   
992.
Objective The aims of this work were to (1) develop a low-cost equine movement tracking collar based on readily available components, (2) conduct preliminary studies assessing the effects of both paddock size and internal fence design on the movements of domestic horses, with and without foals at foot, and (3) describe distances moved by mares and their foals. Additional monitoring of free-ranging feral horses was conducted to allow preliminary comparisons with the movement of confined domestic horses. Procedures A lightweight global positioning system (GPS) data logger modified from a personal/vehicle tracker and mounted on a collar was used to monitor the movement of domestic horses in a range of paddock sizes and internal fence designs for 6.5-day periods. Results In the paddocks used (0.8–16 ha), groups of domestic horses exhibited a logarithmic response in mean daily distance travelled as a function of increasing paddock size, tending asymptotically towards approximately 7.5 km/day. The distance moved by newborn foals was similar to their dams, with total distance travelled also dependent on paddock size. Without altering available paddock area, paddock design, with the exception of a spiral design, did not significantly affect mean daily distance travelled. Feral horses (17.9 km/day) travelled substantially greater mean daily distances than domestic horses (7.2 km/day in 16-ha paddock), even when allowing for larger paddock size. Conclusions Horses kept in stables or small yards and paddocks are quite sedentary in comparison with their feral relatives. For a given paddock area, most designs did not significantly affect mean daily distance travelled.  相似文献   
993.
野生和人工栽培缬草香精油的GC/MS分析对比研究   总被引:4,自引:2,他引:2  
应用GC/MS/DS分析对比研究野生和人工栽培香料油,分别鉴定出34个和28个化合物,两种香精油其相组分21个。占总精油组分的91%-96%,主成分基本相同,占精油总量的78%-85%,其成分上的差异对其在香料工业、日化、食品、饮料、医药工业上的应用没有影响,为缬草开发利用建立家种缬草原料基地,保证缬草香料油工业化生产提供依据。  相似文献   
994.
In this work, we investigated the nutritional status of Ruditapes decussatus juveniles under different rearing conditions, using biochemical indices (RNA/DNA ratio and protein content) and the linear instantaneous growth rate (IGR). Two experiments were conducted to study the effect of starvation, addition of substrate (sand) as rearing support and diet composition on somatic growth and biochemical indices. Results show that biochemical indices of fed juveniles were significantly (P<0.05) higher. Highest RNA/DNA values and protein content were recorded in fed juveniles reared with substrate (P<0.05); conversely, lowest values were recorded in starved ones reared without substrate. In the second experiment, the highest RNA/DNA ratio was recorded in juveniles fed with control algal mixture composed of species commonly used in bivalve nurseries [Isochrysis galbana (clone T-Iso), Chaetoceros calcitrans and Tetraselmis suecica] and lowest in those fed with algal species isolated from Tunisian coasts. This confirms the usefulness of the RNA/DNA ratio and protein content in the clam's nutritional status evaluation. This assay is useful to provide information about the nutritional status and health of the clam in field conditions.  相似文献   
995.
文内介绍了DHF65—39/450型沸腾锅炉安装中主要部件的吊装办法及程序。对同类或近似的锅炉安装具有一定的参考价值。  相似文献   
996.
畜牧项目是海南州农业综合开发项目的主要建设项目之一 ,通过四年的实施 ,已完成计划投资的 1 0 3 .63 %。监测表明 ,项目实施经济效益明显 ,群众经营观念和经营方式有所改变 ,人均收入由项目前的 5 94.43元提高到 1 0 76元 ,增长 81 .1 3 % ;草地生态环境得到初步改善。  相似文献   
997.
二进制是计算机中所采用的数据表示形式,在文章中介绍了基于VRML与JAVA的虚拟动态交互技术,分析了利用Java或JavaScript语言创建出新的内插节点和传感节点来驱动和控制整个虚拟空间的方法,研究了利用VRML和Java的技术去设计动态交互的场景,实现了二进制与十进制之间的动态交互显示,最后指出该系统已被成功运用于教学,文章的创新点在于利用PRO/E优化建模。  相似文献   
998.
就农用飞机作业的航线规划和边界算法提出了一些改进措施,以增强导航系统的性能,有效提高飞机作业的效率。  相似文献   
999.
A retrospective study of the use of intraoperative mesenteric portovenography (IOMP) in the surgical management of congenital portosystemic shunts in 100 dogs and cats was performed. Each portovenogram was scored using a subjective visual analogue scale (VAS) and was assessed for the presence of portal atresia or hypoplasia. VAS scores and portal hypoplasia assessments were obtained for portovenogram images obtained for each animal both before shunt manipulation (preligation) and following the temporary, complete ligation of the vessel (postligation). In each patient, surgical records were reviewed to ascertain the degree of shunt attenuation that was achieved at surgery. Hepatic portal vasculature was significantly different on postligation compared with preligation IOMP. Sixty-two percent of animals had apparent portal hypoplasia or atresia on their preligation IOMP. The majority of these (81%) had an improvement in portal vasculature on postligation IOMP. It was concluded that both preligation and postligation IOMP provided valuable information regarding the morphology of congenital portosystemic shunts. An accurate assessment of an animal's portal vasculature could only be made following the interpretation of a postligation portovenogram, and these findings significantly influenced the surgical management of the patient. Although individuals with high postligation VAS scores were more likely to achieve full shunt attenuation at surgery, there was no quantifiable relationship between IOMP findings and the degree of shunt attenuation achieved.  相似文献   
1000.
GPS技术在林业工作中的应用   总被引:4,自引:0,他引:4  
GPS在林业系统中的应用广泛,具有定位、导航、求积三大功能。GPS求积功能的开发,为三类作业设计调查带来了一种技术上的突破。相对于以往的森林罗盘仪测量有了质的飞跃。  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