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   10468篇
  免费   430篇
  国内免费   1368篇
林业   646篇
农学   1111篇
基础科学   582篇
  1373篇
综合类   4654篇
农作物   701篇
水产渔业   497篇
畜牧兽医   1452篇
园艺   684篇
植物保护   566篇
  2024年   41篇
  2023年   123篇
  2022年   250篇
  2021年   304篇
  2020年   363篇
  2019年   488篇
  2018年   414篇
  2017年   487篇
  2016年   596篇
  2015年   470篇
  2014年   514篇
  2013年   580篇
  2012年   647篇
  2011年   638篇
  2010年   596篇
  2009年   561篇
  2008年   508篇
  2007年   483篇
  2006年   431篇
  2005年   322篇
  2004年   270篇
  2003年   232篇
  2002年   198篇
  2001年   173篇
  2000年   181篇
  1999年   169篇
  1998年   155篇
  1997年   238篇
  1996年   221篇
  1995年   199篇
  1994年   225篇
  1993年   223篇
  1992年   267篇
  1991年   263篇
  1990年   222篇
  1989年   86篇
  1988年   40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 31 毫秒
991.
Takes the motorcycle system design as the target , and researched motorcycle total scheme design and total layout design and kinetics matching optimization to design, KBE's technique , CAX's technique , PDM's technique and to target and so on technique is used in the synthesis , and intelligent CAD/CAE's system of the motorcycle system design work distance traveled by a stream of water and the architectural structure and integrated technique of every application sub system meritorous service capacity composition and CAD/CAE/CAT has been underway research, Open up a model system , and still unite the fund motorcycle produce development , and verifies.  相似文献   
992.
初花期源库比变化对棉花下部"铃-叶系统"生理特征的影响   总被引:11,自引:2,他引:11  
对棉花初花期进行去叶和去蕾处理,研究改变棉花源库比对早期"铃-叶"系统生理状态的影响.结果表明,源库比改变明显地影响着棉叶和棉铃中蛋白质、可溶性糖和淀粉的含量,但蛋白质、可溶性糖、淀粉的含量对源库比变化的反应程度不同.减库处理可以提高叶片中蛋白质、可溶性糖和淀粉的含量,促进子棉中碳水化合物的积累,提高单铃重,并促进后期果枝叶中叶绿素的合成,延长叶片功能期,但是不利于棉子中蛋白质的合成;减源处理促进前期果枝叶中叶绿素含量,但是使后期果枝叶中叶绿素含量快速下降,表现出早衰迹象;并且减源处理不利于叶片中蛋白质的合成.  相似文献   
993.
A class of cubic parameter curve of B-spline type is constructed in this paper with four arbitrary parameters, the cubic Hermite curve, Ball curve, Bezier curve, Timmer curve and B-spline curve as its special cases. The condition in which the curves contact with each edge of characteristic polygon is also given. In order to satisfy the actual riquirements,the shape(i. e. location of end point,direction of convixity and flexion,etc. )of curves can be modified by adjusting the four parameters.  相似文献   
994.
香菜挥发油GC/MS测定   总被引:11,自引:0,他引:11  
对香菜茎叶及根经水蒸汽蒸馏--萃取得到的挥发油进行气相色谱/质谱(GC/MS)分析.其中香菜茎叶中主要成分有:壬烷、癸烷、癸醛、十一醛、环癸烷、十二醛、2-十一烯醛、2-癸烯-1-醇、十三醛、十四醛等,香菜根挥发油中主要成分有:壬烷、辛醛、癸醛、环癸烷、十-醛、十二醛、十四醛等.  相似文献   
995.
In this paper, based on Hill's general variational principle(l958-1959), a variational equation in intitial configuration ( reference configuration) and current configuration ( reference configuration ) is derived using convetced coordinates.The derived variational equation is for large elastoplastic, double nonlinear rate problem and is suited for finite element caculation.As an applied example of the derived variational equation, necking of a plane strain ploymer sheet with a initial geometric inperfection ( initial neck ) is analysed using the finite formulation based on the derived variational equation. Work hardening of materials is considered.The derived variational equation in this paper is suitable for analysing large elastoplastic deformation problem including metal forming processes.  相似文献   
996.
The paper presented a new micomechanics load-transfer model for single-fibre fragmentation test. The interfacial shear streength and the crack will develop along the interface. The fibre stochastic strength distribution was modelled by a two-parameter Wibull distribution. The influence of the interfacial bonding strength and fibre axial-tensile strength on stress states in components of composites was discussed. The interfacial state during the fragmentation process may be one of following three states: 1) fully bonding; 2) partial bonding; 3) fully debonding; according to the fibre axialtensile strength and interfacial properties (including interfacial bounding shear strength, interfacial friction coefficient and matrix radial pressure) after elastic constants and geometrical parameters of composites phases are given. The necessary conditions for these three states were discussed- Finally, a suitable definition of the critical fibre length was given and the critical length was constrouted by two parts :debonding fibre length and bonding fibre length.  相似文献   
997.
In this paper an attempt has been practised to introduce parallel techniques into the structural optimization.Based on space time transform five approaches for parallel computation of structural optimization have been presented.This paper has also pointed out the problems originated from large scale systems of structural optimization (LSSO) and put forward the technical line of design of LSSO.  相似文献   
998.
Summary Freshly cut leaves of Kohleria eriantha and K. x Longwood were exposed to a wide range of gamma irradiation doses and allowed to root and form adventitious buds. K. Eriantha could not be successfully propagated from leaf half cuttings. Longwood produced a small number of adventitious plantlets as compared to other Gesneriads. Colchicine treatments reduced leaf half survival in Longwood by more than 50%. Leaf halves exposed to low and moderate doses of gamma irradiation showed increased overall plantlet production compared to nonirradiated leaf halves.Of the mutation parameters calculated, the number of mutants per 100 surviving leaf halves appears to be the most useful since it relates the number of mutants within a dose to the number of surviving leaf halves, the number of mutants to the number of plants produced, and the number of mutants within doses to control values.Using the criteria, number of mutants per 100 surviving leaf halves, optimum production of all mutants, of useful and of dwarf mutants was obtained at 2.5 kR for noncolchicine treated leaf halves and 1.5 to 3.0 kR for colchicine treated leaf halves.Of the adventitious plantlets produced, 13.8% were classified as mutant types. Mutant plants were found in radiation dose levels up to 5.5 kR. The array of mutants produced was skewed toward plant habit and flower characteristic mutants with several leaf characteristic, lethal, and polyploid mutants observed. A high frequency of mutants, 10 of 93 plants, occurred in the nonirradiated controls. Four sectorial chimeras were identified from both control and irradiation exposed groups. Several potentially useful flower color and dwarf mutants have been selected for further study. Adventitious buds may have developed from callus at petiole and leaf-vein bases. Colchicine treatments severely inhibited adventitious plantlet production and because of this could not be used as a criterion to identify the origin of adventitious buds. Conclusive determination of the organogenesis of adventitious buds from Longwood leaf halves could not be made. Mutant plantlets formed from adventitious buds on detached leaf halves of Longwood appear to arise from single cells as has been found with other Gesneriads.Scientific Journal Series Article No. 10 330 of the Minnesota Agricultural Experiment Station.  相似文献   
999.
It is difficult to establish precise mathematical model in extended Kalman filter data fusion algorithm for GPS/ DR integrated vehicle navigation system, an improved extended Kalman filter data fusion algorithm is put forward based on Discrete stationary wavelet transformation. The simulation results show that the algorithm is better than the extended Kalman filter data fusion algorithm, and is available for GPS/DR integrated vehicle navigation system.  相似文献   
1000.
"米糠油防治储粮害虫研究"表明,毛米糠油是一种对储粮害虫有强引诱力的植物性诱杀剂,用毛米糠油防治储粮害虫,方法简便易行,成本低廉,不污染粮食,不影响粮食的加工和经济价值,不产生公害,特别是消除了害虫的抗药性.它不仅适用于大规模储粮的国库,亦适宜于城乡千家万户的小批量储粮.  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