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   3317篇
  免费   143篇
  国内免费   3篇
林业   237篇
农学   470篇
基础科学   87篇
  856篇
综合类   88篇
农作物   277篇
水产渔业   74篇
畜牧兽医   986篇
园艺   100篇
植物保护   288篇
  2023年   23篇
  2022年   38篇
  2021年   50篇
  2020年   87篇
  2019年   87篇
  2018年   122篇
  2017年   138篇
  2016年   109篇
  2015年   83篇
  2014年   99篇
  2013年   300篇
  2012年   142篇
  2011年   158篇
  2010年   127篇
  2009年   115篇
  2008年   114篇
  2007年   138篇
  2006年   143篇
  2005年   112篇
  2004年   72篇
  2003年   79篇
  2002年   73篇
  2001年   57篇
  2000年   51篇
  1999年   56篇
  1998年   42篇
  1997年   43篇
  1996年   35篇
  1995年   34篇
  1994年   29篇
  1993年   28篇
  1992年   31篇
  1991年   36篇
  1990年   31篇
  1989年   38篇
  1988年   45篇
  1987年   34篇
  1986年   29篇
  1985年   24篇
  1984年   25篇
  1983年   26篇
  1982年   22篇
  1980年   34篇
  1979年   37篇
  1978年   20篇
  1976年   20篇
  1971年   20篇
  1970年   37篇
  1969年   24篇
  1968年   23篇
排序方式: 共有3463条查询结果,搜索用时 31 毫秒
991.
Viruses alter the structure and the function of mitochondria for survival. Electron microscopy analysis of the cells infected with bovine adenovirus 3 revealed extensive damage to the inner mitochondrial membrane characterized by dissolution of the cristae and amorphous appearance of mitochondrial matrix with little or no damage to the outer mitochondrial membrane. There were fewer cristae with altered morphology. Potential patches of protein synthesis machinary around mitochondria could be observed at 12 hours post infection (hpi). At 24 hpi, the multi vascular bodies were evident throughout the infected cell. ATP production, mitochondrial Ca2+ and mitochondrial membrane potential (MMP) peaked at 18 hpi but decreased significantly at 24 hpi. This decrease coincided with the increased production of superoxide (SO) and reactive oxygen species (ROS), at 24 hpi indicating acute oxidative stress in the cells and suggesting a complete failure of the cellular homeostatic machinary. The results reveal an intericate relationship between Ca2+ homeostasis, the ATP generation ability of cells, SO and ROS production, and regulation of MMP following infection by bovine adenovirus 3.  相似文献   
992.
993.
Sodium chlorate (NaClO(3)) crystals are optically active although the molecules of the compound are not chiral. When crystallized from an aqueous solution while the solution is not stirred, statistically equal numbers of levo (L) and dextro (D) NaClO(3) crystals were found. When the solution was stirred, however, almost all of the NaClO(3) crystals (99.7 percent) in a particular sample had the same chirality, either levo or dextro. This result represents an experimental demonstration of chiral symmetry breaking or total spontaneous resolution on a macroscopic level brought about by autocatalysis and competition between L- and D-crystals.  相似文献   
994.
995.
Variable x-ray and γ-ray emission is characteristic of the most extreme physical processes in the universe. We present multiwavelength observations of a unique γ-ray-selected transient detected by the Swift satellite, accompanied by bright emission across the electromagnetic spectrum, and whose properties are unlike any previously observed source. We pinpoint the event to the center of a small, star-forming galaxy at redshift z = 0.3534. Its high-energy emission has lasted much longer than any γ-ray burst, whereas its peak luminosity was ~100 times higher than bright active galactic nuclei. The association of the outburst with the center of its host galaxy suggests that this phenomenon has its origin in a rare mechanism involving the massive black hole in the nucleus of that galaxy.  相似文献   
996.
Chromosomal aberrations induced in barley by LSD   总被引:1,自引:0,他引:1  
Seeds of hulled barley (Hordeum vulgare) were germinated and then treated with LSD. Preparations of squashed root tips stained with Feulgen revealed extensive chromosomal aberrations, most of which were chromosome breaks. Nearly half of the breaks occurred in the region of the primary constriction.  相似文献   
997.
On the basis of an analysis of historic worldwide emissions and removal mechanisms for carbon tetrachloride, a possible precursor for stratospheric ozone destruction, it has been demonstrated that the present atmospheric loading and distribution of carbon tetrachloride is primarily attributable to man-made emissions and no natural sources need be invoked to explain its presence in the atmosphere.  相似文献   
998.
Summary Cowpea and mung bean were subjected to three irrigation schedules during summer dry months (May–June). In general, cowpea had higher rates of net photosynthesis (P n ,Figs. 1, 2), dark respiration (R d ,Table 1), absorption of photosynthetically active radiation (PAR, Table 2), and growth (Fig. 7) than mung bean. Mung bean reflected more PAR and maintained a slightly cooler canopy than cowpea (Table 2). Moisture stress decreased P n , R d ,absorption coefficient of PAR, evaporative cooling and growth in both the species. However, the reductions in P n rates of stressed leaves were more than the decreases in R d .Restoration of water supply to stressed cowpea resulted in a more rapid recovery of growth as compared to mung bean.  相似文献   
999.
Application of SALTMOD in Coastal Clay Soil in India   总被引:1,自引:1,他引:1  
SALTMOD is a simulation model whichpredicts root zone soil salinity, drainagewater quality and water table depth inagricultural land under differentgeo-hydrological conditions and varyingwater management scenarios. The model wasapplied to the data from coastal AndhraPradesh of India where subsurface drainagesystem is laid out at several drainspacings at the experimental site. Fielddata for 1999, 2000 and 2001 were collectedfrom 35 and 55 m drain spacing plots forSALTMOD application. Modelling was doneconsidering two simulation approaches. Thefirst approach (Simulation-I) used the sameinitial values for the entire simulationperiod. In the second approach(Simulation-II), the computations wereperformed year-by-year, giving each yearthe current input values obtained from thesimulation results of the previous year.Results of these two approaches weredifferent from each other. Simulation-IIgave better predictions than that ofSimulation-I in terms of closeness to theobserved values. Simulation results ofsoil salinity in the root zone, drainagewater quality and quantity and depth towater table revealed that the salinity ofroot zone was predicted more accuratelythan that of drainage water quality anddepth to water table. Also throughsimulation, it was found that the salinityof drainage water was relativelyindependent of the root zone soil salinity. Model application study suggests thatSALTMOD can be used with confidence toevaluate various drain spacings of asubsurface drainage system and facilitatereasonable prediction of the reclamationperiod.  相似文献   
1000.
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