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   389篇
  免费   17篇
  国内免费   2篇
林业   18篇
农学   4篇
基础科学   3篇
  59篇
综合类   51篇
农作物   14篇
水产渔业   15篇
畜牧兽医   214篇
园艺   20篇
植物保护   10篇
  2022年   2篇
  2021年   3篇
  2020年   10篇
  2019年   3篇
  2018年   5篇
  2017年   5篇
  2016年   8篇
  2015年   8篇
  2014年   12篇
  2013年   15篇
  2012年   18篇
  2011年   28篇
  2010年   22篇
  2009年   22篇
  2008年   12篇
  2007年   20篇
  2006年   20篇
  2005年   13篇
  2004年   10篇
  2003年   8篇
  2002年   9篇
  2001年   6篇
  2000年   15篇
  1999年   16篇
  1997年   3篇
  1996年   4篇
  1995年   5篇
  1994年   3篇
  1993年   4篇
  1992年   8篇
  1991年   6篇
  1990年   9篇
  1989年   4篇
  1988年   10篇
  1987年   2篇
  1986年   6篇
  1985年   3篇
  1984年   7篇
  1983年   3篇
  1981年   6篇
  1978年   2篇
  1977年   2篇
  1961年   2篇
  1960年   2篇
  1955年   2篇
  1954年   2篇
  1953年   2篇
  1947年   2篇
  1906年   1篇
  1905年   1篇
排序方式: 共有408条查询结果,搜索用时 31 毫秒
101.
This report describes the necropsy findings in 3 cats with a natural T-lymphotropic Lentivirus (FTLV) infection. The changes are compared with the "immunodeficiency syndrome" of humans and monkeys. Furthermore, the pathogenicity of the FTLV and the significance of a dual infection with FTLV and Spumavirus is discussed.  相似文献   
102.
103.
104.
105.
106.
107.
By using data collected during a continuous circumnavigation of the Southern Hemisphere, we observed clear patterns in the population-genetic structure of Prochlorococcus, the most abundant photosynthetic organism on Earth, between and within the three Southern Subtropical Gyres. The same mechanisms that were previously invoked to account for the vertical distribution of ecotypes at local scales accounted for the global (horizontal) patterns we observed. Basin-scale and seasonal variations in the structure and strength of vertical stratification provide a basis for understanding large-scale horizontal distribution in genetic and physiological traits of Prochlorococcus, and perhaps of marine microbial communities in general.  相似文献   
108.
Toxic potential of materials at the nanolevel   总被引:12,自引:0,他引:12  
Nel A  Xia T  Mädler L  Li N 《Science (New York, N.Y.)》2006,311(5761):622-627
Nanomaterials are engineered structures with at least one dimension of 100 nanometers or less. These materials are increasingly being used for commercial purposes such as fillers, opacifiers, catalysts, semiconductors, cosmetics, microelectronics, and drug carriers. Materials in this size range may approach the length scale at which some specific physical or chemical interactions with their environment can occur. As a result, their properties differ substantially from those bulk materials of the same composition, allowing them to perform exceptional feats of conductivity, reactivity, and optical sensitivity. Possible undesirable results of these capabilities are harmful interactions with biological systems and the environment, with the potential to generate toxicity. The establishment of principles and test procedures to ensure safe manufacture and use of nanomaterials in the marketplace is urgently required and achievable.  相似文献   
109.
Density estimators for k-tree distance sampling are sensitive to the amount of extra Poisson variance in distances to the kth tree. To lessen this sensitivity, we propose an adaptive composite estimator (COM). In simulated sampling from 16 test populations, a three-component composite density estimator (COM)–with weights determined by a multinomial logistic function of four readily available ancillary variables–was identified as superior in terms of average relative absolute bias. Results from a different set of nine validation populations–with widely different stem densities and spatial patterns of tree locations—confirmed that relative root mean squared errors (RRMSE) of COM were, on average, considerably lower than those obtained with the three-component k-tree density estimators. The RRMSE performance of COM improved with increasing values of k. With k = 6 and sample sizes of 10, 20, and 30, the average relative bias of COM was between −5 and 5% in seven validation populations but in an open low-density savanna-like population bias reached −12% (1979 data) and 7% (1996 data). For k = 6 and n = 10, the RRMSE of COM was, in six of the nine validation populations, within 3.3 percentage points of the RRMSE for sampling with fixed-area plots. Jackknife estimates of the precision of COM estimates of density were negatively biased, leading to under-coverage (7%) of computed 95% confidence intervals.  相似文献   
110.
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