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   1279篇
  免费   77篇
  2023年   6篇
  2022年   8篇
  2021年   28篇
  2020年   15篇
  2019年   23篇
  2018年   36篇
  2017年   27篇
  2016年   51篇
  2015年   45篇
  2014年   58篇
  2013年   69篇
  2012年   112篇
  2011年   109篇
  2010年   70篇
  2009年   56篇
  2008年   89篇
  2007年   72篇
  2006年   92篇
  2005年   66篇
  2004年   47篇
  2003年   47篇
  2002年   38篇
  2001年   12篇
  2000年   7篇
  1999年   11篇
  1998年   11篇
  1997年   10篇
  1996年   10篇
  1995年   3篇
  1994年   11篇
  1993年   8篇
  1992年   9篇
  1991年   11篇
  1990年   4篇
  1989年   10篇
  1988年   4篇
  1987年   6篇
  1986年   4篇
  1985年   3篇
  1984年   4篇
  1982年   3篇
  1979年   4篇
  1978年   5篇
  1975年   3篇
  1974年   7篇
  1967年   4篇
  1965年   4篇
  1963年   4篇
  1958年   5篇
  1957年   3篇
排序方式: 共有1356条查询结果,搜索用时 15 毫秒
11.
For the first time a complete ostracode and trilobite fauna is described from the Dobrovitá Formation (Ordovician) of Perunica. In contrast to the trilobite fauna the ostracode fauna evidences close relations to both Armorica and Baltica. The trilobite fauna comprises 11 species out of 11 genera, the ostracode fauna comprises 15 (5 new) species out of 15 (1 new) genera.  相似文献   
12.
Gram-positive facultative methylotrophic coryneform bacterium Brevibacterium methylicum was efficiently transformed with various plasmids using electroporation of intact cells. In addition to the plasmid vectors pEC71 and pZ6-1 constructed on the basis of cryptic plasmids from coryneform bacteria, broad-host-range plasmids pLS5 (derivative of plasmid pMV158 from Streptococcus agalactiae) and RSF1010 belonging to the incompatibility group IncQ from Gram-negative bacteria were found to be present as autonomous structurally unchanged DNA molecules in B. methylicum transformants. With the exception of pZ6-1, all these plasmids were stably maintained in B. methylicum cells grown under non-selective conditions. When plasmid DNAs isolated from B. methylicum were used, the highest efficiency of transformation (105 transformants/g DNA) was achieved. Correspondence to: J. Nevera  相似文献   
13.
Relationships between soil chemistry and population chemotype structure of Thymus pulegioides have been studied. The analysis of correlations suggest that an increased carbonate content in soil decreases the chemotype diversity of a population (as calculated by use of the Shannon index): the proportion of linalool chemotype plants rises and that of the phenol chemotype plants declines. In addition, the chemotype diversity decreases with increasing frequency of linalool chemotype, and increases with increase of carvacrol chemotype.  相似文献   
14.
A new covalently-bonded, silica-based stationary phase, using as the chiral selector the 1-(3-aminopropyl) derivative of (+)-(5R,8S,10R)-terguride, has been developed to resolve optically active isomers by HPLC. Good resolution of structurally related racemic ergot alkaloids were obtained using water-methanol mixtures as the eluent. Analysis of the influence of the type and concentration of the organic modifier, and the pH of the buffer in the mobile phase allowed the enantioseparation of these compounds to be optimized. Determination of the optical purity of a lisuride-containig drug is reported. © 1994 Wiley-Liss, Inc.  相似文献   
15.
Intrarectal inoculation of rhesus monkeys with low doses of SIVmac led to a prolonged clinical and virological latency that was not observed for high intrarectal doses or for intravenous inoculation. Animals infected intrarectally with low virus doses remained negative for serum antibody responses to SIV for at least one year even though they readily transferred SIV to naive recipients via transfusion of whole blood.  相似文献   
16.
17.
18.
Radial tree growth is sensitive to environmental conditions, making observed growth increments an important indicator of climate change effects on forest growth. However, unprecedented climate variability could lead to non-stationarity, that is, a decoupling of tree growth responses from climate over time, potentially inducing biases in climate reconstructions and forest growth projections. Little is known about whether and to what extent environmental conditions, species, and model type and resolution affect the occurrence and magnitude of non-stationarity. To systematically assess potential drivers of non-stationarity, we compiled tree-ring width chronologies of two conifer species, Picea abies and Pinus sylvestris, distributed across cold, dry, and mixed climates. We analyzed 147 sites across the Europe including the distribution margins of these species as well as moderate sites. We calibrated four numerical models (linear vs. non-linear, daily vs. monthly resolution) to simulate growth chronologies based on temperature and soil moisture data. Climate–growth models were tested in independent verification periods to quantify their non-stationarity, which was assessed based on bootstrapped transfer function stability tests. The degree of non-stationarity varied between species, site climatic conditions, and models. Chronologies of P. sylvestris showed stronger non-stationarity compared with Picea abies stands with a high degree of stationarity. Sites with mixed climatic signals were most affected by non-stationarity compared with sites sampled at cold and dry species distribution margins. Moreover, linear models with daily resolution exhibited greater non-stationarity compared with monthly-resolved non-linear models. We conclude that non-stationarity in climate–growth responses is a multifactorial phenomenon driven by the interaction of site climatic conditions, tree species, and methodological features of the modeling approach. Given the existence of multiple drivers and the frequent occurrence of non-stationarity, we recommend that temporal non-stationarity rather than stationarity should be considered as the baseline model of climate–growth response for temperate forests.  相似文献   
19.
In a world of accelerating changes in environmental conditions driving tree growth, tradeoffs between tree growth rate and longevity could curtail the abundance of large old trees (LOTs), with potentially dire consequences for biodiversity and carbon storage. However, the influence of tree-level tradeoffs on forest structure at landscape scales will also depend on disturbances, which shape tree size and age distribution, and on whether LOTs can benefit from improved growing conditions due to climate warming. We analyzed temporal and spatial variation in radial growth patterns from ~5000 Norway spruce (Picea abies [L.] H. Karst) live and dead trees from the Western Carpathian primary spruce forest stands. We applied mixed-linear modeling to quantify the importance of LOT growth histories and stand dynamics (i.e., competition and disturbance factors) on lifespan. Finally, we assessed regional synchronization in radial growth variability over the 20th century, and modeled the effects of stand dynamics and climate on LOTs recent growth trends. Tree age varied considerably among forest stands, implying an important role of disturbance as an age constraint. Slow juvenile growth and longer period of suppressed growth prolonged tree lifespan, while increasing disturbance severity and shorter time since last disturbance decreased it. The highest age was not achieved only by trees with continuous slow growth, but those with slow juvenile growth followed by subsequent growth releases. Growth trend analysis demonstrated an increase in absolute growth rates in response to climate warming, with late summer temperatures driving the recent growth trend. Contrary to our expectation that LOTs would eventually exhibit declining growth rates, the oldest LOTs (>400 years) continuously increase growth throughout their lives, indicating a high phenotypic plasticity of LOTs for increasing biomass, and a strong carbon sink role of primary spruce forests under rising temperatures, intensifying droughts, and increasing bark beetle outbreaks.  相似文献   
20.
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