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   185304篇
  免费   8818篇
  国内免费   918篇
  195040篇
  2023年   357篇
  2022年   805篇
  2021年   1660篇
  2020年   942篇
  2019年   1242篇
  2018年   13045篇
  2017年   11745篇
  2016年   9493篇
  2015年   4448篇
  2014年   4664篇
  2013年   5529篇
  2012年   11169篇
  2011年   19284篇
  2010年   15962篇
  2009年   12093篇
  2008年   15234篇
  2007年   16773篇
  2006年   5729篇
  2005年   5797篇
  2004年   6049篇
  2003年   5911篇
  2002年   5380篇
  2001年   1203篇
  2000年   873篇
  1999年   1022篇
  1998年   1310篇
  1997年   905篇
  1996年   823篇
  1995年   768篇
  1994年   661篇
  1993年   764篇
  1992年   680篇
  1991年   598篇
  1990年   570篇
  1989年   550篇
  1988年   523篇
  1987年   482篇
  1986年   435篇
  1985年   609篇
  1984年   687篇
  1983年   634篇
  1982年   721篇
  1981年   658篇
  1980年   660篇
  1979年   387篇
  1978年   465篇
  1977年   396篇
  1976年   399篇
  1972年   456篇
  1971年   457篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.

Introduction  

Total cholesterol (TC) and blood pressure (BP) are likely to take a dynamic course over time in patients with systemic lupus erythematosus (SLE). This would have important implications in terms of using single-point-in-time measurements of these variables to assess coronary artery disease (CAD) risk. The objective of this study was to describe and quantify variability over time of TC and BP among patients with SLE and to determine their correlates.  相似文献   
42.
Plants, when exposed to abiotic or biotic stress, produce several pathogenesis-related proteins to counteract the effects of stress. Osmotin is one of the important pathogenesis-related proteins induced during several stress conditions. We have developed improved salt stress tolerant transgenic chilli pepper plants (Capsicum annum L. var. Aiswarya 2103) by ectopic expression of the Nicotiana tabaccum osmotin gene using Agrobacterium tumefaciens EHA105 as a vector. Four-week-old chilli pepper leaves were used as an explant and A. tumefaciens EHA105 harboring pBINASCOSM plasmid that contains osmotin gene under the control of CaMV 35S promoter and npt II as a selectable marker was used in co-cultivation. Transgene integration and expression were analyzed using molecular, immunochemical, and biochemical assays. PCR and Southern blot analysis confirmed that osmotin gene has been successfully integrated into the genome of chilli pepper plants. The osmotin gene was stably segregated and expressed in T2 generation transgenic chilli pepper plants, and it was confirmed by Western blot analysis. Biochemical assays of these putative transgenic plants revealed enhanced levels of chlorophyll, proline, glycinebetaine, APX, SOD, DHAR, MDHAR, GR, and relative water content. Yield potential of the putative transgenic chilli pepper plants was evaluated under salinity stress conditions in a green house. The putative transgenic chilli pepper plants overexpressing the osmotin gene were morphologically similar to wild-type plants and produced 3.32 kg chilli pepper fruits per plant at 300 mM NaCl concentration.  相似文献   
43.
Connective tissue growth factor (CTGF/CCN2) is overexpressed in diabetes. Diabetic rats possess myocardial and cardiomyocyte hypertrophy. In a recent report, Wang and colleagues (Am J Physiol Cell Physiol. 2009 Jul 22. [Epub ahead of print]) show that CCN2 directly mediates cardiomyocyte hypertrophy as well as that induced by high glucose and fatty acid. CCN2 acted via the TrkA receptor. These data are the subject of this commentary, and emphasize that CCN2 may be an excellent target for therapy in diabetes.  相似文献   
44.
The ability of an invasive plant to occupy new areas is often attributed to both morphological and physiological plasticities that allow them to remain viable over a wide range of environmental conditions. Studies addressing the ecological requirements of Microstegium vimineum often consider soil moisture or soil moisture along with other factors as important explanatory components for the establishment and persistence of this invasive monocot. However, controlled studies specifically targeting water relations in M. vimineum are needed. Therefore, the purpose of this study was to determine how different water availabilities influence the growth and physiological performance of M. vimineum. This study utilized experimental microcosms to achieve different water availabilities including low soil moisture (<15% water), moderate soil moisture (ca. 20–30%), and flooded conditions. While both flooded and low soil moisture resulted in diminished growth, M. vimineum still survived under these conditions. Physiological processes including C4 metabolism, minimum stress under low water conditions, and the ability to increase tissue rigidity may confer some advantages to M. vimineum during periods of limiting water conditions. Similarly, the proportionally low root biomass, shallow root structure, and its ability to maintain stable water relations during flooding and/or soil waterlogging may facilitate M. vimineum’s ability to invade mesic habitats. It is likely, therefore, that the capacity to tolerate both low soil moistures and flooded conditions has enhanced the ability of M. vimineum to populate disturbed systems in central North Carolina.  相似文献   
45.
46.
47.
48.
49.
50.
A new study of divergence in freshwater fish provides strong evidence of rapid, temperature-mediated adaptation. This study is particularly important in the ongoing debate over the extent and significance of evolutionary response to climate change because divergence has occurred in relatively few generations in spite of ongoing gene flow and in the aftermath of a significant genetic bottleneck, factors that have previously been considered obstacles to evolution. Climate change may thus be more likely to foster contemporary evolutionary responses than has been anticipated, and I argue here for the importance of investigating their possible occurrence.  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