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   138002篇
  免费   4571篇
  国内免费   7170篇
  2024年   106篇
  2023年   607篇
  2022年   1256篇
  2021年   2395篇
  2020年   1812篇
  2019年   2242篇
  2018年   13451篇
  2017年   11900篇
  2016年   9351篇
  2015年   3768篇
  2014年   4086篇
  2013年   4215篇
  2012年   8898篇
  2011年   16790篇
  2010年   14467篇
  2009年   10545篇
  2008年   12367篇
  2007年   13671篇
  2006年   2486篇
  2005年   2307篇
  2004年   2427篇
  2003年   2371篇
  2002年   1908篇
  2001年   996篇
  2000年   808篇
  1999年   619篇
  1998年   411篇
  1997年   379篇
  1996年   322篇
  1995年   264篇
  1994年   277篇
  1993年   206篇
  1992年   223篇
  1991年   210篇
  1990年   126篇
  1989年   127篇
  1988年   108篇
  1987年   84篇
  1986年   78篇
  1985年   91篇
  1984年   46篇
  1983年   56篇
  1982年   42篇
  1981年   36篇
  1980年   20篇
  1979年   27篇
  1975年   21篇
  1972年   259篇
  1971年   283篇
  1962年   26篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
151.
152.
153.
154.

Background  

Arsenic contamination is widespread throughout the world and this toxic metalloid is known to cause cancers of organs such as liver, kidney, skin, and lung in human. In spite of a recent surge in arsenic related studies, we are still far from a comprehensive understanding of arsenic uptake, detoxification, and sequestration in plants. Crambe abyssinica, commonly known as 'abyssinian mustard', is a non-food, high biomass oil seed crop that is naturally tolerant to heavy metals. Moreover, it accumulates significantly higher levels of arsenic as compared to other species of the Brassicaceae family. Thus, C. abyssinica has great potential to be utilized as an ideal inedible crop for phytoremediation of heavy metals and metalloids. However, the mechanism of arsenic metabolism in higher plants, including C. abyssinica, remains elusive.  相似文献   
155.

Background  

Meiotic prophase is a critical stage in sexual reproduction. Aberrant chromosome recombination during this stage is a leading cause of human miscarriages and birth defects. However, due to the experimental intractability of mammalian gonads, only a very limited number of meiotic genes have been characterized. Here we aim to identify novel meiotic genes important in human reproduction through computational mining of cross-species and cross-sex time-series expression data from budding yeast, mouse postnatal testis, mouse embryonic ovary, and human fetal ovary.  相似文献   
156.

Background  

Genome sequencing projects have expanded the gap between the amount of known protein sequences and structures. The limitations of current high resolution structure determination methods make it unlikely that this gap will disappear in the near future. Small angle X-ray scattering (SAXS) is an established low resolution method for routinely determining the structure of proteins in solution. The purpose of this study is to develop a method for the efficient calculation of accurate SAXS curves from coarse-grained protein models. Such a method can for example be used to construct a likelihood function, which is paramount for structure determination based on statistical inference.  相似文献   
157.
As compared to cutaneous leishmaniasis, vaccination against visceral leishmaniasis (VL) has received limited attention. In this study, we demonstrate for the first time that an UDP-Galactose: N-acetylglucosamine β 1–4 galactosyltransferase (GenBank Accession No. EF159943) expressing attenuated LD clonal population (A-LD) is able to confer protection against the experimental challenge with the virulent LD AG83 parasite. A-LD was also effective in established leishmania infection. The vaccinated animals showed both cell mediated (in vitro T-cell proliferation, and DTH response) and humoral responses (Th1 type). These results demonstrate the potential of the attenuated clones as an immunotherapeutic and immunoprophylactic agent against visceral leishmaniasis.  相似文献   
158.
159.
Method for production and regeneration of Lactobacillus delbrueckii protoplasts are described. The protoplasts were obtained by treatment with a mixture of lysozyme and mutanolysin in protoplast buffer at pH 6.5 with different osmotic stabilizers. The protoplasts were regenerated on deMan, Rogosa and Sharpe (MRS) with various osmotic stabilizers. Maximum protoplast formation was obtained in protoplast buffer with sucrose as an osmotic stabilizer using a combination of lysozyme (1 mg/ml) and mutanolysin (10 μg/ml). Maximum protoplast regeneration was obtained on MRS medium with sucrose (0.5 M) as an osmotic stabilizer. The regeneration medium was also applicable to other species of lactobacilli as well. This is, to our knowledge, the first report on protoplast formation and efficient regeneration in case of L. delbrueckii.  相似文献   
160.
It is generally perceived that landscape patterns or textures in a given protected area are spatially stationary. The findings of this study suggest that this common perception is only partially correct. Over the course of 52 years, equilibrium in landscape shifting was detected using digital data for the Lassen Volcanic National Park (USA). Vertical aerial photographs taken of the park in 1941 were geo-referenced with the digital orthophoto quarter quadrangle (DOQQ) images of the same area from 1993 to identify landscape compositions and to measure change. Spatial analysis was used to observe pattern changes over time. The results suggested that landscape development maintained equilibrium while patches were in various stages of a successional sequence. The total area of each landscape component held steady, although over time patches throughout the landscape changed—a shifting-mosaic steady state (SMSS). These findings reflect the limitations of contemporary environmental conservation theory. They also suggest the importance of considering landscape change in policies that currently govern park planning and management.  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