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   2761篇
  免费   226篇
  国内免费   3篇
  2990篇
  2023年   10篇
  2022年   20篇
  2021年   55篇
  2020年   30篇
  2019年   35篇
  2018年   38篇
  2017年   35篇
  2016年   73篇
  2015年   107篇
  2014年   120篇
  2013年   159篇
  2012年   256篇
  2011年   206篇
  2010年   141篇
  2009年   132篇
  2008年   192篇
  2007年   175篇
  2006年   194篇
  2005年   152篇
  2004年   195篇
  2003年   146篇
  2002年   137篇
  2001年   32篇
  2000年   14篇
  1999年   33篇
  1998年   34篇
  1997年   28篇
  1996年   17篇
  1995年   22篇
  1994年   18篇
  1993年   29篇
  1992年   17篇
  1991年   11篇
  1990年   17篇
  1989年   17篇
  1988年   11篇
  1987年   10篇
  1986年   13篇
  1985年   13篇
  1984年   5篇
  1983年   7篇
  1982年   11篇
  1981年   5篇
  1980年   4篇
  1979年   4篇
  1978年   1篇
  1976年   4篇
  1974年   2篇
  1969年   1篇
  1964年   1篇
排序方式: 共有2990条查询结果,搜索用时 15 毫秒
1.
2.
3.
Fort Greely, Alaska has an extensive complex of weapon training and testing areas located on lands withdrawn from the public domain under the Military Lands Withdrawal Act (PL106-65). The Army has pledged to implement a program to identify possible munitions contamination. Because of the large size (344,165,000 m2) of the high hazard impact areas, characterization of these constituents will be difficult. We used an authoritative sampling design to find locations most likely to contain explosives-residues on three impact areas. We focused our sampling on surface soils and collected multi-increment and discrete samples at locations of known firing events and from areas on the range that had craters, pieces of munitions, targets, or a designation as a firing point. In the two impact areas used primarily by the Army, RDX was the most frequently detected explosive. In the impact area that was also used by the Air Force, TNT was the most frequently detected explosive. Where detected, the explosives concentrations generally were low (<0.05 mg/kg) except in soils near low-order detonations, where the explosive-filler was in contact with the soil surface. These low-order detonations potentially can serve as localized sources for groundwater contamination if positioned in recharge areas.  相似文献   
4.

Background  

Populations of the Oriental White-backed Vulture (Gyps bengalensis) have declined by over 95% within the past decade. This decline is largely due to incidental consumption of the non-steroidal anti-inflammatory veterinary pharmaceutical diclofenac, commonly used to treat domestic livestock. The conservation status of other Gyps vultures in southern Asia is also of immediate concern, given the lack of knowledge regarding status of their populations and the continuing existence of taxonomic uncertainties. In this study, we assess phylogenetic relationships for all recognized species and the majority of subspecies within the genus Gyps. The continuing veterinary use of diclofenac is an unknown but potential risk to related species with similar feeding habits to Gyps bengalensis. Therefore, an accurate assessment of the phylogenetic relationships among Gyps vultures should aid in their conservation by clarifying taxonomic uncertainties, and enabling inference of their respective relatedness to susceptible G. bengalensis.  相似文献   
5.
6.
Five allozyme polymorphisms were used to analyze the mating system in a Sitka spruce seed orchard in Saanichton, British Columbia. Allelic frequencies differed between the pollen and maternal pools at three of the five loci, with alleles rare in the maternal pool being even rarer in the effective pollen pool. Minor differences in pollen allelic frequencies were observed in the upper vs. lower crown. The multilocus outcrossing rate of the upper crown (tm = 0.909) exceeded that of the lower crown (tm = 0.764). Single-locus estimates of the outcrossing rate were significantly heterogeneous, with the lowest estimate of outcrossing, t = 0.773, observed for PGM-2 locus. Analyses of the mating system for the three maternal PGM-2 genotypes revealed heterogeneous pollen allelic frequencies and heterogeneous outcrossing rates, possibly due to assortative mating at this locus.  相似文献   
7.
A cDNA encoding a soluble sialidase from Chinese hamster ovary(CHO) cells has been cloned and expressed. Completely degenerateoligonucleotide primers, which were based on the amino acidsequence of peptides obtained from the purified sialidase (Warneret al., Glycobiology, 3, 455–463, 1993), and the polymerasechain reaction, with single-stranded cDNA template, were employedto generate a unique oligonucleotide probe. The unique probeof 93 bp was used for screening a  相似文献   
8.
Erratum     
  相似文献   
9.
Strong and regulated promoters in the cyanobacterium Anabaena PCC 7120   总被引:8,自引:0,他引:8  
Abstract The strengths of several promoters were assessed in the cyanobacterium Anabaena PCC 7120 by fusing them to luxAB , encoding bacterial luciferase. Two promoters, P tac and P psbA , with sequences nearly identical to consensus Escherichia coli σ 70 promoters, gave as high or higher expression than the strong Anabaena promoter, P rbc . P npt , the natural promoter driving expression of the kanamycin-resistance determinant from Tn5, was poorly expressed in Anabaena . The Lac repressor partially repressed expression from P tac , permitting regulated expression in Anabaena after induction with isopropyl thiogalactoside to a level 4–5-fold higher than without inducer.  相似文献   
10.
A Soluble sialidase that can degrade recombinant glycoproteinsexpressed in Chinese hamster ovary (CHO) cells has been isolatedand purified to near homogeneity from the cell culture fluidof this host. Purification of  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