全文获取类型
收费全文 | 54573篇 |
免费 | 4438篇 |
国内免费 | 3485篇 |
专业分类
62496篇 |
出版年
2024年 | 124篇 |
2023年 | 595篇 |
2022年 | 1415篇 |
2021年 | 2332篇 |
2020年 | 1571篇 |
2019年 | 1998篇 |
2018年 | 2110篇 |
2017年 | 1748篇 |
2016年 | 2310篇 |
2015年 | 2751篇 |
2014年 | 3488篇 |
2013年 | 3779篇 |
2012年 | 4417篇 |
2011年 | 4177篇 |
2010年 | 2873篇 |
2009年 | 2552篇 |
2008年 | 3020篇 |
2007年 | 2785篇 |
2006年 | 2448篇 |
2005年 | 2110篇 |
2004年 | 1980篇 |
2003年 | 1910篇 |
2002年 | 1587篇 |
2001年 | 1064篇 |
2000年 | 878篇 |
1999年 | 702篇 |
1998年 | 496篇 |
1997年 | 400篇 |
1996年 | 387篇 |
1995年 | 387篇 |
1994年 | 367篇 |
1993年 | 291篇 |
1992年 | 340篇 |
1991年 | 310篇 |
1990年 | 249篇 |
1989年 | 233篇 |
1988年 | 168篇 |
1987年 | 232篇 |
1986年 | 178篇 |
1985年 | 172篇 |
1984年 | 158篇 |
1983年 | 144篇 |
1982年 | 161篇 |
1981年 | 128篇 |
1980年 | 104篇 |
1979年 | 100篇 |
1978年 | 92篇 |
1977年 | 87篇 |
1976年 | 70篇 |
1973年 | 75篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
62.
63.
The possibility of the use of small amounts of chitosan for defatting and clarification of protein solutions prepared by enzymatic hydrolysis was tested. The following treatment conditions were shown to be optimal: a chitosan concentration range, from 1.0 to 1.5 gram per kilogram raw weight; pH of the precipitation medium from 8.0 to 8.5; and duration of the incubation of the protein hydrolysate solution with chitosan, less than 1 h. The hydrolysate defatting grade was found to depend on the degree of chitosan deacetylation. A possible mechanism of the chitosan-induced effects was suggested. The use of chitosan allows the mass fraction of enzyme protein hydrolysates to be reduced fourfold to fivefold. 相似文献
64.
65.
The 13C NMR spectra of geosmin, selina-4(14),7(11)-diene-99-ol and two dihydroeudesmol isomers have been obtained and the individual resonances assigned. Several different empirical correlations developed by others have been combined in simple calculations to predict chemical shift values for sesquiterpenols of the eudesmane group. 相似文献
66.
67.
Aiyi Liu James F. Troendle Kai F. Yu Vivian W. Yuan 《Biometrical journal. Biometrische Zeitschrift》2004,46(6):760-768
We consider estimation after a group sequential test. An estimator that is unbiased or has small bias may have substantial conditional bias (Troendle and Yu, 1999, Coburger and Wassmer, 2001). In this paper we derive the conditional maximum likelihood estimators of both the primary parameter and a secondary parameter, and investigate their properties within a conditional inference framework. The method applies to both the usual and adaptive group sequential test designs. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
68.
Gametic chromosome numbers are reported for 27 collections representing the four species of the Lobelia tupa complex (Campanulaceae, Lobelioideae) in Chile; all are n = 21. This represents the first report of chromosome numbers for L. bridgesii Hook. & Arn., L. excelsa Bonpl., and L. polyphylla Hook. & Arn., and confirms previous reports of this number in L. tupa L. As the basic chromosome number of Lobelioideae is x = 7, these species are interpreted as hexaploids. Higher polyploids are extremely rare among Lobelioideae; most of those previously reported have been either sporadic individuals or populations within an otherwise diploid or tetraploid species, or occasional species within an otherwise diploid and tetraploid lineage. This is the first report of an entire complex of lobelioid species that is uniformly hexaploid. This suggests that the Chilean endemics are relatively derived within Lobelia, and offers some support for the monophyly of the complex. 相似文献
69.
70.
Characterization and in Vivo Cloning of prlC, a Suppressor of Signal Sequence Mutations in Escherichia coli K12 总被引:19,自引:1,他引:18 下载免费PDF全文
The prlC gene of E. coli was originally identified as an allele, prlC1, which suppresses certain signal sequence mutations in the genes for several exported proteins. We have isolated six new alleles of prlC that also confer this phenotype. These mutations can be placed into three classes based on the degree to which they suppress the lamB signal sequence deletion, lamBs78. Genetic mapping reveals that the physical location of the mutations in prlC correlates with the strength of the suppression, suggesting that different regions of the gene can be altered to yield a suppressor phenotype. We also describe an in vivo cloning procedure using lambda placMu9H. The procedure relies on transposition and illegitimate recombination to generate a specialized transducing phage that carries prlC1. This method should be applicable to any gene for which there is a mutant phenotype. 相似文献