全文获取类型
收费全文 | 16909篇 |
免费 | 1754篇 |
国内免费 | 3篇 |
出版年
2022年 | 99篇 |
2021年 | 195篇 |
2020年 | 198篇 |
2019年 | 276篇 |
2018年 | 326篇 |
2017年 | 316篇 |
2016年 | 419篇 |
2015年 | 692篇 |
2014年 | 706篇 |
2013年 | 1008篇 |
2012年 | 1008篇 |
2011年 | 948篇 |
2010年 | 746篇 |
2009年 | 637篇 |
2008年 | 801篇 |
2007年 | 834篇 |
2006年 | 825篇 |
2005年 | 827篇 |
2004年 | 799篇 |
2003年 | 854篇 |
2002年 | 858篇 |
2001年 | 189篇 |
2000年 | 160篇 |
1999年 | 245篇 |
1998年 | 272篇 |
1997年 | 184篇 |
1996年 | 197篇 |
1995年 | 198篇 |
1994年 | 178篇 |
1993年 | 144篇 |
1992年 | 181篇 |
1991年 | 137篇 |
1990年 | 137篇 |
1989年 | 137篇 |
1988年 | 112篇 |
1987年 | 131篇 |
1986年 | 114篇 |
1985年 | 140篇 |
1984年 | 162篇 |
1983年 | 119篇 |
1982年 | 166篇 |
1981年 | 160篇 |
1980年 | 148篇 |
1979年 | 132篇 |
1978年 | 129篇 |
1977年 | 114篇 |
1976年 | 116篇 |
1975年 | 83篇 |
1974年 | 103篇 |
1973年 | 96篇 |
排序方式: 共有10000条查询结果,搜索用时 343 毫秒
151.
C-Glycosylation of 5,7-dihydroxy-3′,4′,5′-trimethoxyflavone was carried out with acetobromo-α-d-glucose, -α-d-galactose, -α-d-xylose, -β-l-arabinose and -αt-L-rhamnose. The respective 6-C-glycosides and 6, 8-di-C-glycosides (excepted for galactose) were isolated and permethylated. MS and TLC comparison confirmed the proposed structures of five natural tricetin-derived C-glycosides. 相似文献
152.
Ginette Pauly Abdelhamid Yani Louis Piovetti Colette Bernard-Dagan 《Phytochemistry》1983,22(4):957-959
The essential oils of the leaves of Cupressus dupreziana and Cupressus sempervirens were compared. The composition of the hydrocarbon fraction showed a great similarity between the two species. 相似文献
153.
6-C-α-l-Arabinopyranosyl- and furanosylacacetins have been synthesized. They are isomerized by short acid treatment to give a mixture of the four anomer/ring size combinations without any Wessely-Moser isomerization. In the same conditions molludistin (8-C-α-l-arabinopyranosylgenkwanin) led only to a mixture of molludistin and 8-C-α-l-arabinofuranosylgenkwanin. This is the first demonstration of ring sugar isomerization in C-glycosylflavones. In usual solvent systems, α-anomers are easily separated from β-anomers, whereas corresponding pyranosyl and furanosyl anomers are not. However, they are easily separated after permethylation and characteristic features are found in the mass spectra of PM 6-C-arabinofuranosyl isomers. 相似文献
154.
Hydrolysis of sphingomyelin and 2-N-(hexade-canoyl)-amino-4-nitrophenyl-phosphorylcholine (HDA-PC), a synthetic analogue of sphingomyelin, by acid and Mg-dependent neutral sphingomyelinases was tested with a homogenate of normal human brain cortex. Results demonstrated quite different substrate specificities for these enzymes. Acid sphingomyelinase, which is neither activated by MgCl2 nor inhibited by EDTA, hydrolyzed both substrates (the hydrolysis ratio of HDA-PC to sphingomyelin is ?2). In contrast, Mg-dependent neutral sphingomyelinase, which is inhibited by EDTA and reactivated by MgCl2, hydrolyzed only sphingomyelin (the hydrolysis ratio of HDA-PC to sphingomyelin is ?0-0.05). This synthetic substrate seems to be useful for selective determination of acid sphingomyelinase and for avoiding interference of Mg-dependent neutral sphingomyelinase. 相似文献
155.
Tn7, a transposon of 14 kb, encodes resistance to trimethoprim (Tp) and streptomycin (Sm). A cleavage site map of this transposon for twenty-two different restriction enzymes as determined by comparison of restriction enzyme cleavage patterns of the plasmids ColE1 and ColE1::Tn7 is presented. The precise localization of these sites was facilitated by the use of two deletion derivatives of ColE1::Tn7: pGB2 and ColE1::Tn7Δ6, and by the use of pOB14 and pOB15 which contain a part of Tn7 cloned into the plasmid pBR322. This map should aid in the study of the structural and genetic organization of this transposon. 相似文献
156.
157.
A study of the chromosomes of Cavia aperea and Galea musteloides has been made following the introduction into captivity of these two cavies from Argentina. The evolutionary relationships of the two genera have been considered, and the possible ancestry of C. porcellus from C. aperea was investigated in hybrids of the two species. 相似文献
158.
159.
Taxonomic Implications of Spore Fine Structure in Clostridium bifermentans 总被引:3,自引:1,他引:2
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Thirty-five strains of Clostridium bifermentans were, in most part, culturally homogeneous by conventional taxonomic criteria but were heterogeneous with respect to spore fine structure. Fourteen of the strains produced spores with appendages, distributed among four distinct ultrastructural types. No consistent correlation existed between spore type and other variable properties of these strains. It is proposed, therefore, that these spore appendage-type strains be considered as "varieties" of C. bifermentans and that they should not be designated as new species. 相似文献
160.