全文获取类型
收费全文 | 16101篇 |
免费 | 1632篇 |
国内免费 | 5篇 |
专业分类
17738篇 |
出版年
2022年 | 114篇 |
2021年 | 212篇 |
2020年 | 176篇 |
2019年 | 270篇 |
2018年 | 338篇 |
2017年 | 351篇 |
2016年 | 475篇 |
2015年 | 727篇 |
2014年 | 737篇 |
2013年 | 992篇 |
2012年 | 1024篇 |
2011年 | 952篇 |
2010年 | 750篇 |
2009年 | 652篇 |
2008年 | 784篇 |
2007年 | 823篇 |
2006年 | 765篇 |
2005年 | 801篇 |
2004年 | 750篇 |
2003年 | 759篇 |
2002年 | 769篇 |
2001年 | 228篇 |
2000年 | 165篇 |
1999年 | 214篇 |
1998年 | 241篇 |
1997年 | 166篇 |
1996年 | 172篇 |
1995年 | 172篇 |
1994年 | 149篇 |
1993年 | 115篇 |
1992年 | 168篇 |
1991年 | 123篇 |
1990年 | 128篇 |
1989年 | 128篇 |
1988年 | 98篇 |
1987年 | 123篇 |
1986年 | 97篇 |
1985年 | 117篇 |
1984年 | 126篇 |
1983年 | 104篇 |
1982年 | 125篇 |
1981年 | 136篇 |
1980年 | 104篇 |
1979年 | 108篇 |
1978年 | 104篇 |
1977年 | 101篇 |
1976年 | 105篇 |
1975年 | 80篇 |
1974年 | 92篇 |
1973年 | 85篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
Widespread mortality of the black sea urchin Diadema antillarum occurred in the Caribbean in 1983; beginning in Panama in January, and having its major impact at Barbados in September. Mortality on ten reefs surveyed in Barbados was 93.2%, with the highest being 99.9% and the lowest 86.9%. Mortality on each reef was independent of the pre-mortality density on the reef. Urchins with test diameters between 20 and 40 mm were more severely affected than smaller or larger urchins. Populations on reefs exposed to incoming oceanic water suffered heavier mortality than those on protected reefs. Mean size of urchins was smallest on high density reefs. This may indicate a negative effect of density on urchin growth. At post-mortality densities, urchins may grow faster and reach sexual maturity sooner. 相似文献
72.
Labuctril 25 (3,5-dibromo-4-hydroxybenzonitrile, LAB) at concentrations up to 100μg/mL inhibits effectively growth, morphology, and pigmentation of most soil streptomycete isolates grown under laboratory conditions. Oxytril CM (OXT), Basagran (BAS) and Faneron 50 WP (FAN) applied at the same concentrations had no detectable effect on growth of substrate mycelium but suppressed both aerial mycelium and pigment formation, the effectivity decreasing in the order OXT—BAS—FAN. The LAB-sensitivity of mutant strains was markedly higher as compared with that of the soil isolates. A wild strain resistant to 100–400μg of LAB per mL (depending on the medium composition) was isolated. It was capable of supporting the growth and development of sensitive strains on the LAB-containing medium. A stimulatory effect of low doses of LAB (10–20μg/mL) on the antibiotic activity of streptomycetes was observed. 相似文献
73.
74.
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
75.
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
76.
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
77.
78.
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
79.
80.
Isolation and Characterization of Rifampin-Resistant and Streptolydigin-Resistant Mutants of Bacillus subtilis with Altered Sporulation Properties 总被引:48,自引:36,他引:12 下载免费PDF全文
Abraham L. Sonenshein Brigitte Cami Jean Brevet Richard Cote 《Journal of bacteriology》1974,120(1):253-265
Mutants of Bacillus subtilis with altered deoxyribonucleic-dependent ribonucleic acid polymerase activity have been isolated and characterized. These mutants, selected as strains resistant to rifampin or streptolydigin, demonstrate drug-resistant in vitro ribonucleic acid synthesis. Sporeforming ability and support of phage infection are altered in many of the mutants. Mutations to rifampin and streptolydigin resistance have been located on the B. subtilis chromosome and ordered relative to the markers cysA14 and str. 相似文献