全文获取类型
收费全文 | 659980篇 |
免费 | 80072篇 |
国内免费 | 339篇 |
专业分类
740391篇 |
出版年
2018年 | 5526篇 |
2016年 | 7226篇 |
2015年 | 9948篇 |
2014年 | 11704篇 |
2013年 | 16669篇 |
2012年 | 18673篇 |
2011年 | 19142篇 |
2010年 | 12821篇 |
2009年 | 12128篇 |
2008年 | 17013篇 |
2007年 | 17557篇 |
2006年 | 16651篇 |
2005年 | 16065篇 |
2004年 | 15642篇 |
2003年 | 15403篇 |
2002年 | 14969篇 |
2001年 | 33237篇 |
2000年 | 33620篇 |
1999年 | 26238篇 |
1998年 | 8575篇 |
1997年 | 9309篇 |
1996年 | 8753篇 |
1995年 | 8149篇 |
1994年 | 7984篇 |
1993年 | 8164篇 |
1992年 | 21626篇 |
1991年 | 21423篇 |
1990年 | 20471篇 |
1989年 | 20183篇 |
1988年 | 18624篇 |
1987年 | 17672篇 |
1986年 | 16412篇 |
1985年 | 16315篇 |
1984年 | 13265篇 |
1983年 | 11547篇 |
1982年 | 8634篇 |
1981年 | 7669篇 |
1980年 | 7391篇 |
1979年 | 12519篇 |
1978年 | 9900篇 |
1977年 | 8973篇 |
1976年 | 8232篇 |
1975年 | 9253篇 |
1974年 | 9598篇 |
1973年 | 9505篇 |
1972年 | 8530篇 |
1971年 | 7876篇 |
1970年 | 6731篇 |
1969年 | 6559篇 |
1968年 | 5821篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
961.
962.
Reconstitution of RecBC DNase activity from purified Escherichia coli RecB and RecC proteins 总被引:8,自引:0,他引:8
I D Hickson C N Robson K E Atkinson L Hutton P T Emmerson 《The Journal of biological chemistry》1985,260(2):1224-1229
The Escherichia coli RecB protein, normally synthesized in low amounts, has been amplified by linkage of the recB gene to the phage lambda leftward promoter in an expression plasmid. From strains harboring this plasmid, RecB protein has been purified to homogeneity by a simple procedure which includes affinity chromatography on a column of RecC protein bound to agarose. The purified RecB protein has DNA-dependent ATPase activity but no exonuclease activity. RecC protein alone has neither ATPase nor exonuclease activity. However, when combined together, the RecB and RecC proteins show the ATP-dependent double-stranded exonuclease properties characteristic of the RecBC DNase. 相似文献
963.
D. Stoyan 《Biometrical journal. Biometrische Zeitschrift》1985,27(4):411-425
This paper presents methods for the stereological analysis of spatial fibre systems on the base of planar or thin sections. Under the assumption that the cross-section figures of the tubular fibres can be measured, the orientation distribution of the fibre system and its line density Lv can be determined from one section only and without distributional assumptions. A simple way to study the degree of randomness of fibre systems consists in the statistical analysis of the point pattern of centres of intersection figures. More sophisticated methods are of stereological nature and yield the spatial reduced second moment measure. Similarly also correlations between two fibre systems can be quantified. The methods are demonstrated by two examples concerning samples of human brain. 相似文献
964.
965.
966.
G. Pellegrino D. Caimi M. E. Noce A. Musacchio 《Plant Systematics and Evolution》2005,251(2-4):119-129
The rewardless orchid Dactylorhiza sambucina shows a stable flower colour polymorphism, with both yellow- and red-flowered morphs growing sympatrically. Pollination biology and breeding system were investigated to examine the effects of density of plants, colour polymorphism, inflorescence dimension, and flower position within inflorescence on male and female reproductive success in three natural populations of D. sambucina. There were significant differences among sites in the number of pollinia removed and in fruit set per inflorescence. Number of removed pollinia and capsule production in D. sambucina were independent from flower and inflorescence size or flower position. As a whole, the red morphs showed the highest number of capsules produced, while the yellow morphs had the greatest male success. The relative male and female reproductive success were independent from plant density but were significantly correlated with the yellow morph frequency at the population level. Overall, our findings show that the contribution to the total reproductive success deriving from the two colour morphs does not conform with the predictions of negative frequency-dependent selection. 相似文献
967.
The infectivity and development of four populations of Meloidogyne hapla were compared, at three temperatures, on tomato and two varieties of cucumber. A population from Canada produced few root-galls on cucumber and, except at 24 C, no larvae developed into adult females and produced egg masses. In contrast, a population with 45 chromosomes from America produced many galls on cucumber and small proportions of larvae became females and produced egg masses at 20 and 24 C. At 18 C this population produced no egg masses on cucumber, but a population from Britain and one from America with 17 chromosomes produced more egg masses at this temperature than at 20 or 24 C. Dissection of the galls showed that on cucumber many larvae died or their growth and development was slowed. 相似文献
968.
A novel assay of single-cell exogenous beta-galactosidase activity in Saccharomyces cerevisiae has been developed. Intracellular fluorescence due to the hydrolysis of resorufin-beta-D-galactopyranoside attains a steady state between production of resorufin and its subsequent leakage from the cell. The cells are permeabilized with Triton X-100, and the assay is performed at 0 degrees C. These conditions were chosen to minimize intercellular fluorescence communication. Free resorufin in the extracellular space is bound by bovine serum albumin to prevent its uptake by cells. Two regimes of fluorescence accumulation are observed, one limited by the rate of diffusion of substrate into the cell, and one limited by the rate of enzymatic cleavage of the substrate. A quantitative correlation between fluorescence and beta-galactosidase activity is obtained under optimized assay conditions. 相似文献
969.
Several human studies have shown an inverse relation between vitamin A intake (and serum concentrations of retinol and carotene) and cancer. Serum cholesterol concentrations have also been reported in inverse relation to cancer. In a study of 3102 people in Evans County, Georgia, who were followed for over 12-14 years to assess the incidence of cancer there was an inverse association between the risk of cancer and both serum retinol and serum cholesterol concentrations. The data also showed an unexpectedly strong correlation between serum retinol and total cholesterol concentrations. The inverse relationship with cancer was stronger with serum retinol than with cholesterol, which suggested that the association with cholesterol might be secondary. This suggestion may also explain the cholesterol-cancer association reported in several other cohort studies. Further studies of the relation between serum concentrations of cholesterol, retinol, and carotene and the incidence of cancer are needed. 相似文献
970.