全文获取类型
收费全文 | 1154篇 |
免费 | 139篇 |
出版年
2021年 | 17篇 |
2020年 | 12篇 |
2019年 | 9篇 |
2018年 | 8篇 |
2017年 | 11篇 |
2015年 | 39篇 |
2014年 | 38篇 |
2013年 | 53篇 |
2012年 | 47篇 |
2011年 | 57篇 |
2010年 | 30篇 |
2009年 | 32篇 |
2008年 | 40篇 |
2007年 | 41篇 |
2006年 | 38篇 |
2005年 | 33篇 |
2004年 | 34篇 |
2003年 | 38篇 |
2002年 | 42篇 |
2001年 | 27篇 |
2000年 | 28篇 |
1999年 | 33篇 |
1998年 | 11篇 |
1997年 | 18篇 |
1996年 | 10篇 |
1995年 | 15篇 |
1994年 | 19篇 |
1992年 | 26篇 |
1991年 | 39篇 |
1990年 | 22篇 |
1989年 | 33篇 |
1988年 | 26篇 |
1987年 | 22篇 |
1986年 | 18篇 |
1985年 | 27篇 |
1984年 | 17篇 |
1983年 | 13篇 |
1982年 | 14篇 |
1981年 | 16篇 |
1980年 | 17篇 |
1979年 | 13篇 |
1978年 | 12篇 |
1977年 | 16篇 |
1976年 | 22篇 |
1975年 | 17篇 |
1974年 | 18篇 |
1973年 | 15篇 |
1972年 | 14篇 |
1968年 | 6篇 |
1967年 | 6篇 |
排序方式: 共有1293条查询结果,搜索用时 15 毫秒
951.
952.
953.
Thomas G. Duthy Lothar H. Staendner Paul A. Manning Michael W. Heuzenroeder 《Journal of bacteriology》1999,181(18):5847-5851
We have sequenced the entire region of DNA required for the biosynthesis of CS5 pili from enterotoxigenic Escherichia coli O115:H40 downstream of the major subunit gene, designated csfA (for coli surface factor five A). Five more open reading frames (ORFs) (csfB, csfC, csfE, csfF, and csfD) which are transcribed in the same direction as the major subunit and are flanked by a number of insertion sequence regions have been identified. T7 polymerase-mediated overexpression of the cloned csf ORFs confirmed protein sizes based on the DNA sequences that encode them. The expression of only the csf region in E. coli K-12 resulted in the hemagglutination of human erythrocytes and the cell surface expression of CS5 pili, suggesting that the cluster contains all necessary information for CS5 pilus biogenesis and function. 相似文献
954.
1. Male chickens (Gallus domesticus) were treated with a single intramuscular injection of oestradiol-17 beta, then changes in the liver and plasma levels of triacylglycerol, phospholipid, nonesterified fatty acids and in the hepatic activities of acetyl-CoA carboxylase and fatty acid synthetase were measured at various times after injection. 2. The results suggest that the initial phase (less than 20 hr) of oestrogen-induced hyperlipidaemia occurs in the absence of changes in the hepatic activities of the major enzymes of fatty acid biosynthesis, but a subsequent increase in these enzyme activities may contribute to the later phase (greater than 20 hr) of oestrogen-induced lipogenesis in avian liver. 相似文献
955.
956.
957.
Sasha N. Jenkins Ian S. Waite Adrian Blackburn Rebecca Husband Steven P. Rushton David C. Manning Anthony G. O’Donnell 《Antonie van Leeuwenhoek》2009,95(4):319-334
Palace Leas, a long-term experiment at Cockle Park Farm, Northumberland, UK was established in winter 1896–1897 since when
the 13 plots have received regular and virtually unchanged mineral fertiliser and farm yard manure inputs. Fertilisers have
had a profound impact on soil pH with the organically fertilised plots showing a significantly higher pH than those receiving
mineral fertiliser where ammonium sulphate has led to soil acidification. Here, we investigate the impact of organic and mineral
fertilisers on the actinobacterial community structure of these soils using terminal restriction fragment length polymorphism
(T-RFLP) and 16S rRNA gene analysis. To differentiate fertiliser effects from seasonal variation, soils were sampled three
times over one growing season between May and September 2004 and January 2005. Community profiles obtained using T-RFLP were
analysed using multivariate statistics to investigate the relationship between community structure, seasonality and fertiliser
management. Soil pH was shown to be the most significant edaphic factor influencing actinobacterial communities. Canonical
correspondence analysis, used to investigate the relationship between the 16S rRNA gene community profiles and the environmental
parameters, showed that actinobacterial communities also responded to soil water content with major changes evident over the
summer months between May and September. Quantitative PCR of the actinobacterial and fungal 16S and 18S rRNA genes, respectively
suggested that fungal rRNA gene copy numbers were negatively correlated (P = 0.0131) with increasing actinobacterial signals. A similar relationship (P = 0.000365) was also evident when fatty acid methyl esters indicative of actinobacterial biomass (10-methyloctadecanoic acid)
were compared with the amounts of fungal octadecadienoic acid (18:2ω9,12). These results show clearly that soil pH is a major
driver of change in actinobacterial communities and that genera such as Arthrobacter and Micrococcus are particularly abundant in soils receiving organic inputs whilst others such as Streptomyces, Acidimicrobium and Actinospica are more prevalent in acid soils. The importance of these findings in terms of fungal abundance and potential disease suppression
are discussed. 相似文献
958.
A direct relationship exists within subjects between midlatency features (<100 ms poststimulus) of respiratory-related evoked potentials and the perceived magnitude of applied oral pressure pulse stimuli. We evaluated perception in 18 normal subjects using cross-modality matching of applied pressure pulses via grip force and estimated mechanoafferent activity in these subjects by computing the global field power (GFP) from respiratory-related evoked potentials recorded over the right side of the scalp. We compared across subjects 1) the predicted magnitude production for a standard pressure pulse and 2) the slope (beta) and 3) the intercept (INT) of the Stevens power law to the summed GFP over 20-100 ms poststimulus. Both the magnitude production for a standard pressure pulse and the beta showed an inverse relationship with the summed GFP over 20-100 ms poststimulus, although there was no relationship between INT and the summed GFP. This may partially reflect characteristics of the mechanosensors and surely includes aspects of cognitive judgment, because we found and corrected for a high correlation between, respectively, beta (and INT) for pressure pulses and beta (and INT) for estimation of line lengths, a nonrespiratory modality. The relatively shallow, even inverse GFP-to-perception relationship suggests that, despite marked differences in the magnitude of afferent traffic, normal subjects seem to perceive things similarly. 相似文献
959.
960.
Six male subjects (19–23 years old) underwent a 7-day control period with respect to diet, temperature (22C), and sleep (7.5 hrs), followed by a 2-day exposure to 15C and a 2-day recovery period (22C). Urine collections were made every 8 hours commencing at 2300 hours; MHPG and VMA were assayed using gas-liquid chromatography. During the control period a diurnal rhythmicity was demonstrated for MHPG and VMA with maxima at 0700–1500 hours. The mean excretory rates for MHPG and VMA were 0.71 ± 0.04 μg and 2.6 ± 0.2 μg per milligram creatinine (± S.E.), respectively. Cold exposure abolished the rhythms for MHPG and VMA and caused an 18% increase in MHPG excretion. In contrast, VMA excretion was not altered. Significant correlations were obtained with MHPG excretion and both urinary cortisol and rectal temperature. The data suggest that MHPG excretion may be indicative of changes in norephinephrine metabolism in the central nervous system, although alterations in peripheral degradative pathways cannot be ruled out. Careful interpretation of changes in MHPG excretion in clinical studies is emphasized due to the relative ease of altering MHPG metabolism. 相似文献