首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1068篇
  免费   106篇
  国内免费   1篇
  1175篇
  2023年   6篇
  2022年   20篇
  2021年   21篇
  2020年   7篇
  2019年   15篇
  2018年   21篇
  2017年   14篇
  2016年   27篇
  2015年   69篇
  2014年   56篇
  2013年   79篇
  2012年   78篇
  2011年   87篇
  2010年   52篇
  2009年   39篇
  2008年   61篇
  2007年   67篇
  2006年   47篇
  2005年   53篇
  2004年   35篇
  2003年   43篇
  2002年   46篇
  2001年   18篇
  2000年   19篇
  1999年   17篇
  1998年   9篇
  1997年   8篇
  1996年   7篇
  1995年   7篇
  1994年   8篇
  1992年   9篇
  1991年   12篇
  1990年   8篇
  1989年   9篇
  1988年   7篇
  1987年   9篇
  1986年   11篇
  1985年   8篇
  1984年   10篇
  1982年   2篇
  1981年   6篇
  1980年   3篇
  1979年   6篇
  1978年   8篇
  1974年   9篇
  1973年   4篇
  1972年   4篇
  1971年   2篇
  1969年   2篇
  1936年   2篇
排序方式: 共有1175条查询结果,搜索用时 0 毫秒
151.
The red species of Cortinarius subgenus Dermocybe in Europe were studied based on morphological and molecular data. Three completely red species were recognized: C. sanguineus (syn. C. sanguineus var. aurantiovaginatus), C. puniceus (syn. C. cruentus, C. rubrosanguineus) and C. vitiosus comb. nov. Cortinarius sanguineus has dusky red to red pileus, reddish yellow mycelium and lacking or with only slightly encrusted hyphae in pileipellis. It occurs in mesic to damp forests with Picea, often on rich soil in the boreal and montane areas of Europe, presumably also in eastern Canada. Cortinarius puniceus differs from C. sanguineus by its stronger purplish red, narrower spores and spot-like encrusted hyphae in pileipellis. It grows with deciduous trees in the temperate zone of Europe. Cortinarius vitiosus is known only from Fennoscandia and occurs in dry to mesic coniferous forests. It has fairly thin, often zonate, dark red to dark reddish brown pileus, pale red mycelium, small spores and encrusted lamellar trama and pileipellis hyphae. In addition to these three species C. fervidus and C. phoeniceus occasionally have red basidiomes. The relationships of the species were inferred by analysis of ITS sequences. Our study suggests that the section Sanguinei, as earlier defined, is polyphyletic. Here the section is limited to include C. sanguineus, C. puniceus and North American D. sierraensis. The relationships with other red species were not determined. Section Dermocybe, including C. cinnamomeus, C. croceus and C. uliginosus, formed a monophyletic group, and the section Malicoriae had some support. A total of 34 new sequences are published including nine from type specimens.  相似文献   
152.

Background

Preclinical studies strongly suggest that accelerated apoptosis in skeletal myocytes may be involved in the pathogenesis of sarcopenia. However, evidence in humans is sparse. In the present study, we investigated whether apoptotic signaling in the skeletal muscle was associated with indices of muscle mass and function in older persons.

Methodology/Principal Findings

Community-dwelling older adults were categorized into high-functioning (HF) or low-functioning (LF) groups according to their short physical performance battery (SPPB) summary score. Participants underwent an isokinetic knee extensor strength test and 3-dimensional magnetic resonance imaging of the thigh. Vastus lateralis muscle samples were obtained by percutaneous needle biopsy and assayed for the expression of a set of apoptotic signaling proteins. Age, sex, number of comorbid conditions and medications as well as knee extensor strength were not different between groups. HF participants displayed greater thigh muscle volume compared with LF persons. Multivariate partial least squares (PLS) regressions showed significant correlations between caspase-dependent apoptotic signaling proteins and the muscular percentage of thigh volume (R2 = 0.78; Q2 = 0.61) as well as gait speed (R2 = 0.81; Q2 = 0.56). Significant variables in the PLS model of percent muscle volume were active caspase-8, cleaved caspase-3, cytosolic cytochrome c and mitochondrial Bak. The regression model of gait speed was mainly described by cleaved caspase-3 and mitochondrial Bax and Bak. PLS predictive apoptotic variables did not differ between functional groups. No correlation was determined between apoptotic signaling proteins and muscle strength or quality (strength per unit volume).

Conclusions/Significance

Data from this exploratory study show for the first time that apoptotic signaling is correlated with indices of muscle mass and function in a cohort of community-dwelling older persons. Future larger-scale studies are needed to corroborate these preliminary findings and determine if down-regulation of apoptotic signaling in skeletal myocytes will provide improvements in the muscle mass and functional status of older persons.  相似文献   
153.
Microbial hydrolysis of polysaccharides is critical to ecosystem functioning and is of great interest in diverse biotechnological applications, such as biofuel production and bioremediation. Here we demonstrate the use of a new, efficient approach to recover genomes of active polysaccharide degraders from natural, complex microbial assemblages, using a combination of fluorescently labeled substrates, fluorescence-activated cell sorting, and single cell genomics. We employed this approach to analyze freshwater and coastal bacterioplankton for degraders of laminarin and xylan, two of the most abundant storage and structural polysaccharides in nature. Our results suggest that a few phylotypes of Verrucomicrobia make a considerable contribution to polysaccharide degradation, although they constituted only a minor fraction of the total microbial community. Genomic sequencing of five cells, representing the most predominant, polysaccharide-active Verrucomicrobia phylotype, revealed significant enrichment in genes encoding a wide spectrum of glycoside hydrolases, sulfatases, peptidases, carbohydrate lyases and esterases, confirming that these organisms were well equipped for the hydrolysis of diverse polysaccharides. Remarkably, this enrichment was on average higher than in the sequenced representatives of Bacteroidetes, which are frequently regarded as highly efficient biopolymer degraders. These findings shed light on the ecological roles of uncultured Verrucomicrobia and suggest specific taxa as promising bioprospecting targets. The employed method offers a powerful tool to rapidly identify and recover discrete genomes of active players in polysaccharide degradation, without the need for cultivation.  相似文献   
154.
155.
Pristine peatlands are carbon (C)‐accumulating wetland ecosystems sustained by a high water table (WT) and consequent anoxia that slows down decomposition. Persistent WT drawdown as a response to climate and/or land‐use change affects decomposition either directly through environmental factors such as increased oxygenation, or indirectly through changes in plant community composition. This study attempts to disentangle the direct and indirect effects of WT drawdown by measuring the relative importance of environmental parameters (WT depth, temperature, soil chemistry) and litter type and/or litter chemical quality on the 2‐year decomposition rates of above‐ and belowground litter (altogether 39 litter types). Consequences for organic matter accumulation were estimated based on the annual litter production. The study sites were chosen to form a three‐stage chronosequence from pristine (undrained) to short‐term (years) and long‐term (decades) WT drawdown conditions at three nutrient regimes. The direct effects of WT drawdown were overruled by the indirect effects through changes in litter type composition and production. Short‐term responses to WT drawdown were small. In long‐term, dramatically increased litter inputs resulted in large accumulation of organic matter in spite of increased decomposition rates. Furthermore, the quality of the accumulated matter greatly changed from that accumulated in pristine conditions. Our results show that the shift in vegetation composition as a response to climate and/or land‐use change is the main factor affecting peatland ecosystem C cycle, and thus dynamic vegetation is a necessity in any model applied for estimating responses of C fluxes to changing environment. We provide possible grouping of litter types into plant functional types that the models could utilize. Furthermore, our results clearly show a drop in soil summer temperature as a response to WT drawdown when an initially open peatland converts into a forest ecosystem, which has not yet been considered in the existing models.  相似文献   
156.
157.
158.

Background

Following male circumcision for HIV prevention, a high proportion of men fail to return for their scheduled seven-day post-operative visit. We evaluated the effect of short message service (SMS) text messages on attendance at this important visit.

Methodology

We enrolled 1200 participants >18 years old in a two-arm, parallel, randomized controlled trial at 12 sites in Nyanza province, Kenya. Participants received daily SMS text messages for seven days (n = 600) or usual care (n = 600). The primary outcome was attendance at the scheduled seven-day post-operative visit. The primary analysis was by intention-to-treat.

Principal Findings

Of participants receiving SMS, 387/592 (65.4%) returned, compared to 356/596 (59.7%) in the control group (relative risk [RR] = 1.09, 95% confidence interval [CI] 1.00–1.20; p = 0.04). Men who paid more than US$1.25 to travel to clinic were at higher risk for failure to return compared to those who spent ≤US$1.25 (adjusted relative risk [aRR] 1.35, 95% CI 1.15–1.58; p<0.001). Men with secondary or higher education had a lower risk of failure to return compared to those with primary or less education (aRR 0.87, 95% CI 0.74–1.01; p = 0.07).

Conclusions

Text messaging resulted in a modest improvement in attendance at the 7-day post-operative clinic visit following adult male circumcision. Factors associated with failure to return were mainly structural, and included transportation costs and low educational level.

Trial Registration

ClinicalTrials.gov NCT01186575  相似文献   
159.
160.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号