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   9621篇
  免费   1011篇
  国内免费   525篇
  2024年   6篇
  2023年   139篇
  2022年   165篇
  2021年   426篇
  2020年   402篇
  2019年   426篇
  2018年   408篇
  2017年   286篇
  2016年   409篇
  2015年   658篇
  2014年   731篇
  2013年   770篇
  2012年   882篇
  2011年   757篇
  2010年   519篇
  2009年   420篇
  2008年   531篇
  2007年   459篇
  2006年   406篇
  2005年   325篇
  2004年   270篇
  2003年   252篇
  2002年   186篇
  2001年   173篇
  2000年   151篇
  1999年   166篇
  1998年   87篇
  1997年   81篇
  1996年   73篇
  1995年   71篇
  1994年   91篇
  1993年   46篇
  1992年   49篇
  1991年   62篇
  1990年   38篇
  1989年   46篇
  1988年   29篇
  1987年   25篇
  1986年   32篇
  1985年   25篇
  1984年   15篇
  1983年   11篇
  1982年   9篇
  1981年   4篇
  1978年   4篇
  1976年   3篇
  1973年   4篇
  1970年   3篇
  1969年   3篇
  1968年   4篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
通过杂交和自交实验,对重唇石斛授粉亲和力和结实性进行分析,并对其开花期间花粉及蕊柱结构的动态变化进行显微观察,以探讨重唇石斛的最佳杂交授粉时间.结果表明:(1)花器表面结构差异较大,唇瓣、药帽、萼片表皮布满突起、钩刺等,花瓣表皮较平滑;(2)杂交时以重唇石斛为父本或母本均具有选择性,但作为父本亲和力强,杂交结实率可达100%,作为母本杂交结实率最高为60%;自交结实率非常低,为4.3%;(3)花粉培养的最佳配方为30%蔗糖+0.15 mg·L-1 CaCl2;(4)花开第4天后花粉发育饱满,第5~12天花粉发芽率稳定大于70%,蕊柱的乳突细胞于开花第6~10 天时发育饱满.研究表明,重唇石斛花开第5~12 天为最佳授粉时间.  相似文献   
2.
Phoebe is an economically important genus from the family Lauraceae. It is widely distributed in tropical and subtropical Asia, but systematics of the genus is unclear, and currently there is no species-level phylogeny. Here, we determined the complete chloroplast genome sequences of two species with long-range PCR and next genome sequencing technologies, and identified mutation sites and highly variable regions. These highly variable sites were used to reconstruct the phylogeny. The plastomes of Phoebe sheareri and P. omeiensis were 152, 876, and 152, 855 bp, respectively. Comparative genomic analysis indicated that there are 222 mutation sites including 146 substitutions, 73 indels, and 3 microinversions in both plastomes. Fifty-six single-nucleotide changes were identified in gene-coding regions, and 45 microsatellite sites were found for use in species identification. Fourteen divergence hotspots of 38 variable regions were located. Phylogeny was reconstructed using a Bayesian and maximum likelihood approach for 12 Phoebe species and other five related Lauraceae based on 15 of the highly variable regions including accD-psaI, atpB-rbcL, ndhC-trnV, ndhF-rpl32, petA-psbJ, psaA, psbA-trnH, rbcL, rps8-rpl14, rps16-trnQ, rpl32-trnL, trnC-petN, trnL-trnF, trnS-trnG, and ycf1 indicated that variability in the chloroplast regions proposed as variable is enough to detect divergence events among 12 taxa of Phoebe, and that maybe also useful to help to elucidate further relationships among other taxa of the genus.  相似文献   
3.
Mesenchymal stem cells (MSCs) are non-hematopoietic cells with multi-lineage potential, which makes them attractive targets for regenerative medicine applications. Efficient gene transfer into MSCs is essential for basic research in developmental biology and for therapeutic applications involving gene-modification in regenerative medicine. Adenovirus vectors (Advs) can efficiently and transiently introduce an exogenous gene into many cell types via their primary receptors, the coxsackievirus and adenovirus receptors (CARs), but not into MSCs, which lack CAR expression. To overcome this problem, an Adv coated with cationic polymer polyethyleneimine (PEI) was developed. In this study, we demonstrated that PEI coating with an optimal ratio can enhance adenoviral transduction of MSCs without cytotoxicity. We also investigated the physicochemical properties and internalization mechanisms of the PEI-coated Adv. These results could help to evaluate the potentiality of the PEI-coated Adv as a prototype vector for efficient and safe transduction into MSCs.  相似文献   
4.
沙门菌对酸压力的应答及其与毒力的关系   总被引:1,自引:0,他引:1  
沙门菌(Salmonella spp.)作为肠道细菌,必须克服胃中酸性环境,才能进一步入侵宿主肠道上皮细胞。已有的研究表明,沙门菌通过进化出多种应答机制,增强自身在酸性环境下的生存。本文回顾了沙门菌的耐酸特性,阐述了抵御酸压力时的几种应答机理,包括胞内pH的维持、调控酸激蛋白的时序表达以及细胞膜特性的改变。这些研究对人类了解和控制沙门菌的感染具有重要的指导意义。  相似文献   
5.
【目的】猪链球菌1、2、14和1/2型间存在单向或双向的交叉抗原性,这种交叉抗原性的产生原因至今未被揭示。【方法】采用Sephacryl S-300凝胶层析柱对猪链球菌14和1/2型荚膜多糖进行分离纯化,经苯酚-硫酸检测和dot-ELISA辅助鉴定,确定荚膜多糖成分。采用高效凝胶渗透色谱法测定14和1/2型猪链球菌荚膜多糖分子量分别为487.38 kDa和512.72 kDa。【结果】经柱前衍生高效液相色谱法、荧光标记液相色谱法和核磁共振测定14和1/2型猪链球菌荚膜多糖单糖组成分别为:Glc/Gal/GlcNAc/Rha/Neu5Ac(1∶2.94∶1.35∶0.24∶0.37)和Glc/Gal/GlcNAc/GalNAc/Rha/Neu5Ac(1∶1.67∶1.05∶0.93∶0.72∶0.7)。并与已知的猪链球菌1、2型荚膜多糖的单糖组成进行比较分析,发现4种血清型荚膜多糖都具有Glc、GlcNAc、Gal和Neu5Ac,但单糖组成和比列并无明显相似性,这种交叉抗原性可能是由于荚膜多糖的空间结构相似性和(或)细胞表面的其他成分引起的。  相似文献   
6.
A serum metabolomics method based on rapid resolution liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (RRLC-Q-TOF-MS) was performed for a holistic evaluation of the metabolic changes of collagen-induced arthritis (CIA) in rats and to assess the interventional effects of type II collagen (CII) in this model. Partial least-squares-discriminant analysis (PLS-DA) was employed to study the metabolic profiling of CIA rats and control rats. Ten metabolites, namely, 12(S)-HHTrE, 12(S)-HEPE, PGE2, TXB2, 12(S)-HETE, LysoPE(16:0), PE(O-18:0/0:0), Lyso-PE(18:2), Lyso-PE(20:4), and Lyso-PC(22:5) were identified as differential metabolites associated with the pathogenesis of CIA. These results suggested that dysregulation of the arachidonic acid (AA) and phospholipid metabolic networks is involved in the pathomechanism of CIA. Differential metabolomics and histopathological analyses demonstrated that CII inhibits the progress of arthritis. Furthermore, the therapeutic effects of CII on CIA may involve regulation of the disordered AA and phospholipid metabolic networks. This metabolomics study provides new insights into the pathogenesis of arthritis and, furthermore, indicates the potential mechanism underlying the significantly increased prevalence of metabolic syndrome, defined as a clustering of cardiovascular disease (CVD) risk factors, in arthritis patients.  相似文献   
7.
A temperature sensor based on hollow fiber (HF) filled with graphene-Ag composite nanowire and liquid is presented. The coupling properties and sensing performance are numerically analyzed by finite element method using wavelength and amplitude interrogations. Results show that the sensor exhibiting strong birefringence with x-polarized peak provides much higher sensitivities and better signal-to-noise ratio (SNR) than y-polarized, which is more suitable for temperature detection. The graphene-Ag composite nanowire can not only solve the oxidation problem but also avoid the metal coating. Moreover, it provides better performance than other similar works like Au-Ag nanowire-filled, Au nanowire-filled, and Ag nanowire-filled sensors. Contrary to the blue shift of traditional SPR temperature sensors, the resonance peak shifts to the longer wavelength in our device when temperature increases and the high sensitivity 9.44 nm/ °C is obtained. The influences of nanowire diameter, grapheme-layer thickness on the designed sensor, are also investigated. This work can provide a reference for developing a high sensitivity, real-time, remote sensing, and distributed temperature SPR sensor.  相似文献   
8.
了解城市内河中病原菌含量对城市生态安全具有重要意义。应用定量PCR( qPCR)方法,对宁波市城区6条重要内河水体中的病原菌含量及主要水质指标进行分析。结果表明,祖关山河、苏家河、南北河、后西河、史魏家河和直落河每升水体中含有病原菌基因的拷贝数分别为:3.34×105、1.47×105、4.68×105、1.75×106、9.65×106和1.33×106;且病原菌基因的含量与水体溶解氧负相关,与总氮、氨氮、浊度、总磷、高锰酸钾指数、叶绿素a和电导率呈正相关关系,但与水体透明度、温度和酸碱度无明显关系。定量PCR分析结果可望为评价城市内河水体质量提供重要参考。  相似文献   
9.

Background

Bipolar disorder types I (BD I) and II (BD II) behave differently in clinical manifestations, normal personality traits, responses to pharmacotherapies, biochemical backgrounds and neuroimaging activations. How the varied emotional states of BD I and II are related to the comorbid personality disorders remains to be settled.

Methods

We therefore administered the Plutchick – van Praag Depression Inventory (PVP), the Mood Disorder Questionnaire (MDQ), the Hypomanic Checklist-32 (HCL-32), and the Parker Personality Measure (PERM) in 37 patients with BD I, 34 BD II, and in 76 healthy volunteers.

Results

Compared to the healthy volunteers, patients with BD I and II scored higher on some PERM styles, PVP, MDQ and HCL-32 scales. In BD I, the PERM Borderline style predicted the PVP scale; and Antisocial predicted HCL-32. In BD II, Borderline, Dependant, Paranoid (-) and Schizoid (-) predicted PVP; Borderline predicted MDQ; Passive-Aggressive and Schizoid (-) predicted HCL-32. In controls, Borderline and Narcissistic (-) predicted PVP; Borderline and Dependant (-) predicted MDQ.

Conclusion

Besides confirming the different predictability of the 11 functioning styles of personality disorder to BD I and II, we found that the prediction was more common in BD II, which might underlie its higher risk of suicide and poorer treatment outcome.  相似文献   
10.

Background

Emergence agitation (EA) is one of the most common postoperative complications in children. The purpose of this meta-analysis is to assess the effect of dexmedetomidine for preventing postoperative agitation in children.

Methods

We searched the Cochrane Central Register of Controlled Trails, MEDLINE, and EMBASE. Randomized controlled trials were included. The following outcome measures were evaluated: incidence of EA, number of patients requiring rescue, time to eye-open, time to extubation, time to discharge from the postanesthesia care unit (PACU).

Results

We analyzed 19 trials (1608 patients) that met the inclusion criteria. Compared with placebo, intravenous dexmedetomidine significantly reduced the incidence of EA [risk ratio (RR) 0.34, 95% confidence interval (CI) 0.25–0.44, P<0.00001). Dexmedetomidine also decreased the incidence of severe pain (RR 0.41, 95% CI 0.27–0.62, P<0.0001) and requirement of a rescue drug (RR 0.31, 95% CI 0.18–0.53, P<0.0001). However, compared with placebo, dexmedetomidine increased the time to eye-open by 0.98 min (P = 0.01) and the time to PACU discharge by 4.63 min (P = 0.02). Dexmedetomidine was also compared with midazolam, propofol, ketamine, and fentanyl, among others. No significant difference was found in the incidence of EA for most of these comparisons, with the exception of fentanyl and propofol, where dexmedetomidine was more beneficial.

Conclusions

Dexmedetomidine was proved effective for preventing EA and for reducing severe pain and the requirement of rescue drugs. It slightly increased the time to eye-open and the time to PACU discharge. Dexmedetomidine was also more beneficial than propofol or fentanyl in preventing EA.  相似文献   
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