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   3581篇
  免费   242篇
  2024年   3篇
  2023年   10篇
  2022年   28篇
  2021年   80篇
  2020年   58篇
  2019年   77篇
  2018年   91篇
  2017年   83篇
  2016年   101篇
  2015年   184篇
  2014年   179篇
  2013年   256篇
  2012年   327篇
  2011年   304篇
  2010年   206篇
  2009年   181篇
  2008年   237篇
  2007年   242篇
  2006年   221篇
  2005年   170篇
  2004年   159篇
  2003年   154篇
  2002年   148篇
  2001年   43篇
  2000年   21篇
  1999年   38篇
  1998年   31篇
  1997年   23篇
  1996年   25篇
  1995年   20篇
  1994年   18篇
  1993年   12篇
  1992年   7篇
  1991年   9篇
  1990年   5篇
  1989年   5篇
  1988年   4篇
  1987年   6篇
  1986年   5篇
  1985年   5篇
  1984年   6篇
  1983年   6篇
  1982年   7篇
  1981年   3篇
  1979年   3篇
  1974年   2篇
  1966年   2篇
  1965年   3篇
  1960年   2篇
  1957年   3篇
排序方式: 共有3823条查询结果,搜索用时 233 毫秒
1.
The European genetic landscape has been shaped by several human migrations occurred since Paleolithic times. The accumulation of archaeological records and the concordance of different lines of genetic evidence during the last two decades have triggered an interesting debate concerning the role of ancient settlers from the Franco-Cantabrian region in the postglacial resettlement of Europe. Among the Franco-Cantabrian populations, Basques are regarded as one of the oldest and more intriguing human groups of Europe. Recent data on complete mitochondrial DNA genomes focused on macrohaplogroup R0 revealed that Basques harbor some autochthonous lineages, suggesting a genetic continuity since pre-Neolithic times. However, excluding haplogroup H, the most representative lineage of macrohaplogroup R0, the majority of maternal lineages of this area remains virtually unexplored, so that further refinement of the mtDNA phylogeny based on analyses at the highest level of resolution is crucial for a better understanding of the European prehistory. We thus explored the maternal ancestry of 548 autochthonous individuals from various Franco-Cantabrian populations and sequenced 76 mitogenomes of the most representative lineages. Interestingly, we identified three mtDNA haplogroups, U5b1f, J1c5c1 and V22, that proved to be representative of Franco-Cantabria, notably of the Basque population. The seclusion and diversity of these female genetic lineages support a local origin in the Franco-Cantabrian area during the Mesolithic of southwestern Europe, ∼10,000 years before present (YBP), with signals of expansions at ∼3,500 YBP. These findings provide robust evidence of a partial genetic continuity between contemporary autochthonous populations from the Franco-Cantabrian region, specifically the Basques, and Paleolithic/Mesolithic hunter-gatherer groups. Furthermore, our results raise the current proportion (≈15%) of the Franco-Cantabrian maternal gene pool with a putative pre-Neolithic origin to ≈35%, further supporting the notion of a predominant Paleolithic genetic substrate in extant European populations.  相似文献   
2.
The association between the red macroalga Jania adhaerens J. V. Lamour. and the sponge Haliclona caerulea is the most successful life‐form between 2 and 4 m depth in Mazatlán Bay (Mexican Pacific). J. adhaerens colonizes the rocky intertidal area and penetrates into deeper areas only when it lives in association with H. caerulea. The aposymbiotic form of the sponge has not been reported in the bay. To understand the ecological success of this association, we examined the capacity of J. adhaerens to acclimate in Mazatlán Bay using transplant experiments. The transplanted aposymbiotic J. adhaerens did not survive the first 2 weeks; however, J. adhaerens when living in association with H. caerulea, acclimated easily to depth, showing no sign of mortality during the 103 d of the experiment. We conclude that the ability of J. adhaerens to colonize in deeper areas in this hydrodynamic environment may in part rely on the protection provided by the sponge to the algal canopy. Both species contribute to the shape of the associated form. Nevertheless, the morphological variation in the association appears to be dominated by the variation in J. adhaerens canopy to regulate pigment self‐shading under light‐limited conditions and/or tissue resistance under high hydrodynamics. Consequently, our results are consistent with light as the abiotic controlling factor, which regulates the lower depth distribution of the association in Mazatlán Bay, through limiting the growth rate of J. adhaerens. Hydrodynamics may determine the upper limit of the association by imposing high mass losses.  相似文献   
3.
4.
Experiments were carried out to evaluate the effects of exposure to nitric oxide on the ability by NADPH‐dependent microsomal electron transfer to generate oxygen radicals. Such interactions could play a role in the potential antioxidant action of nitric oxide (NO). Isolated microsomes from soybean ( Glycine max [L.] Merr. cv. Hood) embryonic axes were exposed to an exogenously added source of nitric oxide (NO) (S‐nitrosoglutathione + dithiothreitol). The O2 generation rate by microsomes exposed to NO decreased significantly as compared to the rate measured in microsomes incubated in the absence of NO. The exposure of the microsomes to the NO donor did not alter the microsomal rate of hydroxyl radical generation. Preincubation of the microsomes with the NO donor affected neither iron reduction rate nor activity of cytochrome c reductase. However, cytochrome P450 activity was significantly inhibited after exposure to NO. This inhibition was completely prevented by hemoglobin. The data are consistent with the hypothesis that NO exhibits a potential antioxidant role in the plant cell by decreasing the rate of generation of superoxide anion. Since endogenous NO was detected in homogenates of soybean embryonic axes by EPR studies, this interaction between NO and cytochrome P450 in soybean embryonic axes could be a factor of relevance for the control of oxidative stress in vivo.  相似文献   
5.
6.
7.
Alzheimer's disease (AD) is a severe age dependent and chronic problem with no cure so far. The available treatments are temporary, acting over short period of time. The main pathological hallmark of the disease includes cholinergic dysfunction, oxidative stress, accumulation of Aβ fibrils and tau tangles. In context with the multi-factorial nature of this disease, two different series of molecules were developed to hit the multifactorial disease targets. Mainly, the molecules were designed to inhibit the AChE and aggregation of Aβ, and also oxidative damage. Two novel series of TAC-fenbufen/menbutone conjugated molecules were designed, synthesized and bio-assayed. All compounds showed inhibition capacity towards AChE, Aβ aggregation and moderate to good radical scavenging capacity. Particularly, five TAC-menbutone molecules showed improved AChE and Aβ aggregation inhibition capacity compared to TAC-fenbufen conjugated molecules. Overall, these novel series of molecules may be potential drug lead molecules in the treatment of AD.  相似文献   
8.
Gas exchange studies in two Portuguese grapevine cultivars   总被引:8,自引:0,他引:8  
Gas exchange characteristics of leaves of Vitis vinifera L. cvs Tinta Amarela and Periquita, two grapevine cultivars grown in distinct climatic regions of Portugal, were studied under natural and controlled conditions. Daily time courses of gas exchange were measured on both a hot, sunny day and a cooler, partly cloudy day. Responses of net photosynthesis to irradiance and internal partial pressure of CO2, were also obtained. A strong correlation between net photosynthesis (PN) and leaf conductance (gs) was found during the diurnal time courses of gas exchange, as well as a relatively constant internal partial pressure of CO2 (Pi), even under non-steady-state conditions. On the cloudless day, both PN and gs were lower in the afternoon than in the morning, despite similar conditions of leaf temperature, air to leaf water vapor deficit and irradiance. The response curves of net photosynthesis to internal CO2 showed linearity up to pi values of 50 Pa, possibly indicating a substantial excess of photosynthetic capacity. When measured at low partial pressures of O2 (1 kPa), PN became inhibited at high CO2 levels. Inhibition of PN at high CO2 was absent under normal levels of O2 (21 kPa). Significant differences in gas exchange characteristics were found between the two cultivars, with T. Amarela having higher rates under similar measurement conditions. In particular, the superior performance of T. Amarela at high temperatures may represent adaptation to the warmer conditions at its place of origin.  相似文献   
9.
Abstract— Lucilia is a South American genus with 23 species restricted to disjunct areas in southeastern Brazil and along the Andes. Lucilia is a monophyletic group defined by the co-occurrence of six characters: herbaceous, alternate-leaved, pappus with scabrid bristles fused at the base into a ring, style-branches with sweeping hairs far down, capitula sessile, and aseptate-flagellate hairs. A dadogram is presented using 41 morphological and anatomical characters arranged into 26 transformation series. The polarity of character states was determined by outgroup comparison with the genus Berroa. The cladistic analysis showed extensive parallel evolution in a number of the more conspicuous characters and produced four unresolved trichotomies. However, basing the hierarchy of Lucilia on the branching pattern produced by cladistic analysis results in a more natural and predicitive classification. Lucilia is divided into three sections, Lucilia, Intermedieae (sect, nov), and Lucilioides [divided into two subsections, Subspicata (subsect. nov.) and Lucilioides]. The latter subsection is subdivided into two series, Lucilioides (ser. nov.) and Paralucilia. The Brazilian species of section Lucilia (acutijolia, linearifolia, ferruginea, tmentosa, recurva, nitens, and flagelliformis) form the most primitive group within the genus. The more derived species of the genus, section lucilioides (plicatifolia, catamarcensis, burkartii, subspkata, lopezmirandae, alpina, pickeringii, piptolepis, santamca, chilensis, schultzii, longifolia, radians, lehmanni, pusilla) are found in the Andes L. eriophora (section Intermedieae) from central Chile bridges these two groups. An explanation for the distribution of the genus is given, based on the ecology of the species in relation to theories of the geologic and climatic history of South America. The present pattern has been determined by the age, geographical range, and vicissitudes of the habitat in which each group occurs. In the Brazilian species group, the habitat is old, and has remained relatively stable since well before the Pleistocene. In the Andean species group, the habitat is young and has undergone numerous rapid alterations since its inception at the end of the Pliocene.  相似文献   
10.
Karyometry and histometry of renal-cell carcinoma   总被引:1,自引:0,他引:1  
In an attempt to more objectively predict the outcome of renal cancers, karyometric and histometric studies were performed using an interactive computer-based system for the quantitative analysis of tissue sections. Analysis showed a significant relationship between patient survival and metastases and the histometric parameters of nuclear elongation, nuclear crowding and mitotic density, as well as tumor grade. Patients who died tended to have a high mitotic density, elongated and crowded nuclei and high-grade tumors. Ploidy showed no significant correlation with prognosis while nuclear elongation and crowding did. Differences in histologic grade were significantly associated with several histometric variables, including nuclear area, shape, crowding, elongation and mitotic density.  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