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   25068篇
  免费   15504篇
  国内免费   3篇
  40575篇
  2023年   13篇
  2022年   88篇
  2021年   393篇
  2020年   2185篇
  2019年   3716篇
  2018年   3818篇
  2017年   4094篇
  2016年   4076篇
  2015年   3980篇
  2014年   3619篇
  2013年   4041篇
  2012年   1702篇
  2011年   1424篇
  2010年   2997篇
  2009年   1747篇
  2008年   627篇
  2007年   225篇
  2006年   219篇
  2005年   271篇
  2004年   248篇
  2003年   242篇
  2002年   232篇
  2001年   246篇
  2000年   181篇
  1999年   129篇
  1998年   7篇
  1997年   5篇
  1996年   6篇
  1995年   7篇
  1994年   9篇
  1993年   4篇
  1992年   7篇
  1991年   4篇
  1990年   1篇
  1989年   2篇
  1988年   1篇
  1986年   1篇
  1984年   1篇
  1978年   1篇
  1977年   1篇
  1974年   1篇
  1889年   1篇
  1882年   1篇
  1881年   1篇
  1873年   1篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Coral reef fish larvae are tiny, exceedingly numerous, and hard to track. They are also highly capable, equipped with swimming and sensory abilities that may influence their dispersal trajectories. Despite the importance of larval input to the dynamics of a population, we remain reliant on indirect insights to the processes influencing larval behavior and transport. Here, we used genetic data (300 independent single nucleotide polymorphisms) derived from a light trap sample of a single recruitment event of Dascyllus abudafur in the Red Sea (N = 168 settlers). We analyzed the genetic composition of the larvae and assessed whether kinship among these was significantly different from random as evidence for cohesive dispersal during the larval phase. We used Monte Carlo simulations of similar‐sized recruitment cohorts to compare the expected kinship composition relative to our empirical data. The high number of siblings within the empirical cohort strongly suggests cohesive dispersal among larvae. This work highlights the utility of kinship analysis as a means of inferring dynamics during the pelagic larval phase.  相似文献   
992.
993.
Ulcerative colitis (UC) is characterized by chronic relapsing intestinal inflammation finally leading to extensive tissue fibrosis and resulting in a stiff colon unable to carry out peristalsis or to resorb fluids. Telocytes, a peculiar type of stromal cells, have been recently identified in the human gastrointestinal tract. Several roles have been proposed for telocytes, including mechanical support, intercellular signalling and modulation of intestinal motility. The aim of the present work was to investigate the presence and distribution of telocytes in colonic specimens from UC patients compared with controls. Archival paraffin‐embedded samples of the left colon from UC patients who underwent elective bowel resection and controls were collected. Tissue sections were stained with Masson's trichrome to detect fibrosis. Telocytes were identified by CD34 immunohistochemistry. In early fibrotic UC cases, fibrosis affected the muscularis mucosae and submucosa, while the muscularis propria was spared. In advanced fibrotic UC cases, fibrosis extended to affect the muscle layers and the myenteric plexus. Few telocytes were found in the muscularis mucosae and submucosa of both early and advanced fibrotic UC colonic wall. In the muscle layers and myenteric plexus of early fibrotic UC, telocytes were preserved in their distribution. In the muscularis propria of advanced fibrotic UC, the network of telocytes was reduced or even completely absent around smooth muscle bundles and myenteric plexus ganglia, paralleling the loss of the network of interstitial cells of Cajal. In UC, a loss of telocytes accompanies the fibrotic remodelling of the colonic wall and might contribute to colonic dysmotility.  相似文献   
994.
Toxoplasma gondii infects virtually all warm‐blooded hosts worldwide. Recently, attention has been focused on the genetic diversity of the parasite to explain its pathogenicity in different hosts. It has been hypothesized that interaction between feral and domestic cycles of T. gondii may increase unusual genotypes in domestic cats and facilitate transmission of potentially more pathogenic genotypes to humans, domestic animals, and wildlife. In the present study, we tested black bear (Ursus americanus), bobcat (Lynx rufus), and feral cat (Felis catus) from the state of Pennsylvania for T. gondii infection. Antibodies to T. gondii were found in 32 (84.2%) of 38 bears, both bobcats, and 2 of 3 feral cats tested by the modified agglutination test (cut off titer 1:25). Hearts from seropositive animals were bioassayed in mice, and viable T. gondii was isolated from 3 of 32 bears, 2 of 2 bobcats, and 2 of 3 feral cats. DNA isolated from culture‐derived tachyzoites of these isolates was characterized using multilocus PCR‐RFLP markers. Three genotypes were revealed, including ToxoDB PCR‐RFLP genotype #1 or #3 (Type II, 1 isolate), #5 (Type 12, 3 isolates), and #216 (3 isolates), adding to the evidence of genetic diversity of T. gondii in wildlife in Pennsylvania. Pathogenicity of 3 T. gondii isolates (all #216, 1 from bear, and 2 from feral cat) was determined in outbred Swiss Webster mice; all three were virulent causing 100% mortality. Results indicated that highly mouse pathogenic strains of T. gondii are circulating in wildlife, and these strains may pose risk to infect human through consuming of game meat.  相似文献   
995.
Almost half of Mexican territory has been classified as environmentally degraded. The main response for the last 60 years has been reforestation to combat soil erosion and loss of forest cover, mostly carried out on private lands where negotiations with local stakeholders were critical. Despite four legal instruments referring to ecological restoration, no specific instrument that defines basic concepts, criteria and standards, required actions, or regulations to implement and evaluate ecological restoration exists. The Ministry of the Environment and Natural Resources is now solely in charge of restoration and only recently have external scientists been invited to be part of the process. Following important national and international events in Latin America and the Caribbean region, the First Mexican Symposium on Ecological Restoration was held in November, 2014. This historic event was the first action undertaken in Mexico to meet Objective 3 of the Global Strategy of Plant Conservation, coordinated in Mexico by the National Council for the Use and Knowledge of Biodiversity. Although mangrove ecosystems are the most endangered ecosystem type in Mexico, they were not well represented at the symposium. In contrast, several other ecosystem types, such as tropical dry forest and islands, have received increased attention. Overall, while the Symposium and above‐cited policy initiatives are important steps, Mexico needs to increase its institutional capacities and social organization of the rural sector with regard to ecological restoration. Better integration of social and natural scientists and increased participation of Mexico internationally is also needed.  相似文献   
996.
For the full activation of cyclin‐dependent kinases (CDKs), not only cyclin binding but also CDK phosphorylation is required. This activating phosphorylation is mediated by CDK‐activating kinases (CAKs). Arabidopsis has four genes showing similarity to vertebrate‐type CAKs, three CDKDs (CDKD;1CDKD;3) and one CDKF (CDKF;1). We previously found that the cdkf;1 mutant is defective in post‐embryonic development, even though the kinase activities of core CDKs remain unchanged relative to the wild type. This raised a question about the involvement of CDKDs in CDK activation in planta. Here we report that the cdkd;1 cdkd;3 double mutant showed gametophytic lethality. Most cdkd;1‐1 cdkd;3‐1 pollen grains were defective in pollen mitosis I and II, producing one‐cell or two‐cell pollen grains that lacked fertilization ability. We also found that the double knock‐out of CDKD;1 and CDKD;3 caused arrest and/or delay in the progression of female gametogenesis at multiple steps. Our genetic analyses revealed that the functions of CDKF;1 and CDKD;1 or CDKD;3 do not overlap, either during gametophyte and embryo development or in post‐embryonic development. Consistent with these analyses, CDKF;1 expression in the cdkd;1‐1 cdkd;3‐1 mutant could not rescue the gametophytic lethality. These results suggest that, in Arabidopsis, CDKD;1 and CDKD;3 function as CAKs controlling mitosis, whereas CDKF;1 plays a distinct role, mainly in post‐embryonic development. We propose that CDKD;1 and CDKD;3 phosphorylate and activate all core CDKs, CDKA, CDKB1 and CDKB2, thereby governing cell cycle progression throughout plant development.  相似文献   
997.
To develop conducting organic polymers (COPs) as luminescent sensors for determination of toxic heavy metals, a new benzene sulfonic acid‐doped polypyrrole (PPy‐BSA) thin film was electrochemically prepared by cyclic voltammetry (CV) on flexible indium tin oxide (ITO) electrode in aqueous solution. PPy‐BSA film was characterized by FTIR spectrometry, X‐ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The optical properties of PPy‐BSA were investigated by ultraviolet (UV)‐visible absorption and fluorescence spectrometry in dimethylsulfoxide (DMSO) diluted solutions. PPy‐BSA fluorescence spectra were strongly quenched upon increasing copper(II) ion (Cu2+) and lead(II) ion (Pb2+) concentrations in aqueous medium, and linear Stern–Volmer relationships were obtained, which indicated the existence of a main dynamic fluorescence quenching mechanism. BSA‐PPy sensor showed a high sensitivity for detection of both metallic ions, Cu2+ and Pb2+, with very low limit of detection values of 3.1 and 18.0 nM, respectively. The proposed quenching‐fluorimetric sensor might be applied to the determination of traces of toxic heavy metallic ions in water samples.  相似文献   
998.
As the importance of personalized therapeutics in aggressive papillary thyroid cancer (PTC) increases, accurate risk stratification is required. To develop a novel prognostic scoring system for patients with PTC (n = 455), we used mRNA expression and clinical data from The Cancer Genome Atlas. We performed variable selection using Network‐Regularized high‐dimensional Cox‐regression with gene network from pathway databases. The risk score was calculated using a linear combination of regression coefficients and mRNA expressions. The risk score and clinical variables were assessed by several survival analyses. The risk score showed high discriminatory power for the prediction of event‐free survival as well as the presence of metastasis. In multivariate analysis, the risk score and presence of metastasis were significant risk factors among the clinical variables that were examined together. In the current study, we developed a risk scoring system that will help to identify suitable therapeutic options for PTC.  相似文献   
999.
In February 2018, the Melanoma Research Foundation and the Moffitt Cancer Center hosted the Second Summit on Melanoma Central Nervous System (CNS) Metastases in Tampa, Florida. In this white paper, we outline the current status of basic science, translational, and clinical research into melanoma brain metastasis development and therapeutic management. We further outline the important challenges that remain for the field and the critical barriers that need to be overcome for continued progress to be made in this clinically difficult area.  相似文献   
1000.
The scope of morphological plasticity of vegetative structures among Podostemoideae (Podostemaceae) is documented for Crenias weddelliana, a neotropical species, Maferria indica, a palaeotropical species, and Sphaerothylax abyssinica, from Kenya, and compared with related taxa. The study highlights intrinsic characters of the widely enigmatic plant body of many species of the subfamily Podostemoideae. These include dorsiventrality of shoots occurring irrespective of gravity, incurvate distichy and one‐sided spirodistichy correlated with shoot dorsiventrality, asymmetric leaves, and several types of positioning of the two prophylls and inflorescence structures. The homogeneity of hairs of the ‘Zeylanidium olivaceum type’ established on the subulate leaves of some Indian species is of taxonomic value. The latter also applies to the stipella (not stipule) on the asymmetric compound leaf in New World species. The morphological data represent a framework of features consistent for the subfamily. © 2002 The Linnean Society of London, Botanical Journal of the Linnean Society, 138 , 63–84.  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