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   36837篇
  免费   2507篇
  国内免费   5188篇
  2024年   67篇
  2023年   1557篇
  2022年   1836篇
  2021年   2090篇
  2020年   1973篇
  2019年   1631篇
  2018年   1849篇
  2017年   1769篇
  2016年   1921篇
  2015年   2554篇
  2014年   3421篇
  2013年   3117篇
  2012年   2834篇
  2011年   2604篇
  2010年   2512篇
  2009年   2491篇
  2008年   1570篇
  2007年   1778篇
  2006年   1461篇
  2005年   930篇
  2004年   579篇
  2003年   447篇
  2002年   385篇
  2001年   347篇
  2000年   286篇
  1999年   318篇
  1998年   208篇
  1997年   165篇
  1996年   199篇
  1995年   178篇
  1994年   156篇
  1993年   157篇
  1992年   145篇
  1991年   172篇
  1990年   132篇
  1989年   127篇
  1988年   126篇
  1987年   29篇
  1986年   19篇
  1985年   26篇
  1984年   26篇
  1983年   56篇
  1982年   19篇
  1981年   28篇
  1980年   39篇
  1979年   36篇
  1978年   18篇
  1977年   26篇
  1976年   33篇
  1974年   23篇
排序方式: 共有10000条查询结果,搜索用时 669 毫秒
1.
《Cell reports》2020,30(4):1152-1163.e4
  1. Download : Download high-res image (170KB)
  2. Download : Download full-size image
  相似文献   
2.
Plant somatic cells have the capability to switch their cell fates from differentiated to undifferentiated status under proper culture conditions, which is designated as totipotency. As a result, plant cells can easily regenerate new tissues or organs from a wide variety of explants. However, the mechanism by which plant cells have such remarkable regeneration ability is still largely unknown. In this study, we used a set of meristem-specific marker genes to analyze the patterns of stem cell differentiation in the processes of somatic embryogenesis as well as shoot or root organogenesis in vitro. Our studies furnish preliminary and important information on the patterns of the de novo stem cell differentiation during various types of in vitro organogenesis.  相似文献   
3.
BackgroundThe utilization of traditional Chinese medicine is a common therapeutic approach for stroke patients in Chinese population, but little is known about the effect of Bu Yang Huan Wu Tang (BYHWT) on post-stroke diabetes.PurposeWe aimed to evaluate the risk of diabetes in stroke patients who used BYHWT.Study designA retrospective cohort study based on a real-world database was conducted.MethodsNewly diagnosed stroke patients receiving inpatient care from 2000 to 2004 were identified using a large-scale insurance database in Taiwan. Propensity score matching was used to select eligible stroke patients who did (n = 9849) and did not (n = 9849) receive BYHWT. These two groups were followed up until the end of 2009 to track incident diabetes. Cox proportional hazard models were used to calculate the adjusted hazard rations (HRs) and 95% confidence intervals (CIs) for post-stroke diabetes associated with BYHWT during the follow-up period.ResultsStroke patients who used BYHWT had a reduced incidence of diabetes (14.1% vs. 19.0%, p < 0.0001) and reduced risk of diabetes (HR 0.77; 95% CI 0.72 to 0.83) compared with the control group. The association between BYHWT and reduced risk of post-stroke diabetes was significant across sexe, age group, and stroke subtype. Additionally, the use of BYHWT was associated with a reduced risk of post-stroke diabetes even after excluding the initial three months of diabetes cases in the sensitivity analysis.ConclusionsStroke patients who received BYHWT therapy had a reduced risk of diabetes, and a positive effect was observed in various subgroups. However, future clinical trials will be necessary to validate the present findings and identify the biochemical mechanism involved.  相似文献   
4.
Stability and procured instability characterize two opposing types of swimming, steady and maneuvering, respectively. Fins can be used to manipulate flow to adjust stability during swimming maneuvers either actively using muscle control or passively by structural control. The function of the dorsal fins during turning maneuvering in two shark species with different swimming modes is investigated here using musculoskeletal anatomy and muscle function. White‐spotted bamboo sharks are a benthic species that inhabits complex reef habitats and thus have high requirements for maneuverability. Spiny dogfish occupy a variety of coastal and continental shelf habitats and spend relatively more time cruising in open water. These species differ in dorsal fin morphology and fin position along the body. Bamboo sharks have a larger second dorsal fin area and proportionally more muscle insertion into both dorsal fins. The basal and radial pterygiophores are plate‐like structures in spiny dogfish and are nearly indistinguishable from one another. In contrast, bamboo sharks lack basal pterygiophores, while the radial pterygiophores form two rows of elongated rectangular elements that articulate with one another. The dorsal fin muscles are composed of a large muscle mass that extends over the ceratotrichia overlying the radials in spiny dogfish. However, in bamboo sharks, the muscle mass is divided into multiple distinct muscles that insert onto the ceratotrichia. During turning maneuvers, the dorsal fin muscles are active in both species with no differences in onset between fin sides. Spiny dogfish have longer burst durations on the outer fin side, which is consistent with opposing resistance to the medium. In bamboo sharks, bilateral activation of the dorsal in muscles could also be stiffening the fin throughout the turn. Thus, dogfish sharks passively stiffen the dorsal fin structurally and functionally, while bamboo sharks have more flexible dorsal fins, which result from a steady swimming trade off. J. Morphol. 274:1288–1298, 2013. © 2013 Wiley Periodicals, Inc.  相似文献   
5.
The functioning of the vertebrate eye depends on its absolute size, which is presumably adapted to specific needs. Eye size variation in lidless and spectacled colubrid snakes was investigated, including 839 specimens belonging to 49 genera, 66 species and subspecies. Variations of adult eye diameters (EDs) in both absolute and relative terms between species were correlated with parameters reflecting behavioral ecology. In absolute terms, eye of arboreal species was larger than in terrestrial and semiaquatic species. For diurnal species, EDs of terrestrial species do not differ from semiaquatic species; for nocturnal species the ED of terrestrial species is larger than fossorial species but not different from semiaquatic species. In relative terms, ED did not differ significantly by habitat for diurnal species. Although the ED of terrestrial species is larger than fossorial species there were no differences for nocturnal species between semiaquatic and fossorial snakes. In contrast to other vertebrates studied to date, colubrid EDs in absolute and relative terms are larger in diurnal than in nocturnal species. These observations suggest that among colubrid snakes, eye size variation reflects adaptation to specific habitats, foraging strategies and daily activities, independently of phylogeny. J. Morphol. 2012. © 2012 Wiley Periodicals, Inc.  相似文献   
6.
Cripto-1 is a protein participating in tissue orientation during embryogenesis but has also been implicated in a wide variety of cancers, such as colon, lung and breast cancer. Cripto-1 plays a role in the regulation of different pathways, including TGF-β/Smad and Wnt/β-catenin, which are highly associated with cell migration both during embryonal development and cancer progression. Little is known about the detailed subcellular localization of cripto-1 and how it participates in the directional movement of cells. In this study, the subcellular localization of cripto-1 in glioblastoma cells was investigated in vitro with high-resolution microscopy techniques. Cripto-1 was found to be localized to dynamic and shed filopodia and transported between cells through tunneling nanotubes. Our results connect the refined subcellular localization of cripto-1 to its functions in cellular orientation and migration.  相似文献   
7.
Lauraceae and Fagaceae are two large woody plant families that are predominant in the low- and middle-altitude regions in Taiwan. The highly interspecific similarity between some species of the family brings limitations on the management and utilization. This work proposed an approach for identifying 15 Lauraceae species and 20 Fagaceae species using leaf images and convolutional neural networks (CNNs). Leaf specimens of 35 species were collected from the northern, central, and southern parts of Taiwan. Images of the leaves were acquired using flat-bed scanners. Three CNN architectures—DenseNet-121, MobileNet V2, and Xception—were trained. Xception achieved the highest mean test accuracy of 99.39%, and MobileNet V2 required the shortest mean test time of 17.1 ms per image using a GPU. The saliency maps revealed that the characteristics learned by models matched the leaf features used by botanists. A pruning algorithm, gate decorator, was applied to the trained models for reducing the number of parameters and number of floating-point operations of the MobileNet V2 by 55.4% and 69.1%, respectively, while the model accuracy was maintained at 92.03%. Thus, MobileNet V2 has the potential to be used for identifying the Lauraceae and Fagaceae species on mobile devices.  相似文献   
8.
Escherichia coli strains MC4100 (parent) and a mutant strain derived from this (IC007) were evaluated for their ability to produce H2 and organic acids (OAs) via fermentation. Following growth, each strain was coated with Pd(0) via bioreduction of Pd(II). Dried, sintered Pd-biomaterials (‘Bio-Pd’) were tested as anodes in a proton exchange membrane (PEM) fuel cell for their ability to generate electricity from H2. Both strains produced hydrogen and OAs but ‘palladised’ cells of strain IC007 (Bio-PdIC007) produced ~threefold more power as compared to Bio-PdMC4100 (56 and 18 mW respectively). The power output used, for comparison, commercial Pd(0) powder and Bio-Pd made from Desulfovibrio desulfuricans, was ~100 mW. The implications of these findings for an integrated energy generating process are discussed.  相似文献   
9.
Implications of Two Different Types of Diffusion for Biological Membranes   总被引:1,自引:0,他引:1  
AS it is not widely appreciated that diffusion within complex media can be strikingly and often qualitatively different from that in simple liquids such as water, there is confusion concerning transport processes across biological membranes1,2. We would like to draw attention to some fundamental differences between the diffusion process in simple liquids and that in more complex media-non-porous networks of hydrophobic polymers and biological membranes.  相似文献   
10.
Increased levels of several human ubiquitin ligases, including ring finger protein 123 (RNF123), in red blood cells with Plasmodium falciparum infection, have been reported. RNF123 is an E3 ubiquitin ligase that is highly expressed in erythroid cells. However, the function of the RNF123 gene and the relationship between the RNF123 gene and malarial parasite has not been clarified in vivo. In this study, we generated RNF123-deficient mice using the CRISPR/Cas9 system, and analyzed malaria susceptibility and erythrocyte morphology. The levels of parasitemia 5 days post-infection and mortality 21 days post-infection with the lethal type of rodent malaria (Plasmodium yoelii 17XL) in RNF123-deficient mice was significantly lower than that in wild-type mice. In contrast, red blood cell morphology in RNF123-deficient mice was almost normal. These results suggest that erythrocytic RNF123 plays a role in susceptibility to rodent malaria infection, but does not play a role in erythrocyte morphology.  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