全文获取类型
收费全文 | 2317篇 |
免费 | 225篇 |
国内免费 | 1篇 |
专业分类
2543篇 |
出版年
2021年 | 31篇 |
2020年 | 24篇 |
2019年 | 29篇 |
2018年 | 32篇 |
2017年 | 16篇 |
2016年 | 43篇 |
2015年 | 87篇 |
2014年 | 106篇 |
2013年 | 134篇 |
2012年 | 115篇 |
2011年 | 128篇 |
2010年 | 86篇 |
2009年 | 65篇 |
2008年 | 108篇 |
2007年 | 115篇 |
2006年 | 100篇 |
2005年 | 116篇 |
2004年 | 102篇 |
2003年 | 85篇 |
2002年 | 83篇 |
2001年 | 42篇 |
2000年 | 36篇 |
1999年 | 42篇 |
1998年 | 32篇 |
1997年 | 18篇 |
1996年 | 24篇 |
1995年 | 20篇 |
1994年 | 17篇 |
1993年 | 27篇 |
1992年 | 37篇 |
1991年 | 22篇 |
1990年 | 18篇 |
1989年 | 26篇 |
1988年 | 23篇 |
1987年 | 15篇 |
1985年 | 21篇 |
1984年 | 19篇 |
1983年 | 23篇 |
1982年 | 28篇 |
1981年 | 24篇 |
1980年 | 28篇 |
1979年 | 22篇 |
1978年 | 22篇 |
1977年 | 20篇 |
1976年 | 15篇 |
1975年 | 15篇 |
1974年 | 18篇 |
1973年 | 18篇 |
1971年 | 25篇 |
1970年 | 14篇 |
排序方式: 共有2543条查询结果,搜索用时 15 毫秒
951.
Hannah Haydock Leon Clarke Elizabeth Craig‐Atkins Rachel Howcroft Jo Buckberry 《American journal of physical anthropology》2013,151(4):604-612
This study investigated stable‐isotope ratio evidence of weaning for the late Anglo‐Saxon population of Raunds Furnells, Northamptonshire, UK. δ15N and δ13C values in rib collagen were obtained for individuals of different ages to assess the weaning age of infants within the population. A peak in δ15N values at about 2‐year‐old, followed by a decline in δ15N values until age three, indicates a change in diet at that age. This change in nitrogen isotope ratios corresponds with the mortality profile from the site, as well as with archaeological and documentary evidence on attitudes towards juveniles in the Anglo‐Saxon period. The pattern of δ13C values was less clear. Comparison of the predicted age of weaning to published data from sites dating from the Iron Age to the 19th century in Britain reveals a pattern of changing weaning practices over time, with increasingly earlier commencement and shorter periods of complementary feeding in more recent periods. Such a change has implications for the interpretation of socioeconomic changes during this period of British history, since earlier weaning is associated with decreased birth spacing, and could thus have contributed to population growth. Am J Phys Anthropol 151:604–612, 2013. © 2013 Wiley Periodicals, Inc. 相似文献
952.
Teresa B. De Leon Elizabeth Karn Kassim Al‐Khatib Luis Espino Timothy Blank Cynthia B. Andaya Virgilio C. Andaya Whitney Brim‐DeForest 《Ecology and evolution》2019,9(10):5835-5848
Control of weeds in cultivated crops is a pivotal component in successful crop production allowing higher yield and higher quality. In rice‐growing regions worldwide, weedy rice (Oryza sativa f. spontanea Rosh.) is a weed related to cultivated rice which infests rice fields. With populations across the globe evolving a suite of phenotypic traits characteristic of weeds and of cultivated rice, varying hypotheses exist on the origin of weedy rice. Here, we investigated the genetic diversity and possible origin of weedy rice in California using 98 simple sequence repeat (SSR) markers and an Rc gene‐specific marker. By employing phylogenetic clustering analysis, we show that four to five genetically distinct biotypes of weedy rice exist in California. Analysis of population structure and genetic distance among individuals reveals diverse evolutionary origins of California weedy rice biotypes, with ancestry derived from indica, aus, and japonica cultivated rice as well as possible contributions from weedy rice from the southern United States and wild rice. Because this diverse parentage primarily consists of weedy, wild, and cultivated rice not found in California, most existing weedy rice biotypes likely originated outside California. 相似文献
953.
Pathway of Salicylic Acid Biosynthesis in Healthy and Virus-Inoculated Tobacco 总被引:19,自引:10,他引:19 下载免费PDF全文
Salicylic acid (SA) is a likely endogenous regulator of localized and systemic disease resistance in plants. During the hypersensitive response of Nicotiana tabacum L. cv Xanthi-nc to tobacco mosaic virus (TMV), SA levels rise dramatically. We studied SA biosynthesis in healthy and TMV-inoculated tobacco by monitoring the levels of SA and its likely precursors in extracts of leaves and cell suspensions. In TMV-inoculated leaves, stimulation of SA accumulation is accompanied by a corresponding increase in the levels of benzoic acid. 14C-Tracer studies with cell suspensions and mock-or TMV-inoculated leaves indicate that the label moves from trans-cinnamic acid to SA via benzoic acid. In healthy and TMV-inoculated tobacco leaves, benzoic acid induced SA accumulation. o-Coumaric acid, which was previously reported as a possible precursor of SA in other species, did not increase SA levels in tobacco. In healthy tobacco tissue, the specific activity of newly formed SA was equal to that of the supplied [14C]benzoic acid, whereas in TMV-inoculated leaves some isotope dilution was observed, presumably because of the increase in the pool of endogenous benzoic acid. We observed accumulation of pathogen-esis-related-1 proteins and increased resistance to TMV in benzoic acid- but not in o-coumaric acid-treated tobacco leaves. This is consistent with benzoic acid being the immediate precursor of SA. We conclude that in healthy and virus-inoculated tobacco, SA is formed from cinnamic acid via benzoic acid. 相似文献
954.
Kochian Leon V. Pence Nicole S. Letham Deborah L.D. Pineros Miguel A. Magalhaes Jurandir V. Hoekenga Owen A. Garvin David F. 《Plant and Soil》2002,247(1):109-119
Plants have evolved sophisticated mechanisms to deal with toxic levels of metals in the soil. In this paper, an overview of recent progress with regards to understanding fundamental molecular and physiological mechanisms underlying plant resistance to both aluminum (Al) and heavy metals is presented. The discussion of plant Al resistance will focus on recent advances in our understanding of a mechanism based on Al exclusion from the root apex, which is facilitated by Al-activated exudation of organic acid anions. The consideration of heavy metal resistance will focus on research into a metal hyperaccumulating plant species, the Zn/Cd hyperaccumulator, Thlaspi caerulescens, as an example for plant heavy metal research. Based on the specific cases considered in this paper, it appears that quite different strategies are used for Al and heavy metal resistance. For Al, our current understanding of a resistance mechanism based on excluding soil-borne Al from the root apex is presented. For heavy metals, a totally different strategy based on extreme tolerance and metal hyperaccumulation is described for a hyperaccumulator plant species that has evolved on naturally metalliferous soils. The reason these two strategies are the focus of this paper is that, currently, they are the best understood mechanisms of metal resistance in terrestrial plants. However, it is likely that other mechanisms of Al and/or heavy metal resistance are also operating in certain plant species, and there may be common features shared for dealing with Al and heavy resistance. Future research may uncover a number of novel metal resistance mechanisms in plants. Certainly the complex genetics of Al resistance in some crop plant species, such as rice and maize, suggests that a number of presently unidentified mechanisms are part of an overall strategy of metal resistance in crop plants. 相似文献
955.
Sales AM Ponce de Leon A Düppre NC Hacker MA Nery JA Sarno EN Penna ML 《PLoS neglected tropical diseases》2011,5(3):e1013
Background
This study aimed to evaluate the risk factors associated with developing leprosy among the contacts of newly-diagnosed leprosy patients.Methodology/Principal Findings
A total of 6,158 contacts and 1,201 leprosy patients of the cohort who were diagnosed and treated at the Leprosy Laboratory of Fiocruz from 1987 to 2007 were included. The contact variables analyzed were sex; age; educational and income levels; blood relationship, if any, to the index case; household or non-household relationship; length of time of close association with the index case; receipt of bacillus Calmette-Guérin (BGG) vaccine and presence of BCG scar. Index cases variables included sex, age, educational level, family size, bacillary load, and disability grade. Multilevel logistic regression with random intercept was applied. Among the co-prevalent cases, the leprosy-related variables that remained associated with leprosy included type of household contact, [odds ratio (OR) = 1.33, 95% confidence interval (CI): 1.02, 1.73] and consanguinity with the index case, (OR = 1.89, 95% CI: 1.42–2.51). With respect to the index case variables, the factors associated with leprosy among contacts included up to 4 years of schooling and 4 to 10 years of schooling (OR = 2.72, 95% CI: 1.54–4.79 and 2.40, 95% CI: 1.30–4.42, respectively) and bacillary load, which increased the chance of leprosy among multibacillary contacts for those with a bacillary index of one to three and greater than three (OR = 1.79, 95% CI: 1.19–2.17 and OR: 4.07–95% CI: 2.73, 6.09), respectively. Among incident cases, household exposure was associated with leprosy (OR = 1.96, 95% CI: 1.29–2.98), compared with non-household exposure. Among the index case risk factors, an elevated bacillary load was the only variable associated with leprosy in the contacts.Conclusions/Significance
Biological and social factors appear to be associated with leprosy among co-prevalent cases, whereas the factors related to the infectious load and proximity with the index case were associated with leprosy that appeared in the incident cases during follow-up. 相似文献956.
Yang XY Yang JL Zhou Y Piñeros MA Kochian LV Li GX Zheng SJ 《Plant, cell & environment》2011,34(12):2138-2148
Al-activated organic acid anion efflux from roots is an important Al resistance mechanism in plants. We have conducted homologous cloning and isolated Vigna umbellata multidrug and toxic compound extrusion (VuMATE), a gene encoding a de novo citrate transporter from rice bean. Al treatment up-regulated VuMATE expression in the root apex, but neither in the mature root region nor in the leaf. The degree of up-regulation of VuMATE was both partially Al concentration and time dependent, consistent with the delay in the onset of the Al-induced citrate efflux in rice bean roots. While La(3+) moderately induced VuMATE expression, Cd(2+) and Cu(2+) did not induce the expression. Electrophysiological analysis of Xenopus oocytes expressing VuMATE indicated this transporter can mediate significant anion efflux across the plasma membrane. [(14) C]citrate efflux experiments in oocytes demonstrated that VuMATE is a H(+) -dependent citrate transporter. In addition, expression of VuMATE in transgenic tomato resulted in increased Al resistance, which correlated with an enhanced citrate efflux. Taken together, these findings suggest that VuMATE is a functional homolog of the known citrate transporters in sorghum, barley, maize and Arabidopsis. The similarities and differences of all the known citrate transporters associated with Al stress in the MATE family are also discussed. 相似文献
957.
Katherine E. Mues Michael Deming David G. Kleinbaum Philip J. Budge Mitch Klein Juan S. Leon Aishya Prakash Jonathan Rout LeAnne M. Fox 《PLoS neglected tropical diseases》2014,8(9)
Background
Lymphedema management programs have been shown to decrease episodes of adenolymphangitis (ADLA), but the impact on lymphedema progression and of program compliance have not been thoroughly explored. Our objectives were to determine the rate of ADLA episodes and lymphedema progression over time for patients enrolled in a community-based lymphedema management program. We explored the association between program compliance and ADLA episodes as well as lymphedema progression.Methodology/Principal Findings
A lymphedema management program was implemented in Odisha State, India from 2007–2010 by the non-governmental organization, Church''s Auxiliary for Social Action, in consultation with the Centers for Disease Control and Prevention. A cohort of patients was followed over 24 months. The crude 30-day rate of ADLA episodes decreased from 0.35 episodes per person-month at baseline to 0.23 at 24 months. Over the study period, the percentage of patients who progressed to more severe lymphedema decreased (P-value = 0.0004), while those whose lymphedema regressed increased over time (P-value<0.0001). Overall compliance to lymphedema management, lagged one time point, appeared to have little to no association with the frequency of ADLA episodes among those without entry lesions (RR = 0.87 (0.69, 1.10)) and was associated with an increased rate (RR = 1.44 (1.11, 1.86)) among those with entry lesions. Lagging compliance two time points, it was associated with a decrease in the rate of ADLA episodes among those with entry lesions (RR = 0.77 (95% CI: 0.59, 0.99)) and was somewhat associated among those without entry lesions (RR = 0.83 (95% CI: 0.64, 1.06)). Compliance to soap was associated with a decreased rate of ADLA episodes among those without inter-digital entry lesions.Conclusions/Significance
These results indicate that a community-based lymphedema management program is beneficial for lymphedema patients for both ADLA episodes and lymphedema. It is one of the first studies to demonstrate an association between program compliance and rate of ADLA episodes. 相似文献958.
959.
Vernica A. Segarra Clara Primus Graciela A. Unguez Ashanti Edwards Candice Etson Sonia C. Flores Catherine Fry Ashley N. Guillory Susan L. Ingram Mark Lawson Richard McGee Stephanie Paxson Laura Phelan Kirsta Suggs Leticia R. Vega Elizabeth Vuong J. Christopher Havran Alfredo Leon Michael D. Burton J. Luis Lujan Marina Ramirez-Alvarado 《Molecular biology of the cell》2020,31(23):2495
Scientific societies aiming to foster inclusion of scientists from underrepresented (UR) backgrounds among their membership often delegate primary responsibility for this goal to a diversity-focused committee. The National Science Foundation has funded the creation of the Alliance to Catalyze Change for Equity in STEM Success (ACCESS), a meta-organization bringing together representatives from several such STEM society committees to serve as a hub for a growing community of practice. Our goal is to coordinate efforts to advance inclusive practices by sharing experiences and making synergistic discoveries about what works. ACCESS has analyzed the approaches by which member societies have sought to ensure inclusivity through selection of annual meeting speakers. Here we discuss how inclusive speaker selection fosters better scientific environments for all and identify challenges and promising practices for societies striving to maximize inclusivity of speakers in their scientific programming. 相似文献
960.
Leon W. Bone 《Invertebrate reproduction & development.》2013,57(6):311-321
The literature suggests that chemical communication by primer and releaser pheromones may be extensive within the helminths. Although our detailed knowledge of these events is sparse, the number of reports indicates that a chemical mediation of developmental, reproductive, and behavioral processes may be mandatory in the nematodes, acanthocephala, and other aschelminths. Accordingly, this review presents a summary of our current status. 相似文献