全文获取类型
收费全文 | 497篇 |
免费 | 20篇 |
专业分类
517篇 |
出版年
2023年 | 7篇 |
2021年 | 4篇 |
2020年 | 11篇 |
2019年 | 19篇 |
2018年 | 13篇 |
2017年 | 18篇 |
2016年 | 24篇 |
2015年 | 10篇 |
2014年 | 9篇 |
2013年 | 118篇 |
2012年 | 21篇 |
2011年 | 17篇 |
2010年 | 14篇 |
2009年 | 21篇 |
2008年 | 9篇 |
2007年 | 11篇 |
2006年 | 7篇 |
2005年 | 7篇 |
2004年 | 12篇 |
2003年 | 4篇 |
2002年 | 8篇 |
2001年 | 6篇 |
2000年 | 3篇 |
1999年 | 12篇 |
1998年 | 5篇 |
1997年 | 5篇 |
1996年 | 4篇 |
1995年 | 10篇 |
1994年 | 6篇 |
1993年 | 4篇 |
1991年 | 5篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 7篇 |
1983年 | 10篇 |
1982年 | 6篇 |
1981年 | 2篇 |
1980年 | 5篇 |
1979年 | 8篇 |
1978年 | 6篇 |
1977年 | 3篇 |
1976年 | 7篇 |
1975年 | 6篇 |
1973年 | 3篇 |
1972年 | 4篇 |
1971年 | 2篇 |
1970年 | 2篇 |
1965年 | 2篇 |
排序方式: 共有517条查询结果,搜索用时 0 毫秒
101.
The effect of charring and burial on the biochemical composition of cereal grains: investigating the integrity of archaeological plant material 总被引:1,自引:0,他引:1
A.K. Styring H. Manning R.A. Fraser M. Wallace G. Jones M. Charles T.H.E. Heaton A. Bogaard R.P. Evershed 《Journal of archaeological science》2013
Stable isotope analysis of charred archaeobotanical cereal grains has the potential to provide direct evidence of crop growing conditions in the past and to refine palaeodietary predictions. If isotope values of archaeobotanical material are to be considered robust, it is necessary to characterise the compositional changes associated with their charring and burial. This study used a suite of analytical techniques, including FT-IR and solid state 13C NMR, to characterise changes in the biochemical composition of modern einkorn grains with heating at 230 °C for 2 h, 4 h, 8 h and 24 h, encompassing conditions that replicate their undistorted ancient counterparts. The biochemical composition of archaeobotanical charred einkorn grains was also investigated by FT-IR and solid state 13C NMR in order to assess the changes in composition which occur during burial. Results of FT-IR and solid-state 13C NMR show that heating of modern einkorn grains resulted in Maillard reactions between cereal proteins and starch, forming high molecular weight melanoidins, which contain both alkyl and aromatic carbon. Loss of low molecular weight carbon and nitrogen-containing volatiles resulted in a slight but non-systematic increase in the δ13C values and a systematic increase of 0.8‰ in the δ15N values of the charred einkorn grains. Solid-state 13C NMR shows that the ancient charred einkorn grains consisted entirely of aromatic carbon and retained a similar proportion of nitrogen to their modern 24 h charred counterparts, despite a significantly lower concentration of amino acids. This indicates that the amino acid nitrogen in the ancient charred grains was retained in the stable melanoidins whose polymeric structure makes them resistant to subsequent degradation. 相似文献
102.
Donald W. Jones 《Journal of regional science》1988,28(3):317-327
ABSTRACT. Previous work has developed a method for studying noninfinitesimal operational units, called “plantations,” with the Thüen model. In that model, increasing returns to scale generate large operational units, but the potential market power conferred by the scale economies is sidestepped as an issue. The present work introduces finite supply elasticities and examines their locational impacts. The profit-maximizing monopsonistic plantation is smaller, and the shipment distance for its processed output is shorter, than for a comparable competitive plantation. The present approach does not involve spatial competition strategies. 相似文献
103.
104.
105.
Ron Johnston Charles Pattie Todd Hartman David Manley David Rossiter Kelvyn Jones 《Scottish Geographical Journal》2018,134(1-2):24-38
The rapid expansion in support for the Scottish National Party (SNP) between the 2010 and 2015 general elections substantially changed the country’s electoral geography, as again did its relative decline at the next election in 2017. At that last contest, however, the SNP won many seats with fewer than 40% of the votes cast, a situation very different from that in the rest of Great Britain. That difference – which had a considerable impact on the formation of a government in June 2017 – came about because of the nature of the competition in individual seats. 相似文献
106.
Jennifer E. Jones 《Conservation and Management of Archaeological Sites》2018,20(3):143-164
The archaeological remains of ships in the beach zone are part of a complex and dynamic system. Being periodically exposed and reburied, they vary between being both visible and frequently forgotten features of the physical and cultural coastal landscape. These limited and non-renewable resources are highly susceptible to instability within the landscape. Challenges to stability result in these resources being damaged, ignored, or forgotten, leading to a potential loss of pertinent social, economic, and physical information. Although little can be done to prevent certain physical actions, a better understanding of physical and cultural processes allows for their mitigation and better management practices for the beached shipwreck resource. This paper looks at how stability is defined in relation to beached shipwreck sites and how these definitions impact the management of the resource of two case study sites in Maine and North Carolina. 相似文献
107.
An analysis of Forest Service-related legislative activity found that increasingly Congress is becoming involved in agency activities and decisionmaking. Congress's growing scrutiny and management of the Forest Service is indicated by rising trends in requests for agency testimony at congressional hearings, in the number of Forest Service-related bills and amendments introduced in Congress, and in the amount of Forest Service-related legislation enacted. Anecdotal evidence of direct political pressure imposed upon agency personnel by congressional members also indicates that much congressional influence occurs outside of these legislative activities. An examination of activities such as these and directives attached to annual appropriation bills also shows that not all congressional actions are productive in fostering positive agency change. The results of this study indicate the likelihood that Congress has been playing a major role in instigating change within the agency. 相似文献
108.
Interest groups involved in the Forest Service's 1990 Resources Planning Act (RPA) forest planning process and representatives from accredited forestry schools were surveyed via a written questionnaire on their perceptions of change within the Forest Service since 1976. Respondents felt that the agency had changed, particularly during the late 1980s and early 1990s. Consumptive users perceived the change to be negative, while non-consumptive user groups and forestry schools felt it was somewhat positive, although they still felt the Forest Service had much further to go in this direction before achieving these groups' desired priorities for National Forest management. These results indicate that the agency is undergoing a net shift in direction towards increased emphasis on non-commodity forest uses. In general, external forces, such as Congress and the courts, were seen as having a greater impact on agency change than were sources of change within the agency itself. Forestry schools agreed most often with non-consumptive users, indicating that these programs may be undergoing a change away from their traditional focus on timber production. In addition to providing valuable insights on the subject of change in the Forest Service, these survey results locate areas of agreement and disagreement, and provide valuable feedback to the agency regarding its constituent groups' views about its performance in managing the National Forests. 相似文献
109.
Contingency,Realism, and the Expansion Method 总被引:1,自引:0,他引:1
This paper examines the relationship between scientific realism and the expansion method. The frame of reference for our investigation is their joint conceptualization of “contingency.” We first explore the varied historical meanings of contingency; this is followed by a survey of its differential deployment within geographic thought. Attention then turns to theorizations of contingency in realism and the expansion method. The final portion of the paper assesses the potential for integrating these two analytic traditions within geographic research. 相似文献
110.
With states making more substantive decisions in technically complex policy areas, the capacity of state legislatures to gather and use technical information and analysis is crucial. Applying a "market" metaphor, the article examines the demand for and the supply of technical information and analysis in state legislatures. The demand among legislators and staff is widespread and increasing, and the supply is varied in origin, importance, and quality. Reliance on sources external to the legislature—usually perceived as biased—is related to a lack of professionalization in legislatures, although the level of satisfaction in the available technical information and analysis is not. Identifiable barriers on both the demand side and the supply side prevent better use of technical information and analysis. There are ways to strengthen the relationship between state legislatures and a broad array of sources, as well as to educate the legislative consumer of technical information and analysis. 相似文献