全文获取类型
收费全文 | 606605篇 |
免费 | 74019篇 |
国内免费 | 218篇 |
专业分类
680842篇 |
出版年
2018年 | 5359篇 |
2016年 | 7412篇 |
2015年 | 11051篇 |
2014年 | 12989篇 |
2013年 | 17720篇 |
2012年 | 20553篇 |
2011年 | 20513篇 |
2010年 | 13401篇 |
2009年 | 12705篇 |
2008年 | 18176篇 |
2007年 | 18946篇 |
2006年 | 17562篇 |
2005年 | 17080篇 |
2004年 | 16880篇 |
2003年 | 16240篇 |
2002年 | 15587篇 |
2001年 | 23533篇 |
2000年 | 23855篇 |
1999年 | 19502篇 |
1998年 | 7774篇 |
1997年 | 7819篇 |
1996年 | 7539篇 |
1995年 | 7240篇 |
1994年 | 7121篇 |
1993年 | 7115篇 |
1992年 | 16821篇 |
1991年 | 16620篇 |
1990年 | 16303篇 |
1989年 | 15863篇 |
1988年 | 14811篇 |
1987年 | 14215篇 |
1986年 | 13302篇 |
1985年 | 13589篇 |
1984年 | 11574篇 |
1983年 | 10243篇 |
1982年 | 8089篇 |
1981年 | 7606篇 |
1980年 | 7176篇 |
1979年 | 11318篇 |
1978年 | 8945篇 |
1977年 | 8207篇 |
1976年 | 7803篇 |
1975年 | 8539篇 |
1974年 | 8878篇 |
1973年 | 8715篇 |
1972年 | 8157篇 |
1971年 | 7096篇 |
1970年 | 6212篇 |
1969年 | 5877篇 |
1968年 | 5324篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
161.
David A. Kideckel 《Dialectical Anthropology》1987,12(4):399-411
Conclusion To summarize the uses of economic images in the socialist transformation, change in local and state forms both modeled changes in national class relations and sought to shape that same process. Clearly, though, state images have triumphed for the moment. Though first less than compelling when opposed to the symbolically expressed solidarity of the local community, they have since come to dominate the ideological evaluations of local workers. Simultaneously the local community witnessed a massive social transformation where such unity as it existed, however artificial, was replaced by a more atomized social structure partially eroded by the promise of material plenty.Thus, at first, state images assisted the formation of the Romanian proletariate as a class in itself. Today, however, with political stability generally assured and increased production an absolute necessity, one can observe the reformulation of class boundaries via state-promulgated differentiation as well as the erosion of working class consciousness, class for itself, under consumerist pressures.Workers' solidarity, purposefully engendered in post-war state images, was rarely achieved in the Fgra region. Since the mid-1960s the expansion of consumerist images especially prevents its formation. Over time the counterhegemonic qualities of local images are replaced by those which accept the socialist system and question only certain of its material results. Until the recent debt crisis local images have been hauntingly silent about production relations.This need not be permanent. The disaggregation of the local community, assisted by state image manipulaton, is a historical moment. The hegemony of state iconography also carries the possibility of its own dissolution. The spectra of Romania as socialist and luxurious may yet foment worker solidarity due to unfulfilled expectations. As the debt crisis deepens, dependency intensifies, and workers experience more of the rationing and shortages common of late, the legitimacy of Romania's hyper-industrialization may once again be questioned.It is in great measure through economic iconography that Romania's socialist governments have legitimized themselves. As hierarchy is elaborated and on-line production relations made stringent to address the current fiscal crisis, it is through industry, as expressed in local folk images, that such legitimacy may be undermined.David A. Kideckel is Associate Professor of Anthropology, Central Connecticut State University. 相似文献
162.
Expression of a beta thalassemia gene with abnormal splicing. 总被引:3,自引:1,他引:2
C Lapoumeroulie S Acuto F Rouabhi D Labie R Krishnamoorthy A Bank 《Nucleic acids research》1987,15(20):8195-8204
Expression of a cloned human beta thalassemia gene with a single base change at position 5 of IVS 1 has been analyzed 48 hours after transfer of the gene into HeLa cells (transient expression). Little or no normal beta globin mRNA accumulates in the presence of the abnormal beta gene in contrast to significantly more normal beta mRNA produced with other mutations at this same position. By contrast, large amounts of an abnormal beta globin mRNA are present; this is due to the use of a cryptic 5' splice site in exon 1 rather than the normal 5' splice site of IVS 1. The results indicate the variability of the effect on RNA splicing of different single base defects within IVS. 相似文献
163.
A transport resistance model for shootroot partitioningby Thornley and a more aggregated partitioning model by Reynoldsand Thornley are compared. Three functional forms of substrateutilization are applied, corresponding to different assumptionson the ability of carbon and nitrogen to compensate one anotherin promoting structural growth. On the basis of simulationsat balanced exponential growth, it is shown that the Reynoldsand Thornley model (in optimal form) is embedded in the Thornleymodel. Davidson's functional balance is studied as a functionof the degree of carbon-nitrogen compensation. The applicabilityof the models and the utilization functions is discussed. Model, shoot-root partitioning, substrate utilization, functional balance 相似文献
164.
165.
Cervical biopsy samples were taken from 79 patients who had various grades of cervical intraepithelial neoplasia or who showed evidence, in the form of koilocytosis, of human papillomavirus infection of the uterine cervix and from 10 women with normal cervices. The DNA content of the cells in the samples was analysed by flow cytometry. Analysis of the data obtained showed that the biopsy samples from women with cervical intraepithelial neoplasia and human papillomavirus lesions contained significantly more dividing cells (31.2% of cells from human papillomavirus lesions with no cervical intraepithelial neoplasia and 33.06%, 29.89%, and 31.76% of cells from cervical intraepithelial neoplasia grades I, II, and III, respectively) than those from women with normal cervices (21.6%). The proportion of aneuploid samples from the group who showed evidence of human papillomavirus infection only (18.2%) did not differ significantly from the group with cervical intraepithelial neoplasia grade III (21.2%). Aneuploidy and an increased rate of cellular proliferation are recognised characteristics of malignancy. These results therefore support the view that human papillomavirus plays an important part in the aetiology of cervical carcinoma and are relevant to the clinical management of patients. 相似文献
166.
The claim that list sizes in general practice should continue to fall towards a national average of 1700 patients rests heavily on the assumption that the extra time available to doctors would be used mainly for longer consultations, resulting in better standards of care. Evidence suggests, however, that the time is more likely to be used to increase rates of consultation in surgeries and home visits and to reduce the length of the working week. A national, random sample of 2104 principals in general practice in England and Wales were questioned about their allocation and use of time. The response rate was 67%, and no large biases in response were detected. The smaller their personal list size the less time general practitioners spent on all aspects of their work and the higher their rates of consultation and home visiting. The effects of further reductions in list sizes would be haphazard, being differentially distributed across the range of list sizes. Longer consultations would probably result, but most of the extra time would probably be used in higher rates of consultation in surgeries and home visits and some would be taken as free time. 相似文献
167.
Hydrolysis of N3-methyl-2'-deoxycytidine: model compound for reactivity of protonated cytosine residues in DNA 总被引:1,自引:0,他引:1
Protonation of cytosine residues at physiological pH may occur in DNA as a consequence of both alkylation and aberrant base-pair formation. When cytosine derivatives are protonated, they undergo hydrolysis reactions at elevated rates and can either deaminate to form the corresponding uracil derivatives or depyrimidinate generating abasic sites. The kinetic parameters for reaction of protonated cytosine are derived by studying the hydrolysis of N3-methyl-2'-deoxycytidine (m3dC), a cytosine analogue which is predominantly protonated at physiological pH. Both deamination and depyrimidimation reaction rates are shown to be linearly dependent upon the fraction of protonated molecules. We present here thermodynamic parameters which allow determination of hydrolysis rates of m3dC as functions of pH and temperature. Protonation of cytosine residues in DNA, as induced by aberrant base-pair formation or base modification, may accelerate the rate of both deamination and depyrimidation up to several thousand-fold under physiological conditions. 相似文献
168.
A new class of low molecular weight, aminomethylimidazol-4-one (IQ-"like") mutagens have been produced by the reaction of creatinine with the amino acid L-threonine, in liquid-reflux models, mimicking cooking, of diethylene glycol:5% distilled water (2 h at 150 degrees C). Two mutagens, 2-amino-1-methyl-5-propylideneimidazol-4-one (AMPI) and 2-amino-5-ethylidene-1-methylimidazol-4-one (AEMI) were isolated and characterized by UV absorption spectra, mass spectra, and 1H-NMR. The mutagen AEMI was identical to that obtained from the reaction of creatinine with acetaldehyde. These mutagens were positive in all IQ-sensitive Ames tester strains and were not inactivated by acidic nitrosation at pH 1.0. Products displaying mutagenicity were also obtained by refluxing creatinine with other hydroxyamino acids such as L-serine, L-homoserine, and L-4-amino-3-hydroxybutyric acid, and aldehydes such as glyoxal, methylglyoxal, glycolaldehyde, but not formaldehyde. Simple model systems such as creatinine and acetaldehyde may be useful in more clearly defining the exact mechanism of formation of IQ-type mutagens (aminomethylimidazo-quinolines and -quinoxalines) produced during cooking, as well as in screening for potential inhibitors of IQ-type mutagen formation, and elucidating the mechanism of such inhibition. 相似文献
169.
Palaeosols in Westphalian Coal-bearing and red-bed sequences, central and northern England 总被引:3,自引:0,他引:3
Westphalian A-C coal-bearing strata and red-beds in the Pennine Basin of Central and Northern England contain a wide variety of palaeosols. In coal-bearing facies associations alluvial, gley, semi-gley, organic and rare well-drained palaeosols reflect the predominance of poor drainage conditions during deposition. In red-beds more evolved ferruginous and ferallitic palaeosols testify to free drainage conditions during deposition. Catenary relationships can be inferred between most of the palaeosols in both facies associations, providing further information on the palaeogeomorphology of the depositional systems, and assisting in the understanding of the environments and conditions of coal seam formation. 相似文献
170.
Seasonal changes in the pigments, carbohydrates and growth of red spruce as affected by ozone 总被引:9,自引:7,他引:2
R. G. ALSCHER R. G. AMUNDSON J. R. CUMMNG S. FELLOWS J. FINCHER G. RUBIN P. VAN LEUKEN L. H. WEINSTEIN 《The New phytologist》1989,113(2):211-223
Two-year-old red spruce seedlings were exposed to various levels ot ozone, from 0.4 to 3 times ambient levels, in open-top chambers in Ithaca, NY, USA. Exposures, which varied with changes in day length, commenced on May 30, 1987 and continued until December 14, 1987. Seedling biomass, carbohydrate contents, pigment contents, and rate of electron transport were assessed twice monthly during the fumigation period. Orthogonal quadratic or cubic polynomials were used to model the response through time each variable measured. A one-way analysis of variance model was fitted to every regression coefficient in each polynomial model to test for ozone effects on seasonal physiological patterns. Ozone did not influence growth, foliar pigment content, foliar starch content, root carbohydrate content, or rate of electron transport. The seasonal change of needle raffinose content differed between exposed to low (0.4 ×, 1×) and high (2×, 3×) ozone levels. There was also a trend towards reduced total soluble sugar content foliage during late autumn in higher ozone treatments. 相似文献