全文获取类型
收费全文 | 7025篇 |
免费 | 433篇 |
专业分类
7458篇 |
出版年
2023年 | 57篇 |
2022年 | 95篇 |
2021年 | 179篇 |
2020年 | 114篇 |
2019年 | 134篇 |
2018年 | 198篇 |
2017年 | 143篇 |
2016年 | 230篇 |
2015年 | 375篇 |
2014年 | 352篇 |
2013年 | 528篇 |
2012年 | 575篇 |
2011年 | 548篇 |
2010年 | 347篇 |
2009年 | 328篇 |
2008年 | 395篇 |
2007年 | 387篇 |
2006年 | 390篇 |
2005年 | 362篇 |
2004年 | 327篇 |
2003年 | 286篇 |
2002年 | 286篇 |
2001年 | 47篇 |
2000年 | 33篇 |
1999年 | 47篇 |
1998年 | 66篇 |
1997年 | 68篇 |
1996年 | 33篇 |
1995年 | 51篇 |
1994年 | 42篇 |
1993年 | 34篇 |
1992年 | 25篇 |
1991年 | 34篇 |
1990年 | 16篇 |
1989年 | 19篇 |
1988年 | 24篇 |
1987年 | 17篇 |
1986年 | 9篇 |
1985年 | 15篇 |
1984年 | 31篇 |
1983年 | 17篇 |
1982年 | 19篇 |
1981年 | 23篇 |
1980年 | 24篇 |
1979年 | 16篇 |
1978年 | 17篇 |
1977年 | 11篇 |
1976年 | 11篇 |
1974年 | 9篇 |
1965年 | 8篇 |
排序方式: 共有7458条查询结果,搜索用时 0 毫秒
41.
Bartolomeo Lercari Franco Tognoni Roberto Lorenzi Alberto Manetti 《Plant biosystems》2013,147(3):147-156
Abstract Growth response of coleoptile sections from four Italian cultivars of wheat to some growth regulators.—The following Italian wheat varieties were used to obtain the coleoptile sections used throughout the experiments: Funo, S. Pastore, Abbondanza, Generoso. The growth substances tested were Indolacetic acid (IAA), Abscisic acid (ABA), Kinetin and Gibberellic acid. The colcoptile response of each variety was tested with three different concentrations for each growth regulator in combination with two sucrose concentrations and three different pH levels. The experiments indicated that the coleoptiles from the variety « Funo » react linearly to incresing IAA and ABA concentration resulting the best one to be used in this bioassay. 相似文献
42.
Prof. Alberto Chiarugi 《Plant biosystems》2013,147(4):572-575
Riassunto L'A. illustra la morfologia e l'anatomia di Solenostemma Argel con particolare considerazione dei caratteri del legno secondario in rapporto all'abito sempreverde della pianta. 相似文献
43.
Turbellarians, which typically feed on bacteria, algae, rotifers, oligochaetes, dipteran larvae, microcrustaceans, and other organisms, are abundant in diverse types of wetlands. Despite their importance, abundance, and species richness in freshwater environments, turbellarians are seldom considered in studies on biodiversity. We analyzed the structure of turbellarian communities in shore areas of three categories of permanent wetlands classified according to their perimeter as small, intermediate, and large during an annual cycle. In total, 1847 turbellarians were collected representing 42 species and 15 genera, from the orders Catenulida, Macrostomida, Lecithoepitheliata, Rhabdocoela, and Tricladida. Sixteen species were common to the three categories of wetlands, whereas nineteen species were unique to a particular category. Species composition varied among wetlands of different sizes; small, intermediate, and large wetlands had different dominant species. We found seasonal differences in community composition over the year, but no significant differences in mean values of observed species richness among wetlands with different sizes and among seasonal samples. The estimated species richness was, however, higher in the small wetlands, followed by the large and intermediate wetlands. In the summer, abundance was significantly lower in the small water bodies than in the intermediate and large bodies of water. Our results reinforce the need for conservation of wetlands of different sizes. 相似文献
44.
Giuseppe Battistoni Fabrizio Cappucci Nicola Bertolino Maria Grazia Brambilla Hae Song Mainardi Alberto Torresin 《Physica medica : PM : an international journal devoted to the applications of physics to medicine and biology : official journal of the Italian Association of Biomedical Physics (AIFB)》2013,29(6):656-661
The purpose of this work is to investigate the capability of the FLUKA Monte Carlo (MC) code to simulate the Elekta Leksell Gamma Knife Perfexion (LGK-PFX) and reproduce the Treatment Planning System (TPS) Leksell GammaPlan version 8.2 (LGP) dose calculations for the case of a water equivalent phantom target. Thanks to the collaboration with Elekta Instruments AB, the collimation system geometry, the source positions and all the involved material have been simulated in detail. The relative linear dose distribution along the three coordinate axes, for each collimator size, and the Relative Output Factors (ROF) have been investigated. The simulation has been validated comparing simulated linear dose profiles with measurements performed with EBT radiochromic films. The acceptance criterion between experimental data and FLUKA results is based on the gamma index (GI) method. The FLUKA MC calculation for the ROF provided the values of 0.920 for the 8 mm collimators and 0.800 for the 4 mm collimators. These values are in good agreement with the Elekta reference data of 0.924 and 0.805 respectively. The percentage difference between calculated and reference values for the ROF is under 1% and within the FLUKA uncertainty. Also the simulated relative dose profiles show a good agreement with the LGP calculation expressed by means of the gamma index method. This established accuracy proves that FLUKA is a suitable and powerful tool in order to reproduce successfully the LGP calculations for the homogeneous media. 相似文献
45.
Valentina Sanghez Maria Teresa Russo Maria Antonietta Ajmone‐Cat Emanuele Cacci Alberto Martire Patrizia Popoli Germana Falcone Flavia Michelini Marco Crescenzi Paolo Degan Luisa Minghetti Margherita Bignami Gemma Calamandrei 《Aging cell》2013,12(4):695-705
The contribution that oxidative damage to DNA and/or RNA makes to the aging process remains undefined. In this study, we used the hMTH1‐Tg mouse model to investigate how oxidative damage to nucleic acids affects aging. hMTH1‐Tg mice express high levels of the hMTH1 hydrolase that degrades 8‐oxodGTP and 8‐oxoGTP and excludes 8‐oxoguanine from both DNA and RNA. Compared to wild‐type animals, hMTH1‐overexpressing mice have significantly lower steady‐state levels of 8‐oxoguanine in both nuclear and mitochondrial DNA of several organs, including the brain. hMTH1 overexpression prevents the age‐dependent accumulation of DNA 8‐oxoguanine that occurs in wild‐type mice. These lower levels of oxidized guanines are associated with increased longevity and hMTH1‐Tg animals live significantly longer than their wild‐type littermates. Neither lipid oxidation nor overall antioxidant status is significantly affected by hMTH1 overexpression. At the cellular level, neurospheres derived from adult hMTH1‐Tg neural progenitor cells display increased proliferative capacity and primary fibroblasts from hMTH1‐Tg embryos do not undergo overt senescence in vitro. The significantly lower levels of oxidized DNA/RNA in transgenic animals are associated with behavioral changes. These mice show reduced anxiety and enhanced investigation of environmental and social cues. Longevity conferred by overexpression of a single nucleotide hydrolase in hMTH1‐Tg animals is an example of lifespan extension associated with healthy aging. It provides a link between aging and oxidative damage to nucleic acids. 相似文献
46.
Alessandra Vacca Roberta Montisci Pietro Garau Paolo Siotto Matteo Piga Alberto Cauli Massimo Ruscazio Luigi Meloni Sabino Iliceto Alessandro Mathieu 《Arthritis research & therapy》2013,15(1):R8
Introduction
Microcirculation dysfunction is a typical feature of systemic sclerosis (SSc) and represents the earliest abnormality of primary myocardial involvement. We assessed coronary microcirculation status by combining two functional tests in SSc patients and estimating its impact on disease outcome.Methods
Forty-one SSc patients, asymptomatic for coronary artery disease, were tested for coronary flow velocity reserve (CFR) by transthoracic-echo-Doppler with adenosine infusion (A-TTE) and for left ventricular wall motion abnormalities (WMA) by dobutamine stress echocardiography (DSE). Myocardial multi-detector computed tomography (MDCT) enabled the presence of epicardial stenosis, which could interfere with the accuracy of the tests, to be excluded. Patient survival rate was assessed over a 6.7- ± 3.5-year follow-up.Results
Nineteen out of 41 (46%) SSc patients had a reduced CFR (≤2.5) and in 16/41 (39%) a WMA was observed during DSE. Furthermore, 13/41 (32%) patients showed pathological CFR and WMA. An inverse correlation between wall motion score index (WMSI) during DSE and CFR value (r = -0.57, P <0.0001) was observed; in addition, CFR was significantly reduced (2.21 ± 0.38) in patients with WMA as compared to those without (2.94 ± 0.60) (P <0.0001). In 12 patients with abnormal DSE, MDCT was used to exclude macrovasculopathy. During a 6.7- ± 3.5-year follow-up seven patients with abnormal coronary functional tests died of disease-related causes, compared to only one patient with normal tests.Conclusions
A-TTE and DSE tests are useful tools to detect non-invasively pre-clinical microcirculation abnormalities in SSc patients; moreover, abnormal CFR and WMA might be related to a worse disease outcome suggesting a prognostic value of these tests, similar to other myocardial diseases. 相似文献47.
Alberto G. O. P. Barretto Göran Berndes Gerd Sparovek Stefan Wirsenius 《Global Change Biology》2013,19(6):1804-1815
Does agricultural intensification reduce the area used for agricultural production in Brazil? Census and other data for time periods 1975–1996 and 1996–2006 were processed and analyzed using Geographic Information System and statistical tools to investigate whether and if so, how, changes in yield and stocking rate coincide with changes in cropland and pasture area. Complementary medium‐resolution data on total farmland area changes were used in a spatially explicit assessment of the land‐use transitions that occurred in Brazil during 1960–2006. The analyses show that in agriculturally consolidated areas (mainly southern and southeastern Brazil), land‐use intensification (both on cropland and pastures) coincided with either contraction of both cropland and pasture areas, or cropland expansion at the expense of pastures, both cases resulting in farmland stability or contraction. In contrast, in agricultural frontier areas (i.e., the deforestation zones in central and northern Brazil), land‐use intensification coincided with expansion of agricultural lands. These observations provide support for the thesis that (i) technological improvements create incentives for expansion in agricultural frontier areas; and (ii) farmers are likely to reduce their managed acreage only if land becomes a scarce resource. The spatially explicit examination of land‐use transitions since 1960 reveals an expansion and gradual movement of the agricultural frontier toward the interior (center‐western Cerrado) of Brazil. It also indicates a possible initiation of a reversed trend in line with the forest transition theory, i.e., agricultural contraction and recurring forests in marginally suitable areas in southeastern Brazil, mainly within the Atlantic Forest biome. The significant reduction in deforestation that has taken place in recent years, despite rising food commodity prices, indicates that policies put in place to curb conversion of native vegetation to agriculture land might be effective. This can improve the prospects for protecting native vegetation by investing in agricultural intensification. 相似文献
48.
A. Joan Levine Alberto Salvan Glenn Talaska Mark F. Boeniger Anthony Suruda Paul A. Schulte 《Biomarkers》2013,18(2):135-138
Epithelial-cell micronuclei (MN) are potentially useful markers of occupational exposure to genotoxicants. With intermittent exposures, cells sampled either before or after a specific time interval, reflecting the time it takes for damaged cells to become available at the epithelial surface, are unlikely to be exposure-related. It may then be important to conduct an exposure-window analysis, with the goal of identifying the relevant exposures.We re-analysed individual exposure data from a previous study (Suruda et al. 1993) of MN formation in 22 male mortuary science students exposed to formaldehyde during a 90-day embalming class. We conducted an exposurewindow analysis and compared the results with those obtained with 90-day cumulative exposure. The window widths varied between 7 and 25 days, in 1 day increments, assuming a constant 7-day cell-cycle. We assessed the fit (likelihood-ratio test) of a linear regression model, regressing the change in buccal MN prevalence on formaldehyde exposure, using both asymptotic and non-asymptotic methods. Exposures defined from 7-15 to 7-18 days before specimen collection provided a slightly better fit than the 90-day cumulative exposure, with a doubling of the regression coefficient for the exposure effect (for the 7-16-days window LR = 5.32, p = 0.032, coefficient = 0.088 MN per 1000 cells per ppm-hr; 95% CI = 0.014, 0.16; for the 90-day cumulative exposure LR = 4.44, p = 0.048, coefficient = 0.045 MN per 1000 cells per ppm-hr, 95% CI = 0.0038, 0.086). Although hampered by the small number of subjects, these results reinforce the potential importance of exposure timing. 相似文献
49.
This article explores commercial, academic, and national initiatives aimed at using sequencing technologies to generate “actionable” genomic results that can be applied to the clinical management of oncology patients. We argue that the term “actionable” is not merely a buzzword, but signals the emergence of a distinctive sociotechnical regime of genomic medicine in oncology. Unlike other regimes of genomic medicine that are organized around assessing and managing inherited risk for developing cancer (e.g. BRCA testing), actionable regimes aim to generate predictive relationships between genetic information and drug therapies, thereby generating new kinds of clinical actions. We explore how these genomic results are made actionable by articulating them with existing clinical routines, clinical trials, regulatory regimes, and health care systems; and in turn, how clinical sequencing programs have begun to reconfigure knowledge and practices in oncology. Actionability regimes confirm the emergence of bio-clinical decision-making in oncology, whereby the articulation of molecular hypotheses and experimental therapeutics become central to patient care. 相似文献