全文获取类型
收费全文 | 21010篇 |
免费 | 1210篇 |
国内免费 | 5篇 |
专业分类
22225篇 |
出版年
2023年 | 169篇 |
2022年 | 268篇 |
2021年 | 503篇 |
2020年 | 386篇 |
2019年 | 471篇 |
2018年 | 741篇 |
2017年 | 645篇 |
2016年 | 879篇 |
2015年 | 1006篇 |
2014年 | 1174篇 |
2013年 | 1552篇 |
2012年 | 1466篇 |
2011年 | 1581篇 |
2010年 | 984篇 |
2009年 | 806篇 |
2008年 | 1109篇 |
2007年 | 1161篇 |
2006年 | 1123篇 |
2005年 | 923篇 |
2004年 | 805篇 |
2003年 | 769篇 |
2002年 | 650篇 |
2001年 | 427篇 |
2000年 | 372篇 |
1999年 | 291篇 |
1998年 | 165篇 |
1997年 | 88篇 |
1996年 | 84篇 |
1995年 | 85篇 |
1994年 | 77篇 |
1993年 | 60篇 |
1992年 | 108篇 |
1991年 | 112篇 |
1990年 | 87篇 |
1989年 | 71篇 |
1988年 | 71篇 |
1987年 | 68篇 |
1986年 | 64篇 |
1985年 | 66篇 |
1984年 | 63篇 |
1983年 | 47篇 |
1982年 | 44篇 |
1981年 | 42篇 |
1980年 | 37篇 |
1979年 | 50篇 |
1978年 | 43篇 |
1977年 | 26篇 |
1975年 | 32篇 |
1974年 | 22篇 |
1952年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
141.
Catanozi S Rocha JC Passarelli M Guzzo ML Alves C Furukawa LN Nunes VS Nakandakare ER Heimann JC Quintão EC 《Journal of lipid research》2003,44(4):727-732
This study aimed at measuring the influence of a low salt diet on the development of experimental atherosclerosis in moderately hyperlipidemic mice. Experiments were carried out on LDL receptor (LDLR) knockout (KO) mice, or apolipoprotein E (apoE) KO mice on a low sodium chloride diet (LSD) as compared with a normal salt diet (NSD). On LSD, the rise of the plasma concentrations of TG and nonesterified fatty acid (NEFA) was, respectively, 19% and 34% in LDLR KO mice, and 21% and 35% in apoE KO mice, and that of plasma cholesterol was limited to the LDLR KO group alone (15%). Probably due to the apoE KO severe hypercholesterolemia, the arterial inner-wall fat storage was not influenced by the diet salt content and was far more abundant in the apoE KO than in the LDLR KO mice. However, in the less severe hypercholesterolemia of the LDLR KO mice, lipid deposits on the LSD were greater than on the NSD. Arterial fat storage correlated with NEFA concentrations in the LDLR KO mice alone (n = 14, P = 0.0065). Thus, dietary sodium chloride restriction enhances aortic wall lipid storage in moderately hyperlipidemic mice. 相似文献
142.
Species–area relationships (SARs) of European butterfly species (Rhopalocera) appear to follow power functions with Mediterranean butterflies having a much higher slope value (z=0.49) compared to the slope for the northern and eastern European countries (z=0.10). A simulated process of species extinction by a stepwise density dependent random elimination of species affected species–area patterns differently. For Mediterranean countries SAR slopes decreased, for other European countries slopes increased during the extinction process. Comparisons of species numbers before and after extinction with those predicted by a classical SAR approach differed widely and revealed that SARs are not able to predict future species numbers at local scales. For Mediterranean countries the classical SAR approach underestimated the number of species remaining after simulated extinction, for all other European countries SARs highly overestimated species numbers. These contrasting patterns indicate that changes in SAR patterns do not unequivocally point to changes in species diversity or community structure as assumed by current theory. On the other hand, the results strongly indicate that simplified applications of SARs for forecasting might give misimpressions about species loss and future biodiversity if the initial community structure, especially relative densities and numbers of species with restricted range size, are not taken into account. 相似文献
143.
Rowena Spence Robert J. Wootton Iain Barber Mirosław Przybylski Carl Smith 《Ecology and evolution》2013,3(6):1717-1726
The central assumption of evolutionary theory is that natural selection drives the adaptation of populations to local environmental conditions, resulting in the evolution of adaptive phenotypes. The three‐spined stickleback (Gasterosteus aculeatus) displays remarkable phenotypic variation, offering an unusually tractable model for understanding the ecological mechanisms underpinning adaptive evolutionary change. Using populations on North Uist, Scotland we investigated the role of predation pressure and calcium limitation on the adaptive evolution of stickleback morphology and behavior. Dissolved calcium was a significant predictor of plate and spine morph, while predator abundance was not. Stickleback latency to emerge from a refuge varied with morph, with populations with highly reduced plates and spines and high predation risk less bold. Our findings support strong directional selection in three‐spined stickleback evolution, driven by multiple selective agents. 相似文献
144.
Rafaela Magalhães Brandão Maria das Graças Cardoso Luís Roberto Batista Alex Rodrigues Silva Caetano Ana Carolina Cortez Lemos Maria Alice Martins David Lee Nelson Juliano Elvis De Oliveira 《Letters in applied microbiology》2022,74(5):765-776
Poly(lactic acid) (PLA) nanofibres containing different proportions of the essential oils from Ocimum basilicum L. and Ocimum gratissimum L. were prepared by solution blow spinning method. The essential oils were extracted by hydrodistillation and characterized by gas chromatography. MEV, contact angle, DSC and FTIR were used to characterize the nanofibres. The effect of bioative nanofibres on the growth of the fungus and on the production of ochratoxin A were evaluated using the fumigation test. Linalool, 1·8-cineole and camphor were the principal components of the essential oil from O. basilicum, and eugenol was the principal constituent in the oil from O. gratissimum. An increase in the average diameter of the nanofibres was observed with the addition of the essential oils. The essential oils acted as a plasticizer, resulting in a reduction in the crystallinity of the PLA. The encapsulation of essential oils in PLA nanofibres was verified by FTIR. An effective antifungal and antimicotoxygenic activity against Aspergillus ochraceus and Aspergillus westerdjikiae was observed for the bioative nanofibres. These results confirm the potential of PLA nanofibres containing the essential oils for the control of toxigenic fungi that cause the deterioration of food and are harmful to human health. 相似文献
145.
146.
Potential mineralization of nitrogen from organic wastes to ryegrass and wheat crops 总被引:8,自引:0,他引:8
Two-pot experiments with ryegrass and wheat plants were conducted in a Cambic Arenosol to test the reliability of N fate predicted by incubation experiments previously performed, with the same soil, to assess potentially mineralizable nitrogen from six organic wastes (municipal solid waste compost, secondary pulp mill sludge, horn meal, poultry manure, solid phase from pig slurry and composted pig manure). Two treatments, corresponding to 80 and 160 kgN/ha were tested, with or without mineral N fertilization. Experimental data obtained in the pot trials was consistent with nitrogen net mineralization trend observed in the aerobic incubations with all the wastes tested. Values of potentially mineralizable nitrogen (N(0)) from the equations obtained by model fitting, to the incubation data, were well correlated to ryegrass and wheat N uptake. Poultry manure was the most efficient N supplier to crops. 相似文献
147.
The presented comprehensive review of current knowledge about genetic factors predisposing to Graves' disease (GD) put emphasis on functional significance of observed associations. In particular, we discuss recent efforts aimed at refining diseases associations found within the HLA complex and implicating HLA class I as well as HLA-DPB1 loci. We summarize data regarding non-HLA genes such as PTPN22, CTLA4, CD40, TSHR and TG which have been extensively studied in respect to their role in GD. We review recent findings implicating variants of FCRL3 (gene for FC receptor-like-3 protein), SCGB3A2 (gene for secretory uteroglobin-related protein 1- UGRP1) as well as other unverified possible candidate genes for GD selected through their documented association with type 1 diabetes mellitus: Tenr-IL2-IL21, CAPSL (encoding calcyphosine-like protein), IFIH1(gene for interferon-induced helicase C domain 1), AFF3, CD226 and PTPN2. We also review reports on association of skewed X chromosome inactivation and fetal microchimerism with GD. Finally we discuss issues of genotype-phenotype correlations in GD. 相似文献
148.
Pereira-Lorenzo S Costa RM Ramos-Cabrer AM Ciordia-Ara M Ribeiro CA Borges O Barreneche T 《Génome》2011,54(4):301-315
This is a large-scale molecular study based on simple sequence repeat (SSR) loci of the diversification process in chestnut cultivars from Portugal and Spain, from the northern Iberian Peninsula to the Canary Islands and the Azores. A total of 593 grafted chestnut trees (Castanea sativa Mill.) were analysed with 10 SSRs: 292 from Portugal and 301 from Spain. Some of the trees studied were more than 300 years old. Accessions were analysed using a model-based Bayesian procedure to assess the geographical structure and to assign individuals to reconstructed populations based on the SSR genotypes. We found 356 different genotypes with a mean value of clonality of 33% owing to grafting. Mutations accounted for 6%, with hybridization being the main diversification process that can explain the great diversity found. Ten main cultivar groups were detected: four in northern Spain, five in the centre of the Iberian Peninsula, and one in southern Spain related to the centre of the Iberian Peninsula. This work demonstrated that cultivar origin and the diversification process was a combination of clonal propagation of selected seedlings, hybridization, and mutations, which allowed high levels of diversity to be maintained with respect to selected clones for fruit production. Furthermore, seedlings and graft sticks facilitated the transport to new destinations in the colonization process, transporting sometimes more than 3000 km if we consider the Azores and the Canary Islands. 相似文献
149.
150.
Merete Bjørgan Schrøder Emilio Flaño Lars Pilström Trond Ø. Jørgensen 《Fish & shellfish immunology》1998,8(8):565-576
Localisation of immunoglobulin heavy chain (IgH) producing cells was determined in sections from head kidney, spleen, thymus, gills, gut, skin, heart and liver from the Atlantic cod. In general, IgH mRNA positive cells were detected in all organs examined and were mainly located to the connective tissue surrounding the vascular system in these organs. In the head kidney and spleen, IgH mRNA positive cells appeared as single distributed cells or as dense clusters, whereas in the thymus only single distributed positive cells were observed. The percentage of Ig heavy chain mRNA positive (plasma) cells in the head kidney, spleen and thymus was estimated at about 1% of the total cell mass. The number of IgH mRNA positive cells was lower than this in all the other organs examined. 相似文献