全文获取类型
收费全文 | 46669篇 |
免费 | 3463篇 |
国内免费 | 1208篇 |
专业分类
51340篇 |
出版年
2023年 | 461篇 |
2022年 | 877篇 |
2021年 | 1542篇 |
2020年 | 1029篇 |
2019年 | 1286篇 |
2018年 | 1450篇 |
2017年 | 1153篇 |
2016年 | 1637篇 |
2015年 | 2477篇 |
2014年 | 2657篇 |
2013年 | 3256篇 |
2012年 | 3701篇 |
2011年 | 3489篇 |
2010年 | 2057篇 |
2009年 | 1838篇 |
2008年 | 2368篇 |
2007年 | 2204篇 |
2006年 | 1962篇 |
2005年 | 1808篇 |
2004年 | 1717篇 |
2003年 | 1501篇 |
2002年 | 1354篇 |
2001年 | 932篇 |
2000年 | 820篇 |
1999年 | 749篇 |
1998年 | 416篇 |
1997年 | 337篇 |
1996年 | 349篇 |
1995年 | 356篇 |
1994年 | 286篇 |
1993年 | 245篇 |
1992年 | 405篇 |
1991年 | 412篇 |
1990年 | 333篇 |
1989年 | 319篇 |
1988年 | 293篇 |
1987年 | 283篇 |
1986年 | 225篇 |
1985年 | 283篇 |
1984年 | 215篇 |
1983年 | 184篇 |
1982年 | 132篇 |
1981年 | 147篇 |
1980年 | 122篇 |
1979年 | 168篇 |
1978年 | 164篇 |
1977年 | 118篇 |
1975年 | 116篇 |
1974年 | 119篇 |
1973年 | 101篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
Li A Ma Y Yu X Mort RL Lindsay CR Stevenson D Strathdee D Insall RH Chernoff J Snapper SB Jackson IJ Larue L Sansom OJ Machesky LM 《Developmental cell》2011,21(4):722-734
Highlights? Rac1 and the Scar/WAVE complex drive pseudopod-based motility of melanoblasts ? Rac1-depleted melanoblasts move using unique actin-based stubs and not blebs ? Rac1 controls pseudopod frequency but is dispensable for pseudopod formation ? Loss of Rac1 delays melanoblast cell-cycle progression and cytokinesis 相似文献
942.
da Silva KE Martins SV Ribeiro CA Santos NT de Azevedo CP Matos FD do Amaral IL 《Revista de biología tropical》2011,59(4):1927-1938
The Amazon region is one of the most diverse areas in the world. Research on high tropical forest diversity brings up relevant contributions to understand the mechanisms that result and support such diversity. In the present study we describe the species composition and diversity of 15 one-ha plots in the Amazonian terra firme dense forest in Brazil, and compare the floristic similarity of these plots with other nine one-ha plots. The 15 plots studied were randomly selected from permanent plots at the Embrapa Experimental site, Amazonas State in 2005. The diversity was analysed by using species richness and Shannon's index, and by applying the Sorensen's index for similarity and unweighted pair-group average (UPGMA) as clustering method. Mantel test was performed to study whether the differences in species composition between sites could be explained by the geographic distance between them. Overall, we identified 8 771 individuals, 264 species and 51 plant families. Most of the species were concentrated in few families and few had large number of individuals. Families presenting the highest species richness were Fabaceae (Faboideae: 22spp., Mimosoideae: 22spp.), Sapotaceae: 22spp., Lecythidaceae: 15 and Lauraceae: 13. Burseraceae had the largest number of individuals with 11.8% of the total. The ten most abundant species were: Protium hebetatum (1 037 individuals), Eschweilera coriacea (471), Licania oblongifolia (310), Pouteria minima (293), Ocotea cernua (258), Scleronema micranthum (197), Eschweilera collina (176), Licania apelata (172), Naucleopsis caloneura (170) and Psidium araca (152), which represented 36.5% of all individuals. Approximately 49% of species had up to ten individuals and 13% appeared only once in all sampled plots, showing a large occurrence of rare species. Our study area is on a forest presenting a high tree species diversity with Shannon's diversity index of 4.49. The dendrogram showed two groups of plots with low similarity between them (less than 0.25), and the closer the plots were one to another, more similar in species composition (Mantel R = 0.3627, p < 0.01). The 15 plots in our study area share more than 50% of their species composition and represent the group of plots that have the shortest distance between each other. Overall, our results highlight the high local and regional heterogeneity of environments in terra firme forests, and the high occurrence of rare species, which should be considered in management and conservation programs in the Amazon rainforest, in order to maintain its structure on the long run. 相似文献
943.
Wang PC Ran XH Chen R Luo HR Ma QY Liu YQ Hu JM Huang SZ Jiang HZ Chen ZQ Zhou J Zhao YX 《化学与生物多样性》2011,8(10):1908-1913
Two new guaiane-type sesquiterpenoids, valerol A (1) and kessyl 3-acetate (2), together with nine known compounds, valeracetate (3), anismol A (4), orientalol C (5), spatulenol (6), 4α,10α-epoxyaromadendrane (7), (+)-8-hydroxypinoresinol (8), pinorespiol (9), pinoresinol 4-O-β-D-glucopyranoside (10), and 8-hydroxypinoresinol 4'-O-β-D-glucopyranoside (11) were isolated from the roots of Valeriana officinalis. The structures and relative configurations of 1 and 2 were elucidated on the basis of spectroscopic methods (1D- and 2D-NMR, MS, UV, and IR). These compounds were evaluated for inhibitory activity on acetylcholinesterase (AChE) and enhancing activity on nerve growth factor (NGF)-mediated neurite outgrowth in PC12 cells. 相似文献
944.
报道了十字花科(Brassicaceae)四齿芥属(Tetracme Bunge)一中国新记录种——帕米尔四齿芥(Tetracme pamirica Vassilcz.)。标本采自西藏自治区札达县底雅乡,生于干燥的河谷坡地。该种与相近种四齿芥的区别是,角果长10~13 mm,种子长1~2 mm。凭证标本保存在内蒙古大学植物标本馆(HIMC)。 相似文献
945.
Marie-Hélène Greffard Émilie Saulnier-Talbot Irene Gregory-Eaves 《Ecological Indicators》2011,11(6):1541-1551
The analysis of sedimentary chironomid assemblages is an approach that has been widely adopted for inferring past environmental conditions. However, there is an ongoing discussion in the literature about whether this approach could become more sensitive at detecting past environmental changes if paleolimnologists conducted finer taxonomic analyses of their specimens. To advance this discussion, we conducted comparative analyses of larval chironomid assemblages resolved to two levels of taxonomic resolution. For this exercise, we chose to use live assemblages (as opposed to sub-fossil assemblages) because fine taxonomic resolution of live assemblages is more easily obtained. Our specific aims were to (i) determine if finely resolved taxa comprising a coarsely resolved group have similar ecological niches, (ii) determine if different environmental predictors of community composition are identified when two different levels of taxonomic resolution are applied and (iii) evaluate whether the variance explained by environmental variables differs substantially between levels of taxonomic resolution. We found that there was substantial dispersion among finely resolved taxa belonging to a single coarse group, which suggests that the merging of these taxa results in the loss of ecological information, and therefore warrants higher taxonomic precision. However, the identification of significant environmental predictors and the proportion of variance explained by these did not differ greatly between our two levels of taxonomic resolution. Overall our results show that coarse-resolution analyses may be adequate for some applications, but if the aim is to infer subtle environmental changes (as is the case in most paleolimnological studies) we recommend the highest possible level of taxonomic resolution. 相似文献
946.
947.
Maurice S Coroller L Debaets S Vasseur V Le Floch G Barbier G 《Journal of applied microbiology》2011,111(6):1436-1446
Aims: To predict the risk factors for building infestation by Serpula lacrymans, which is one of the most destructive fungi causing timber decay in buildings. Methods and Results: The growth rate was assessed on malt extract agar media at temperatures between 1·5 and 45°C, at water activity (aw) over the range of 0·800–0·993 and at pH ranges from 1·5 to 11·0. The radial growth rate (μ) and the lag phase (λ) were estimated from the radial growth kinetics via the plots radius vs time. These parameters were then modelled as a function of the environmental factors tested. Models derived from the cardinal model (CM) were used to fit the experimental data and allowed an estimation of the optimal and limit values for fungal growth. Optimal growth rate occurred at 20°C, at high aw level (0·993) and at a pH range between 4·0 and 6·0. The strain effect on the temperature parameters was further evaluated using 14 strains of S. lacrymans. The robustness of the temperature model was validated on data sets measured in two different wood‐based media (Quercus robur L. and Picea abies). Conclusions: The two‐step procedure of exponential model with latency followed by the CM with inflection gives reliable predictions for the growth conditions of a filamentous fungus in our study. The procedure was validated for the study of abiotic factors on the growth rate of S. lacrymans. Significance and Impact of the Study: This work describes the usefulness of evaluating the effect of physico‐chemical factors on fungal growth in predictive building mycology. Consequently, the developed mathematical models for predicting fungal growth on a macroscopic scale can be used as a tool for risk assessment of timber decay in buildings. 相似文献
948.
Hashim A Wang L Juneja K Ye Y Zhao Y Ming LJ 《Bioorganic & medicinal chemistry letters》2011,21(21):6430-6432
Cu(II) complexes of Alzheimer's disease-related β-amyloid (Aβ) peptides exhibit metal-centered oxidation chemistry. The metallo-Aβ complexes are the hallmark of the disease and have been attributed to the generation of reactive oxygen species (ROS), causing oxidative stress. In this communication, the inhibitions of the oxidative activity of Cu(II)-Aβ by vitamin B6 compounds pyridoxamine (PM), pyridoxine (PN), pyridoxal (PL), and pyridoxal-5'-phosphate (PLP) are presented. These B6's are competitive inhibitors toward dopamine oxidation by Cu(II)-Aβ(1-20), with K(i) values of 1.4, 8.3, 1.2, and 0.2mM, respectively. The phospho-moiety in PLP seems to exhibit cooperative inhibition, affording a clue for future design of inhibitors. 相似文献
949.
Plant growth promoting rhizobacteria and endophytes accelerate phytoremediation of metalliferous soils 总被引:8,自引:0,他引:8
Technogenic activities (industrial—plastic, textiles, microelectronics, wood preservatives; mining—mine refuse, tailings, smelting; agrochemicals—chemical fertilizers, farm yard manure, pesticides; aerosols—pyrometallurgical and automobile exhausts; biosolids—sewage sludge, domestic waste; fly ash—coal combustion products) are the primary sources of heavy metal contamination and pollution in the environment in addition to geogenic sources. During the last two decades, bioremediation has emerged as a potential tool to clean up the metal-contaminated/polluted environment. Exclusively derived processes by plants alone (phytoremediation) are time-consuming. Further, high levels of pollutants pose toxicity to the remediating plants. This situation could be ameliorated and accelerated by exploring the partnership of plant-microbe, which would improve the plant growth by facilitating the sequestration of toxic heavy metals. Plants can bioconcentrate (phytoextraction) as well as bioimmobilize or inactivate (phytostabilization) toxic heavy metals through in situ rhizospheric processes. The mobility and bioavailability of heavy metal in the soil, particularly at the rhizosphere where root uptake or exclusion takes place, are critical factors that affect phytoextraction and phytostabilization. Developing new methods for either enhancing (phytoextraction) or reducing the bioavailability of metal contaminants in the rhizosphere (phytostabilization) as well as improving plant establishment, growth, and health could significantly speed up the process of bioremediation techniques. In this review, we have highlighted the role of plant growth promoting rhizo- and/or endophytic bacteria in accelerating phytoremediation derived benefits in extensive tables and elaborate schematic sketches. 相似文献
950.