全文获取类型
收费全文 | 29782篇 |
免费 | 15954篇 |
国内免费 | 4篇 |
出版年
2022年 | 100篇 |
2021年 | 467篇 |
2020年 | 2224篇 |
2019年 | 3761篇 |
2018年 | 3858篇 |
2017年 | 4151篇 |
2016年 | 4175篇 |
2015年 | 4136篇 |
2014年 | 3781篇 |
2013年 | 4254篇 |
2012年 | 1979篇 |
2011年 | 1693篇 |
2010年 | 3178篇 |
2009年 | 1906篇 |
2008年 | 841篇 |
2007年 | 442篇 |
2006年 | 421篇 |
2005年 | 470篇 |
2004年 | 443篇 |
2003年 | 400篇 |
2002年 | 396篇 |
2001年 | 394篇 |
2000年 | 334篇 |
1999年 | 228篇 |
1998年 | 82篇 |
1997年 | 73篇 |
1996年 | 46篇 |
1995年 | 56篇 |
1994年 | 48篇 |
1993年 | 47篇 |
1992年 | 87篇 |
1991年 | 57篇 |
1990年 | 82篇 |
1989年 | 82篇 |
1988年 | 63篇 |
1987年 | 47篇 |
1986年 | 55篇 |
1985年 | 70篇 |
1984年 | 43篇 |
1983年 | 38篇 |
1980年 | 33篇 |
1979年 | 59篇 |
1978年 | 49篇 |
1977年 | 50篇 |
1976年 | 33篇 |
1974年 | 48篇 |
1973年 | 38篇 |
1972年 | 43篇 |
1970年 | 31篇 |
1969年 | 34篇 |
排序方式: 共有10000条查询结果,搜索用时 251 毫秒
951.
952.
Amy M. Iler David W. Inouye Toke T. Høye Abraham J. Miller‐Rushing Laura A. Burkle Eleanor B. Johnston 《Global Change Biology》2013,19(8):2348-2359
Variation in species’ responses to abiotic phenological cues under climate change may cause changes in temporal overlap among interacting taxa, with potential demographic consequences. Here, we examine associations between the abiotic environment and plant–pollinator phenological synchrony using a long‐term syrphid fly–flowering phenology dataset (1992–2011). Degree‐days above freezing, precipitation, and timing of snow melt were investigated as predictors of phenology. Syrphids generally emerge after flowering onset and end their activity before the end of flowering. Neither flowering nor syrphid phenology has changed significantly over our 20‐year record, consistent with a lack of directional change in climate variables over the same time frame. Instead we document interannual variability in the abiotic environment and phenology. Timing of snow melt was the best predictor of flowering onset and syrphid emergence. Snow melt and degree‐days were the best predictors of the end of flowering, whereas degree‐days and precipitation best predicted the end of the syrphid period. Flowering advanced at a faster rate than syrphids in response to both advancing snow melt and increasing temperature. Different rates of phenological advancements resulted in more days of temporal overlap between the flower–syrphid community in years of early snow melt because of extended activity periods. Phenological synchrony at the community level is therefore likely to be maintained for some time, even under advancing snow melt conditions that are evident over longer term records at our site. These results show that interacting taxa may respond to different phenological cues and to the same cues at different rates but still maintain phenological synchrony over a range of abiotic conditions. However, our results also indicate that some individual plant species may overlap with the syrphid community for fewer days under continued climate change. This highlights the role of interannual variation in these flower–syrphid interactions and shows that species‐level responses can differ from community‐level responses in nonintuitive ways. 相似文献
953.
João de Pina‐Cabral 《The journal of the Royal Anthropological Institute》2013,19(1):75-101
This article is a study of personal naming using drawings made by secondary school students from two small towns in coastal Bahia (Brazil) of the persons who gave them their names. The article explores the relation between namer and named as one of continued identity created by imaginative processes of surrogation akin to pretend‐play. In early ontogeny, the person is constituted through a series of mutualities of being that come to constitute a core of affects. The article aims to contribute towards a better understanding of the processes of constitution of personhood in the context of human relatedness. 相似文献
954.
955.
Aims
To evaluate the interaction between selected yeasts and bacteria and associate their metabolic activity with secondary cucumber fermentation.Methods and Results
Selected yeast and bacteria, isolated from cucumber secondary fermentations, were inoculated as single and mixed cultures in a cucumber juice model system. Our results confirmed that during storage of fermented cucumbers and in the presence of oxygen, spoilage yeasts are able to grow and utilize the lactic and acetic acids present in the medium, which results in increased brine pH and the chemical reduction in the environment. These conditions favour opportunistic bacteria that continue the degradation of lactic acid. Lactobacillus buchneri, Clostridium bifermentans and Enterobacter cloacae were able to produce acetic, butyric and propionic acids, respectively, when inoculated in the experimental medium at pH 4·6. Yeast and bacteria interactions favoured the survival of Cl. bifermentans and E. cloacae at the acidic pH typical of fermented cucumbers (3·2), but only E. cloacae was able to produce a secondary product.Conclusions
The methodology used in this study confirmed that a complex microbiota is responsible for the changes observed during fermented cucumber secondary fermentation and that certain microbial interactions may be essential for the production of propionic and butyric acids.Significance and Impact of the Study
Understanding the dynamics of the development of secondary cucumber fermentation aids in the identification of strategies to prevent its occurrence and economic losses for the pickling industry. 相似文献956.
E.G. Santos F.G.P. Costa J.H.V. Silva T.D.D. Martins D.F. Figueiredo‐Lima M. Macari C.J.B. Oliveira P.E.N. Givisiez 《Journal of applied microbiology》2013,114(4):1158-1165
Aims
To evaluate mannan oligosaccharide (MOS) and threonine effects on performance, small intestine morphology and Salmonella spp. counts in Salmonella Enteritidis‐challenged birds.Methods and Results
One‐day‐old chicks (1d) were distributed into five treatments: nonchallenged animals fed basal diet (RB‐0), animals fed basal diet and infected with Salmonella Enteritidis (RB‐I), animals fed high level of threonine and infected (HT‐I), birds fed basal diet with MOS and infected (MOS‐I), birds fed high level of threonine and MOS and infected (HT+MOS‐I). Birds were inoculated at 2d with Salmonella Enteritidis, except RB‐0 birds. Chicks fed higher dietary threonine and MOS showed performance similar to RB‐0 and intestinal morphology recovery at 8 dpi. Salmonella counts and the number of Salmonella‐positive animals were lower in HT+MOS‐I compared with other challenged groups.Conclusion
Mannan oligosaccharides and threonine act synergistically, resulting in improved intestinal environment and recovery after Salmonella inoculation.Significance and Impact of the Study
Nutritional approaches may be useful to prevent Salmonella infection in the first week and putative carcass contamination at slaughter. This is the first report on the possible synergistic effect of mannan oligosaccharides and threonine, and further studies should be performed including performance, microbiota evaluation, composition of intestinal mucins and immune assessment. 相似文献957.
958.
959.
960.