全文获取类型
收费全文 | 618篇 |
免费 | 64篇 |
专业分类
682篇 |
出版年
2022年 | 7篇 |
2021年 | 11篇 |
2020年 | 12篇 |
2019年 | 7篇 |
2018年 | 21篇 |
2017年 | 10篇 |
2016年 | 24篇 |
2015年 | 22篇 |
2014年 | 18篇 |
2013年 | 35篇 |
2012年 | 38篇 |
2011年 | 37篇 |
2010年 | 31篇 |
2009年 | 31篇 |
2008年 | 32篇 |
2007年 | 24篇 |
2006年 | 28篇 |
2005年 | 16篇 |
2004年 | 15篇 |
2003年 | 16篇 |
2002年 | 15篇 |
2001年 | 15篇 |
2000年 | 11篇 |
1999年 | 14篇 |
1998年 | 12篇 |
1997年 | 5篇 |
1996年 | 10篇 |
1995年 | 5篇 |
1994年 | 9篇 |
1993年 | 9篇 |
1992年 | 11篇 |
1991年 | 11篇 |
1990年 | 13篇 |
1988年 | 5篇 |
1987年 | 9篇 |
1986年 | 7篇 |
1985年 | 11篇 |
1984年 | 10篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1978年 | 6篇 |
1977年 | 10篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1974年 | 5篇 |
1972年 | 4篇 |
1970年 | 2篇 |
1940年 | 2篇 |
排序方式: 共有682条查询结果,搜索用时 15 毫秒
101.
Evenness is an important property of communities. Species richness alone does not capture the fact that one or a few species may dominate total abundance and biomass of a community. This in turn has important consequences for ecosystem functioning and species interactions. Evenness has been observed to vary systematically along environmental and productivity gradients. However, a truly general theory about which factors control evenness in a community has yet to emerge. Prior research on evenness has suggested that high richness, biomass and abundance should lead to lower community evenness in our study system of bats in Panama. However, only few empirical studies examine the simultaneous effects of species richness, biomass or abundance on evenness. For the first time, we applied path analysis in the study of evenness to tease apart the relative importance and direction (positive or negative) of causality among these three factors. As predicted, we found that evenness decreases with increasing species richness, abundance and biomass. The negative effect of abundance was mediated by the positive joint effect of biomass and richness. The selected models varied in the strength of the correlation between the three variables with evenness but their direction was consistent. Overall, we argue that rarity, high mobility and differences in resource availability at sites with lower environmental stress can explain the negative effects of richness on evenness. 相似文献
102.
Through a transrectal ultrasonography (Aloka SSD 500 microplus equipment, linear transducer of 7.5 MHz) a total of 267 observations were performed in a group of 50 females mainly Bos indicus, raised under humid tropical conditions and pregnant from day 20 every other day until day 40 post-artificial insemination (AI). The objective was to measure the size of the amniotic vesicle and the longitudinal and transversal axis of the embryo, to determine the day in which it is possible to measure this structure and detect the heart contractions in the screened embryos. The accuracy in identifying the amniotic vesicle was 60% (12/20) on day 20 of the first screening with embryos having an average length of 0.44 +/- 0.20 cm. Accuracy in detecting the embryo was 66.6 and 100% between days 20 and 24 post-breeding, respectively. The mean length of the embryos on day 20 was of 0.20 +/- 0.07 cm and the transversal shape had a mean length of 0.07 +/- 0.02 cm. The accuracy of detecting heart contractions was 50% (9/18) on day 24 post-breeding and for day 26 the accuracy was 100%. At the end of the screening (day 40), embryos had means of 1.92 +/- 0.24 (longitudinal) and 0.85 +/- 0.11 cm (transverse). The embryo can be detected from day 24 post-AI and the heart contractions from day 26 with 100% reliability. 相似文献
103.
Martin J Blaser Maria G Dominguez-Bello Monica Contreras Magda Magris Glida Hidalgo Isidoro Estrada Zhan Gao Jose C Clemente Elizabeth K Costello Rob Knight 《The ISME journal》2013,7(1):85-95
The human skin harbors complex bacterial communities. Prior studies showing high inter-individual variation focused on subjects from developed countries. We therefore compared cutaneous bacterial communities of Amerindians in the Venezuelan Amazon with subjects in the United States. Forearm skin specimens were studied from healthy Amerindians in Platanillal village in Amazonas State, and from healthy persons in New York and Colorado. All skin sampling used similar swab/buffer techniques. Multiplexed V2-targeted 16S rRNA gene pyrosequencing yielded high quality sequences from 112 samples. The results show 20 phyla, with three (Proteobacteria, Firmicutes, Actinobacteria) predominating. US residents and Venezuelan Amerindians had significantly different forearm skin bacterial community compositions, with United States dominated by Propionibacterium. Among the Amerindians, there was a deep split based on bacterial community membership, with 30 and 42 samples, respectively, falling into each of the two groups, not associated with age, gender, or body mass index. One Amerindian group had diversity similar to the United States, but was dominated by Staphylococcus rather than Propionibacterium. The other Amerindian group was significantly more diverse and even than the US or the other Amerindian group, and featured a broad range of Proteobacteria. The results provide evidence that ethnicity, lifestyle and/or geography are associated with the structure of human cutaneous bacterial communities. 相似文献
104.
In this paper evidence of anthropogenic influence over the warming of the 20th century is presented and the debate regarding the time-series properties of global temperatures is addressed in depth. The 20th century global temperature simulations produced for the Intergovernmental Panel on Climate Change’s Fourth Assessment Report and a set of the radiative forcing series used to drive them are analyzed using modern econometric techniques. Results show that both temperatures and radiative forcing series share similar time-series properties and a common nonlinear secular movement. This long-term co-movement is characterized by the existence of time-ordered breaks in the slope of their trend functions. The evidence presented in this paper suggests that while natural forcing factors may help explain the warming of the first part of the century, anthropogenic forcing has been its main driver since the 1970’s. In terms of Article 2 of the United Nations Framework Convention on Climate Change, significant anthropogenic interference with the climate system has already occurred and the current climate models are capable of accurately simulating the response of the climate system, even if it consists in a rapid or abrupt change, to changes in external forcing factors. This paper presents a new methodological approach for conducting time-series based attribution studies. 相似文献
105.
106.
BACKGROUND: The objective of this study was to identify factors motivating women to take part in endometriosis research and to determine if these factors differ for women participating in clinical versus basic science studies. METHODS: A consecutive series of 24 women volunteering for participation in endometriosis-related research were asked to indicate, in their own words, why they chose to volunteer. In addition, the women were asked to rate, on a scale of 0 to 10, sixteen potentially motivating factors. The information was gathered in the form of an anonymous self-administered questionnaire. RESULTS: Strong motivating factors (mean score > 8) included potential benefit to other women's health, improvement to one's own condition, and participation in scientific advancement. Weak motivating factors (mean score < 3) included financial compensation, making one's doctor happy, and use of 'natural' products. No difference was detected between clinical and basic science study participants. CONCLUSION: This study is the first study to specifically investigate the factors that motivate women to take part in endometriosis research. Understanding why women choose to take part in such research is important to the integrity of the informed consent process. The factors most strongly motivating women to participate in endometriosis research related to improving personal or public health; the weakest, to financial compensation and pleasing the doctor. 相似文献
107.
Enrique Samper Lucia Morgado Juan C. Estrada Antonio Bernad Alan Hubbard Susana Cadenas Simon Melov 《Free radical biology & medicine》2009,46(3):387-396
Lymphomas adapt to their environment by undergoing a complex series of biochemical changes that are currently not well understood. To better define these changes, we examined the gene expression and gene ontology profiles of thymic lymphomas from a commonly used model of carcinogenesis, the p53?/? mouse. These tumors show a highly significant upregulation of mitochondrial biogenesis, mitochondrial protein translation, mtDNA copy number, reactive oxygen species, antioxidant defenses, proton transport, ATP synthesis, hypoxia response, and glycolysis, indicating a fundamental change in the bioenergetic profile of the transformed T cell. Our results suggest that T cell tumorigenesis involves a simultaneous upregulation of mitochondrial biogenesis, mitochondrial respiration, and glycolytic activity. These processes would allow cells to adapt to the stressful tumor environment by facilitating energy production and thereby promote tumor growth. Understanding these adaptations is likely to result in improved therapeutic strategies for this tumor type. 相似文献
108.
The cellular prion protein (PrPC) is a membrane-bound glycoprotein especially abundant in the central nervous system (CNS). The scrapie prion protein (PrPSc, also termed prions) is responsible of transmissible spongiform encephalopathies (TSE), a group of neurodegenerative diseases which affect humans and other mammal species, although the presence of PrPC is needed for the establishment and further evolution of prions.The present work compares the expression and localization of PrPC between healthy human brains and those suffering from Alzheimer disease (AD).In both situations we have observed a rostrocaudal decrease in the amount of PrPC within the CNS, both by immunoblotting and immunohistochemistry techniques. PrPC is higher expressed in our control brains than in AD cases. There was a neuronal loss and astogliosis in our AD cases. There was a tendency of a lesser expression of PrPC in AD cases than in healthy ones. And in AD cases, the intensity of the expression of the unglycosylated band is higher than the di- and monoglycosylated bands.With regards to amyloid plaques, those present in AD cases were positively labeled for PrPC, a result which is further supported by the presence of PrPC in the amyloid plaques of a transgenic line of mice mimicking AD.The work was done according to Helsinki Declaration of 1975, and approved by the Ethics Committee of the Faculty of Medicine of the University of Navarre.Key words: cellular prion protein, Alzheimer disease, transgenic mice 相似文献
109.
110.
Labeed FH Lu J Mulhall HJ Marchenko SA Hoettges KF Estrada LC Lee AP Hughes MP Flanagan LA 《PloS one》2011,6(9):e25458