全文获取类型
收费全文 | 2922篇 |
免费 | 183篇 |
专业分类
3105篇 |
出版年
2023年 | 22篇 |
2022年 | 40篇 |
2021年 | 75篇 |
2020年 | 47篇 |
2019年 | 63篇 |
2018年 | 79篇 |
2017年 | 61篇 |
2016年 | 96篇 |
2015年 | 150篇 |
2014年 | 178篇 |
2013年 | 174篇 |
2012年 | 235篇 |
2011年 | 192篇 |
2010年 | 134篇 |
2009年 | 103篇 |
2008年 | 147篇 |
2007年 | 153篇 |
2006年 | 125篇 |
2005年 | 115篇 |
2004年 | 115篇 |
2003年 | 108篇 |
2002年 | 103篇 |
2001年 | 25篇 |
2000年 | 15篇 |
1999年 | 11篇 |
1998年 | 33篇 |
1997年 | 14篇 |
1996年 | 15篇 |
1995年 | 25篇 |
1994年 | 13篇 |
1993年 | 14篇 |
1992年 | 25篇 |
1991年 | 15篇 |
1990年 | 14篇 |
1989年 | 16篇 |
1988年 | 16篇 |
1987年 | 12篇 |
1986年 | 10篇 |
1985年 | 14篇 |
1984年 | 12篇 |
1981年 | 15篇 |
1980年 | 12篇 |
1978年 | 10篇 |
1976年 | 9篇 |
1973年 | 14篇 |
1971年 | 9篇 |
1967年 | 7篇 |
1966年 | 7篇 |
1935年 | 7篇 |
1917年 | 7篇 |
排序方式: 共有3105条查询结果,搜索用时 15 毫秒
61.
62.
63.
64.
Anake Kijjoa Astréa M. Giesbrecht Otto R. Gottlieb Hugo E. Gottlieb 《Phytochemistry》1981,20(6):1385-1388
The trunk woods of two Amazonian Myristicaceae, Virola minutiflora and V. elongata, contain respectively 1-(2-hydroxy-4-methoxyphenyl)-3-(3,4-m 相似文献
65.
Marden A. De Alvarenga J. Jerônimo Da Silva Hugo E. Gottlieb Otto R. Gottlieb 《Phytochemistry》1981,20(5):1159-1161
The structure of micrandrol-C from Micrandropsis scleroxylon (Euphorbiaceae) is revised to 2,6-dihydroxy-7-methyl-1-methylthiophenanthrene. This and other micrandrols are probably diterpenes in view of their co-occurrence with micrandrol-D, the hemiketal of 1,2,3,4,9,10-hexahydro-6-hydroxy-4a-hydroxymethyl-1,1,7-trimethy-2-oxophenanthrene. 相似文献
66.
Because of their rarity, primary malignant tumors of the heart continue to evade clinical preoperative diagnosis. In five cases of malignant heart tumor that were reviewed from the diagnostic point of view, the clinical symptoms and radiographic abnormalities were found to be similar to those produced by more common heart diseases. The diagnosis of malignant cardiac tumor should be considered in a patient with heart disease who has (1) an unusual cardiomediastinal silhouette, (2) an atypical cardiac calcification, and (3) clinical or radiographic cardipulmonary findings which do not conform to the expected behavior of the common forms of heart disease. 相似文献
67.
68.
Francisco Amorim Orly Razgour Vanessa A. Mata Susana Lopes Raquel Godinho Carlos Ibáñez Javier Juste Stephen J. Rossiter Pedro Beja Hugo Rebelo 《Journal of Zoological Systematics and Evolutionary Research》2020,58(1):499-518
The Mediterranean Basin is a global biodiversity hotspot, hosting a number of native species belonging to families that are found almost exclusively in tropical climates. Yet, whether or not these taxa were able to survive in the Mediterranean region during the Quaternary climatic oscillations remains unknown. Focusing on the European free-tailed bat (Tadarida teniotis), we aimed to (a) identify potential ancient populations and glacial refugia; (b) determine the post-glacial colonization routes across the Mediterranean; and (c) evaluate current population structure and demography. Mitochondrial and nuclear markers were used to understand T. teniotis evolutionary and demographic history. We show that T. teniotis is likely restricted to the Western Palearctic, with mitochondrial phylogeny suggesting a split between an Anatolian/Middle East clade and a European clade. Nuclear data pointed to three genetic populations, one of which is an isolated and highly differentiated group in the Canary Islands, another distributed across Iberia, Morocco, and France, and a third stretching from Italy to the east, with admixture following a pattern of isolation by distance. Evolutionary and demographic reconstruction supports a pre-Last Glacial Maximum (LGM) colonization of Italy and the Anatolian/Middle East, while the remaining populations were colonized from Italy after the Younger Dryas. We also found support for demographic expansion following the Iberian colonization. The results show that during the LGM T. teniotis persisted in Mediterranean refugia and has subsequently expanded to its current circum-Mediterranean range. Our findings raise questions regarding the physiological and ecological traits that enabled species with tropical affinities to survive in colder climates. 相似文献
69.
In addition to its builders, termite nests are known to house a variety of secondary opportunistic termite species so‐called inquilines, but little is known about the mechanisms governing the maintenance of these symbioses. In a single nest, host and inquiline colonies are likely to engage in conflict due to nestmate discrimination, and an intriguing question is how both species cope with each other in the long term. Evasive behaviour has been suggested as one of the mechanisms reducing the frequency of host‐inquiline encounters, yet, the confinement imposed by the nests' physical boundaries suggests that cohabiting species would eventually come across each other. Under these circumstances, it is plausible that inquilines would be required to behave accordingly to secure their housing. Here, we show that once inevitably exposed to hosts individuals, inquilines exhibit nonthreatening behaviours, displaying hence a less threatening profile and preventing conflict escalation with their hosts. By exploring the behavioural dynamics of the encounter between both cohabitants, we find empirical evidence for a lack of aggressiveness by inquilines towards their hosts. Such a nonaggressive behaviour, somewhat uncommon among termites, is characterised by evasive manoeuvres that include reversing direction, bypassing and a defensive mechanism using defecation to repel the host. The behavioural adaptations we describe may play an important role in the stability of cohabitations between host and inquiline termite species: by preventing conflict escalation, inquilines may improve considerably their chances of establishing a stable cohabitation with their hosts. 相似文献
70.
Integrated requirement of non‐specific and sequence‐specific DNA binding in Myc‐driven transcription