全文获取类型
收费全文 | 6632篇 |
免费 | 423篇 |
国内免费 | 2篇 |
专业分类
7057篇 |
出版年
2023年 | 41篇 |
2022年 | 103篇 |
2021年 | 198篇 |
2020年 | 140篇 |
2019年 | 155篇 |
2018年 | 214篇 |
2017年 | 186篇 |
2016年 | 282篇 |
2015年 | 360篇 |
2014年 | 387篇 |
2013年 | 476篇 |
2012年 | 566篇 |
2011年 | 520篇 |
2010年 | 351篇 |
2009年 | 285篇 |
2008年 | 370篇 |
2007年 | 364篇 |
2006年 | 314篇 |
2005年 | 282篇 |
2004年 | 248篇 |
2003年 | 232篇 |
2002年 | 228篇 |
2001年 | 84篇 |
2000年 | 52篇 |
1999年 | 75篇 |
1998年 | 60篇 |
1997年 | 39篇 |
1996年 | 34篇 |
1995年 | 35篇 |
1994年 | 32篇 |
1993年 | 31篇 |
1992年 | 29篇 |
1991年 | 35篇 |
1990年 | 26篇 |
1989年 | 19篇 |
1988年 | 14篇 |
1987年 | 11篇 |
1986年 | 15篇 |
1985年 | 19篇 |
1984年 | 15篇 |
1983年 | 20篇 |
1982年 | 13篇 |
1981年 | 8篇 |
1980年 | 12篇 |
1979年 | 8篇 |
1977年 | 8篇 |
1976年 | 6篇 |
1975年 | 5篇 |
1971年 | 5篇 |
1969年 | 5篇 |
排序方式: 共有7057条查询结果,搜索用时 15 毫秒
31.
Helena Mata Carla Suertegaray Fontana Giovanni Nachtigall Maurício Marcos Ricardo Bornschein Marcelo Ferreira de Vasconcelos Sandro L. Bonatto 《Molecular phylogenetics and evolution》2009,53(2):450-462
Scytalopus and the recently erected Eleoscytalopus are among the Neotropical groups of birds whose taxonomy is most difficult to resolve given their very conservative morphology. We investigated the phylogeny and species limits of Eleoscytalopus and the eastern Scytalopus using two mitochondrial genes and two nuclear introns of multiple individuals from all species of these groups. The eastern Scytalopus are separated in three well defined clades also supported by morphological or vocal characteristics, although the relationships between these clades could not be resolved. We found several allopatric and very divergent lineages in these genera whose characteristics are consistent with species-level divergence, especially in S. speluncae. The great divergence between E. psychopompus and its sister species supports the former as a valid species. Our results corroborate the importance of the Bahia refuge as an avian center of endemism. 相似文献
32.
JoÃo B. Fernandes Otto R. Gottlieb Lucia M. Xavier 《Biochemical Systematics and Ecology》1978,6(1):55-58
The trunk wood of Aniba riparia (Nees) Mez (Lauraceae) contains flavokawin-B, (2S)-pinostrombin, (2S)-5, 7-di-O-methylpinocembrin, (2R, 3R)-5, 7-di-O-methylpinobanksin, izalpinin and 3,5, 7-tri-O-methylgalangin. Structural comparison of these flavonoids with the pyrones and neolignans, which characterized all previously examined Aniba spp., leads to a chemical classification of the genus. 相似文献
33.
2-(2-Furoyl)-4(5)-(2-furanyl)-1H-imidazole (FFI) is a fluorescent molecule which was originally discovered in chloroform extract of ammoniacal solution of acid-hydrolyzed glycated proteins and proposed to represent a protein cross-link. The absence of a lysyl residue side chain and other observations promoted a detailed study of its mechanism of formation. Glycated alpha-t-Butoxycarbonyllysine was incubated for 29 days and periodically assayed for FFI and FFI-like fluorescence. Whereas fluorescence increased over time, FFI recovery was unexpectedly highest on day 0 and lowest on day 29, suggesting that FFI was directly derived from Amadori products. FFI was also recovered from hydrolysates of glycated neopentylamine, furosine, and browned poly-L-lysine but was virtually undetectable in similar solutions basified with NaOH, triethylamine, or pyridine instead of ammonia. Gas chromatography-mass spectrometry analysis of FFI from similar hydrolysates basified in the presence of 15N-enriched NH4Cl revealed for all precursors a parent ion peak at 230 instead of 228 m/e units, suggesting that the two imidazole nitrogen atoms had been incorporated from free ammonia into FFI. Spontaneous FFI synthesis occurred when furosine was reacted with aqueous ammonia at room temperature. These results do not support the proposition that FFI is an advanced glycosylation end product or a protein cross-link. They suggest that FFI is formed from ammonia and furosine which are by-products of acid-hydrolyzed glycated proteins. 相似文献
34.
Rossana Saracino Chiara Capponi Sara Di Persio Carla Boitani Silvia Masciarelli Francesco Fazi Stefania Fera Elena Vicini 《Molecular reproduction and development》2020,87(4):419-429
Glial cell line‐derived neurotrophic factor (GDNF) and retinoic acid (RA) are two molecules crucial for the regulation of the spermatogonial compartment of the testis. During the cycle of the seminiferous epithelium, their relative concentration oscillates with lower GDNF levels in stages where RA levels are high. It has been recently shown that RA negatively regulates Gdnf expression but the mechanisms behind are so far unknown. Here, we show that RA directly downregulates Gdnf mRNA levels in primary murine Sertoli cells through binding of RARα to a novel DR5‐RARE on Gdnf promoter. Pharmacological inhibition and chromatin immunoprecipitation–quantitative polymerase chain reaction analysis suggested that the underlying mechanism involved histone deacetylase activity and epigenetic repression of Gdnf promoter upon RA treatment. 相似文献
35.
Ann Carla Staver Paulo M. Brando Jos Barlow Douglas C. Morton C.E. Timothy Paine Yadvinder Malhi Alejandro Araujo Murakami Jhon del Aguila Pasquel 《Ecology letters》2020,23(1):99-106
Understory fires represent an accelerating threat to Amazonian tropical forests and can, during drought, affect larger areas than deforestation itself. These fires kill trees at rates varying from < 10 to c. 90% depending on fire intensity, forest disturbance history and tree functional traits. Here, we examine variation in bark thickness across the Amazon. Bark can protect trees from fires, but it is often assumed to be consistently thin across tropical forests. Here, we show that investment in bark varies, with thicker bark in dry forests and thinner in wetter forests. We also show that thinner bark translated into higher fire‐driven tree mortality in wetter forests, with between 0.67 and 5.86 gigatonnes CO2 lost in Amazon understory fires between 2001 and 2010. Trait‐enabled global vegetation models that explicitly include variation in bark thickness are likely to improve the predictions of fire effects on carbon cycling in tropical forests. 相似文献
36.
Funck Graciele Daiana de Lima Marques Juliana da Silva Dannenberg Guilherme dos Santos Cruxen Claudio Eduardo Sehn Carla Pohl Prigol Marina Silva Márcia Rósula Poetini da Silva Wladimir Padilha Fiorentini Ângela Maria 《Probiotics and antimicrobial proteins》2020,12(1):91-101
Probiotics and Antimicrobial Proteins - This study aimed to characterize, evaluate toxicity and optimize the conditions for the growth and production of bacteriocin-like substances by Lactobacillus... 相似文献
37.
Carla Martins Lopes Marta De Barba Frdric Boyer Cline Mercier Daniel Galiano Bruno Busnello Kubiak Renan Maestri Pedro Joel Silva da Silva Filho Ludovic Gielly Eric Coissac Thales Renato Ochotorena de Freitas Pierre Taberlet 《Molecular ecology》2020,29(16):3143-3153
Knowledge of how animal species use food resources available in the environment can increase our understanding of many ecological processes. However, obtaining this information using traditional methods is difficult for species feeding on a large variety of food items in highly diverse environments. We amplified the DNA of plants for 306 scat and 40 soil samples, and applied an environmental DNA metabarcoding approach to investigate food preferences, degree of diet specialization and diet overlap of seven herbivore rodent species of the genus Ctenomys distributed in southern and midwestern Brazil. The metabarcoding approach revealed that these species consume more than 60% of the plant families recovered in soil samples, indicating generalist feeding habits of ctenomyids. The family Poaceae was the most common food resource retrieved in scats of all species as well in soil samples. Niche overlap analysis indicated high overlap in the plant families and molecular operational taxonomic units consumed, mainly among the southern species. Interspecific differences in diet composition were influenced, among other factors, by the availability of resources in the environment. In addition, our results provide support for the hypothesis that the allopatric distributions of ctenomyids allow them to exploit the same range of resources when available, possibly because of the absence of interspecific competition. 相似文献
38.
39.
40.
dos Santos Fernanda Marcelia Pflüger Pricila Fernandes Lazzarotto Leticia Uczay Mariana de Aguida Wesley Roberto da Silva Lisiane Santos Boaretto Fernanda Brião Menezes de Sousa Jayne Torres Picada Jaqueline Nascimento da Silva Torres Iraci Lucena Pereira Patrícia 《Neurochemical research》2021,46(8):2066-2078
Neurochemical Research - Gamma-decanolactone (GD) has been shown to reduce epileptic behavior in different models, inflammatory decreasing, oxidative stress, and genotoxic parameters. This study... 相似文献