全文获取类型
收费全文 | 4617篇 |
免费 | 477篇 |
国内免费 | 628篇 |
出版年
2024年 | 23篇 |
2023年 | 137篇 |
2022年 | 159篇 |
2021年 | 237篇 |
2020年 | 226篇 |
2019年 | 273篇 |
2018年 | 223篇 |
2017年 | 190篇 |
2016年 | 212篇 |
2015年 | 230篇 |
2014年 | 245篇 |
2013年 | 384篇 |
2012年 | 167篇 |
2011年 | 171篇 |
2010年 | 162篇 |
2009年 | 241篇 |
2008年 | 252篇 |
2007年 | 245篇 |
2006年 | 246篇 |
2005年 | 174篇 |
2004年 | 159篇 |
2003年 | 138篇 |
2002年 | 155篇 |
2001年 | 129篇 |
2000年 | 89篇 |
1999年 | 84篇 |
1998年 | 94篇 |
1997年 | 58篇 |
1996年 | 60篇 |
1995年 | 63篇 |
1994年 | 57篇 |
1993年 | 49篇 |
1992年 | 43篇 |
1991年 | 32篇 |
1990年 | 35篇 |
1989年 | 45篇 |
1988年 | 16篇 |
1987年 | 28篇 |
1986年 | 22篇 |
1985年 | 34篇 |
1984年 | 27篇 |
1983年 | 16篇 |
1982年 | 17篇 |
1981年 | 15篇 |
1980年 | 12篇 |
1979年 | 11篇 |
1978年 | 17篇 |
1977年 | 7篇 |
1976年 | 8篇 |
1975年 | 4篇 |
排序方式: 共有5722条查询结果,搜索用时 203 毫秒
131.
JOHNJ. PIZZIMENTI ROB DE SALLE 《Biological journal of the Linnean Society. Linnean Society of London》1981,15(4):339-354
The distributional abundance of 13 species of southern Peruvian rodents is significantly correlated with altitude and patterns of vegetation but vegetation is a better predictor than altitude. Coincidental reversals in the altitudinal trends of both vegetation and rodents demonstrate that rodents are responding to vegetation patterns. Abundance and diversity of rodents are greatest around 4000 m elevation which coincides with the region of greatest vegetational abundance and seasonally heavy rainfall.
The rodent communities consist of two separate feeding guilds, omnivores and insectivores and these guilds show distinctly different distributional patterns. Insectivorous species are strongly associated with the zone of seasonally heavy rainfall (4000 m) whereas omnivorous rodents are relatively abundant over a broader spectrum of elevations and habitats. In a previous study of these same communities, we showed a correspondence between rodent morphology and their diets. This study reveals a correspondence between distributional abundance of the various species and their diets. We conclude that the distributional abundance of southern Peruvian rodents is related to the physiological constraints imposed by altitude (climate) and the distributional abundance of food resources. 相似文献
The rodent communities consist of two separate feeding guilds, omnivores and insectivores and these guilds show distinctly different distributional patterns. Insectivorous species are strongly associated with the zone of seasonally heavy rainfall (4000 m) whereas omnivorous rodents are relatively abundant over a broader spectrum of elevations and habitats. In a previous study of these same communities, we showed a correspondence between rodent morphology and their diets. This study reveals a correspondence between distributional abundance of the various species and their diets. We conclude that the distributional abundance of southern Peruvian rodents is related to the physiological constraints imposed by altitude (climate) and the distributional abundance of food resources. 相似文献
132.
133.
The suitability of bead mills for the release of intracellular bacterial enzymes has been studied using the Dyno-Mill Model KDL. The effect of cell concentration, bead size and agitation speed on the release of beta-lactamase from Enterobacter cloacae P99 was examined. Scale-up considerations included, the best operational values for these parameters were 1 g cell paste suspended in 2.5 ml buffer, 0.25 mm diameter glass beads and 15 ms −1 agitation speed. These conditions proved suitable for the release of enzymes from other Gram-negative bacteria in both batch and continuous processes. 相似文献
134.
S L Marcus S W Smith C I Jarowski M J Modak 《Biochemical and biophysical research communications》1976,70(1):37-44
Guanosine, rather than its methylated derivative, was found to be the inverted nucleoside present in the 5′ terminal capping structure of insect oocyte messenger RNA. Since methylation of the terminal guanosine at the 7 position is necessary for the initiation of protein synthesis in eukaryotes, this evidence suggests that the translational inactivity of the mRNA prior to fertilization may be associated with the absence of methylation. 相似文献
135.
D. Sharp 《Plant Ecology》1976,31(2):103-119
Summary The vegetation of cultivated, fallow and ‘old’ fields of Scotland County, N.C. was both randomly and subjectively sampled
throughout the growing season of 1973. Relevés were placed in each of the above management units, and the data collected in
this relevé analysis were sorted into phytosociological charts, with the aid of the digital computer.
Weed associations delimited in the charts reflected the importance of edaphic factors, time (aspect), and man's agricultural
activities as causal factors in the determination of characteristic assemblages of weeds. Several character and differential
species were denoted, in a tentative classification of the weed associations. The cultivated and fallow stands in the spring
aspect were noted to be dominated by a group of spring annuals. Summer annuals were more prevalent in homologous situations
in the summer. It was determined that perennials, appearing mainly as seedlings in cultivated and fallow fields, attained
dominance in ‘old’ fields, one to five years in age. Within each of the major spheres of management, and in each season, associations
of weeds mainly reflected differences in edaphic factors. Many character species were noted to maintain affinities to an assortment
of soil types that ranged from moist loams to xeric, sterile sands.
A study of roadsides, hedgerows, field margins and other ecotonal areas inhabited by weeds was carried throughout the growing
season of 1973, also. Species presence was documented in each of these habitat types and reported in Sharp (1974).
The weed flora of Scotland County was found to be rich in species. A total of 506 species representing 248 genera and 80 families
were encountered throughout the study. Of the total number of species listed, approximately 25 percent were of foreign origin,
and 28 percent represented additions to the flora of Scotland County.
This paper is the short version of a masters thesis for the Department of Botany, University of North Carolina at Chapel Hill.
The author would like to extend his appreciation and gratitude to Professors H. Lieth, A. E. Radford and J. Hardin for their
generous advice and encouragement extended through the duration of the study. 相似文献
136.
P. E. MALONEY S. C. LYNCH S. F. KANE C. E. JENSEN D. M. RIZZO 《Journal of Ecology》2005,93(5):899-905
137.
Sang-Im Yun Byung-Hak Song Jin-Kyoung Kim Young-Min Lee 《Journal of visualized experiments : JoVE》2015,(106)
Reverse genetics, an approach to rescue infectious virus entirely from a cloned cDNA, has revolutionized the field of positive-strand RNA viruses, whose genomes have the same polarity as cellular mRNA. The cDNA-based reverse genetics system is a seminal method that enables direct manipulation of the viral genomic RNA, thereby generating recombinant viruses for molecular and genetic studies of both viral RNA elements and gene products in viral replication and pathogenesis. It also provides a valuable platform that allows the development of genetically defined vaccines and viral vectors for the delivery of foreign genes. For many positive-strand RNA viruses such as Japanese encephalitis virus (JEV), however, the cloned cDNAs are unstable, posing a major obstacle to the construction and propagation of the functional cDNA. Here, the present report describes the strategic considerations in creating and amplifying a genetically stable full-length infectious JEV cDNA as a bacterial artificial chromosome (BAC) using the following general experimental procedures: viral RNA isolation, cDNA synthesis, cDNA subcloning and modification, assembly of a full-length cDNA, cDNA linearization, in vitro RNA synthesis, and virus recovery. This protocol provides a general methodology applicable to cloning full-length cDNA for a range of positive-strand RNA viruses, particularly those with a genome of >10 kb in length, into a BAC vector, from which infectious RNAs can be transcribed in vitro with a bacteriophage RNA polymerase. 相似文献
138.
The interrelation of Alpine topography with the micro − and mesobiota is still poorly understood. We investigated the effects of ground cover type and slope exposure on the soil microbial biomass (double-stranded DNA, dsDNA) and abundances (real time PCR, qPCR); hydrolytic enzyme activities; and enchytraeid community structure in top soils (2.5-cm increments depth) in subalpine forests in the Italian Alps. Dominant ground covers were grass, moss, litter and woody debris at the north- and the south-facing slopes. The autochthonous soil microbiota (bacteria, fungi and archaea) was quantified by qPCR in the extracellular (eDNA) and intracellular fraction (iDNA) of the total soil DNA pool. A higher eDNA/iDNA ratio indicative of lower microbial activity was recorded in the deepest layer of the grass plots at the north-facing slope. This can be related to a lower degradation of eDNA and/or to an accumulation of eDNA with increasing depth as a result of leaching. The exposure effect was enzyme-specific and higher activities occurred under woody debris primarily at the south-facing slope. These plots also showed a higher nutrient content and a greater microbial biomass assessed as dsDNA yields. Total microannelid abundance was elevated on north-facing slopes on account of strong acidity indicator species. This was related to soil pH being one unit lower compared to the south-facing slope. The thickness of the organic layer (OL + OF + OH) was elevated at the north-facing slope due to a considerably thicker OH-horizon. The vast majority of microannelids at this slope occurred in the organic layer, while at south exposure they were almost evenly distributed between the organic layer and the mineral soil (A-horizon). Exposure was found to be more determinative for the composition of microannelid assemblages than the ground cover type. 相似文献
139.
Elsa Leitão Ana Catarina Costa Claudia Brito Lionel Costa Rita Pombinho 《Cell cycle (Georgetown, Tex.)》2014,13(6):928-940
Listeria monocytogenes (Lm) is a human intracellular pathogen widely used to uncover the mechanisms evolved by pathogens to establish infection. However, its capacity to perturb the host cell cycle was never reported. We show that Lm infection affects the host cell cycle progression, increasing its overall duration but allowing consecutive rounds of division. A complete Lm infectious cycle induces a S-phase delay accompanied by a slower rate of DNA synthesis and increased levels of host DNA strand breaks. Additionally, DNA damage/replication checkpoint responses are triggered in an Lm dose-dependent manner through the phosphorylation of DNA-PK, H2A.X, and CDC25A and independently from ATM/ATR. While host DNA damage induced exogenously favors Lm dissemination, the override of checkpoint pathways limits infection. We propose that host DNA replication disturbed by Lm infection culminates in DNA strand breaks, triggering DNA damage/replication responses, and ensuring a cell cycle delay that favors Lm propagation. 相似文献
140.