全文获取类型
收费全文 | 116179篇 |
免费 | 2814篇 |
国内免费 | 814篇 |
出版年
2023年 | 113篇 |
2022年 | 140篇 |
2021年 | 485篇 |
2020年 | 287篇 |
2019年 | 345篇 |
2018年 | 12161篇 |
2017年 | 10979篇 |
2016年 | 8046篇 |
2015年 | 1748篇 |
2014年 | 1441篇 |
2013年 | 1715篇 |
2012年 | 5997篇 |
2011年 | 14495篇 |
2010年 | 13042篇 |
2009年 | 9137篇 |
2008年 | 11006篇 |
2007年 | 12630篇 |
2006年 | 1499篇 |
2005年 | 1697篇 |
2004年 | 2075篇 |
2003年 | 1988篇 |
2002年 | 1696篇 |
2001年 | 665篇 |
2000年 | 511篇 |
1999年 | 362篇 |
1998年 | 250篇 |
1997年 | 201篇 |
1996年 | 171篇 |
1995年 | 148篇 |
1994年 | 152篇 |
1993年 | 168篇 |
1992年 | 237篇 |
1991年 | 243篇 |
1990年 | 199篇 |
1989年 | 202篇 |
1988年 | 193篇 |
1987年 | 197篇 |
1986年 | 174篇 |
1985年 | 184篇 |
1984年 | 156篇 |
1983年 | 154篇 |
1982年 | 118篇 |
1981年 | 107篇 |
1979年 | 122篇 |
1978年 | 102篇 |
1977年 | 129篇 |
1976年 | 97篇 |
1974年 | 130篇 |
1972年 | 325篇 |
1971年 | 370篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
951.
952.
Norihide Hinomoto Tomomi Higaki Junichiro Abe Masafumi Yamane Eizi Yano 《Applied Entomology and Zoology》2012,47(2):165-171
We have developed and characterized 21 microsatellite markers in the aphidophagous gall midge Aphidoletes aphidimyza (Rondani) (Diptera: Cecidomyiidae). All 21 loci tested were polymorphic: the number of alleles ranged from 2 to 17. Allelic
richness and observed heterozygosities were higher in females than in males. Several loci had no heterozygosity in males,
suggesting that the loci were located on sex chromosomes or E-chromosomes, common to cecidomyiids. The high polymorphism detected
in this study suggests the markers will be of value in analyzing genetic structure of field populations. 相似文献
953.
Eric M. Lind Elizabeth Borer Eric Seabloom Peter Adler Jonathan D. Bakker Dana M. Blumenthal Mick Crawley Kendi Davies Jennifer Firn Daniel S. Gruner W. Stanley Harpole Yann Hautier Helmut Hillebrand Johannes Knops Brett Melbourne Brent Mortensen Anita C. Risch Martin Schuetz Carly Stevens Peter D. Wragg 《Ecology letters》2013,16(4):513-521
Plant growth can be limited by resource acquisition and defence against consumers, leading to contrasting trade‐off possibilities. The competition‐defence hypothesis posits a trade‐off between competitive ability and defence against enemies (e.g. herbivores and pathogens). The growth‐defence hypothesis suggests that strong competitors for nutrients are also defended against enemies, at a cost to growth rate. We tested these hypotheses using observations of 706 plant populations of over 500 species before and following identical fertilisation and fencing treatments at 39 grassland sites worldwide. Strong positive covariance in species responses to both treatments provided support for a growth‐defence trade‐off: populations that increased with the removal of nutrient limitation (poor competitors) also increased following removal of consumers. This result held globally across 4 years within plant life‐history groups and within the majority of individual sites. Thus, a growth‐defence trade‐off appears to be the norm, and mechanisms maintaining grassland biodiversity may operate within this constraint. 相似文献
954.
955.
956.
957.
A molecular phylogeny of New World emballonurid bats based on parsimony and Bayesian analyses of loci from the three different
nuclear genetic transmission pathways in mammals (autosomal, X, and Y chromosomes) is well supported and independently corroborated by each individual gene tree. This is in contrast to a single
most parsimonious but poorly supported tree based on morphological data, which has only one intergeneric or higher relationship
shared with the molecular phylogeny. Combining the morphological and molecular data partitions results in a tree similar to
the molecular tree suggesting a high degree of homoplasy and low phylogenetic signal in the morphological data set. Behavioral
data are largely incomplete and likewise produce a poorly resolved tree. Nonetheless, patterns of evolution in morphology
and behavior can be investigated by using the molecular tree as a phylogenetic framework. Character optimization of the appearance
of dorsal fur and preferred roosting sites maps consistently and are correlated on the phylogeny. This suggests an association
of camouflage for bats with unusual appearance (two dorsal stripes in Rhynchonycteris and Saccopteryx, or pale fur in Cyttarops and Diclidurus) and roosting in exposed sites (tree trunks or under palm leaves). In contrast, the ancestral states for Old and New World
emballonurids are typically uniform brown or black, and they usually roost in sheltered roosts such as caves and tree hollows.
Emballonuridae is the only family of bats that has a sac-like structure in the wing propatagium, which is found in four New
World genera. Mapping the wing sac character states onto the phylogeny indicates that wing sacs evolved independently within
each genus and that there may be a phylogenetic predisposition for this structure. Ear orientation maps relatively consistently
on the molecular phylogeny and is correlated to echolocation call parameters and foraging behavior, suggesting a phylogenetic
basis for these character systems. 相似文献
958.
959.
Leonardo Mata Helena Gaspar Fátima Justino Rui Santos 《Journal of applied phycology》2011,23(5):827-832
The genus Asparagopsis is a prolific source of halogenated metabolites. Due to its commercial applications, it has been intensively cultivated in
southern Portugal. In the present study, we assess if the internal levels of the major halogenated metabolites (bromoform
and dibromoacetic acid) in Asparagopsis taxiformis can be increased with hydrogen peroxide (H2O2) addition. Previous studies with red algae showed that the production/release of bromoform can be enhanced by exogenously
supplying H2O2. However, no study has assessed if H2O2 supply enhances the content of secondary metabolites within the biomass. This detail is important as the objective of the
proposed research is to enhance the content of these valuable metabolites in the produced biomass. Both the activity of the
haloperoxidase enzyme and the metabolite content were assessed on short-term and long-term incubation periods to H2O2. To determine the susceptibility of A. taxiformis photosynthetic performance to the imposed oxidative stress, the in vivo fluorescence of photosystem II was monitored. A. taxiformis was shown to be physiologically vulnerable to H2O2, given the observed decrease of the maximum quantum yield of photosynthesis (F
v/F
m). Contrary to what was expected, the presence of H2O2 inhibited the activity of the iodoperoxidase enzyme. Nevertheless, the extracted halogenated metabolites were higher over
the first hours of exposure to H2O2, decreasing after 48 h. These results are probably related to the prosthetic group of the halogenated enzyme in A. taxiformis and the long-term oxidative stress damage of H2O2 exposure. Considering the objective of the proposed research, addition of H2O2 to the cultures, prior (3 h) to biomass harvesting, increases the metabolite content. 相似文献
960.
Food restriction (FR) is hypothesized to decrease body fat content of an animal and thus prevent obesity. However, the response
of energy budget to a continuous (CFR) or discontinuous FR (DFR) remains inconsistent. In the present study, effects of CFR
or DFR and refeeding on energy budget and behavior were examined in male Swiss mice. CFR significantly decreased the energy
expenditure associated with basal metabolic rate (BMR) and activity behavior, but not sufficiently to compensate for energy
deficit and thus resulted in lower body mass and fat content. DFR mice had a significantly higher food intake on ad libitum
days and showed increases in BMR and activity after 4 weeks’ DFR, which might resulted in lower body mass and less body fat
than controls. After being refed ad libitum, both CFR and DFR mice had similar body mass, BMR, and behavioral patterns to
controls but had 95% and 75% higher fat content. This suggested that not only CFR but also DFR would be a significant factor
in the process of obesity for animals that were refed ad libitum. It also indicated that food restriction interrupted many
times by periods of ad libitum feeding had the same long-term effects like continuous underfeeding. 相似文献