全文获取类型
收费全文 | 126248篇 |
免费 | 3111篇 |
国内免费 | 2699篇 |
专业分类
132058篇 |
出版年
2024年 | 69篇 |
2023年 | 395篇 |
2022年 | 937篇 |
2021年 | 1638篇 |
2020年 | 1040篇 |
2019年 | 1255篇 |
2018年 | 12739篇 |
2017年 | 11277篇 |
2016年 | 8587篇 |
2015年 | 2593篇 |
2014年 | 2633篇 |
2013年 | 2826篇 |
2012年 | 7179篇 |
2011年 | 15318篇 |
2010年 | 13494篇 |
2009年 | 9517篇 |
2008年 | 11305篇 |
2007年 | 12708篇 |
2006年 | 1599篇 |
2005年 | 1634篇 |
2004年 | 1958篇 |
2003年 | 1859篇 |
2002年 | 1505篇 |
2001年 | 931篇 |
2000年 | 838篇 |
1999年 | 678篇 |
1998年 | 394篇 |
1997年 | 359篇 |
1996年 | 368篇 |
1995年 | 333篇 |
1994年 | 332篇 |
1993年 | 245篇 |
1992年 | 371篇 |
1991年 | 307篇 |
1990年 | 313篇 |
1989年 | 264篇 |
1988年 | 206篇 |
1987年 | 184篇 |
1986年 | 152篇 |
1985年 | 134篇 |
1984年 | 128篇 |
1983年 | 112篇 |
1982年 | 79篇 |
1981年 | 61篇 |
1979年 | 75篇 |
1978年 | 55篇 |
1977年 | 53篇 |
1975年 | 64篇 |
1972年 | 287篇 |
1971年 | 310篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
261.
The energetic incubation constraint hypothesis (EICH) for clutch size states that birds breeding in poor habitat may free
up resources for future reproduction by laying a smaller clutch. The eider (Somateria mollissima) is considered a candidate for supporting this hypothesis. Clutch size is smaller in exposed nests, presumably because of
faster heat loss and higher incubation cost, and, hence, smaller optimal clutch size. However, an alternative explanation
is partial predation: the first egg(s) are left unattended and vulnerable to predation, which may disproportionately affect
exposed nests, so clutch size may be underestimated. We experimentally investigated whether predation on first-laid eggs in
eiders depends on nest cover. We then re-evaluated how nesting habitat affects clutch size and incubation costs based on long-term
data, accounting for confounding effects between habitat and individual quality. We also experimentally assessed adult survival
costs of nesting in sheltered nests. The risk of egg predation in experimental nests decreased with cover. Confounding between
individual and habitat quality is unlikely, as clutch size was also smaller in open nests within individuals, and early and
late breeders had similar nest cover characteristics. A trade-off between clutch and female safety may explain nest cover
variation, as the risk of female capture by us, mimicking predation on adults, increased with nest cover. Nest habitat had
no effect on female hatching weight or weight loss, while lower temperature during incubation had an unanticipated positive
relationship with hatching weight. There were no indications of elevated costs of incubating larger clutches, while clutch
size and colony size were positively correlated, a pattern not predicted by the ‘energetic incubation constraint’ hypothesis.
Differential partial clutch predation thus offers the more parsimonious explanation for clutch size variation among habitats
in eiders, highlighting the need for caution when analysing fecundity and associated life-history parameters when habitat-specific
rates of clutch predation occur. 相似文献
262.
The rare sugar xylitol is a five-carbon polyol (pentitol) that has beneficial health effects. Xylitol has global markets and,
therefore, it represents an alternative to current dominant sweeteners. The research on microbial reduction of d-xylose to xylitol has been focused on metabolically engineered Saccharomycess cerevisiae and Candida strains. The Candida strains have an advantage over the metabolically engineered S. cerevisiae in terms of d-xylose uptake and maintenance of the intracellular redox balance. Due to the current industrial scale production of xylitol,
it has become an inexpensive starting material for the production of other rare sugar. The first part of this mini-review
concentrates on the biochemistry of xylitol biosynthesis and the problems related to intracellular redox balance. 相似文献
263.
Celia G. Jerez Carolina B. García Agustín Rearte Félix L. Figueroa 《Journal of applied phycology》2016,28(3):1635-1648
In order to estimate microalgal carbon assimilation or production of Chlorella fusca cultures based on electron transport rate (ETR) as in vivo chlorophyll a fluorescence, it is necessary to determine the photosynthetic yield and the absorbed quanta by measuring the incident irradiance and the fraction of absorbed light, i.e., absorptance or absorption coefficient in the photosynthetic active radiation (PAR) region of the spectra. Due to difficulties associated with the determination of light absorption, ETR is commonly expressed as relative units (rETR) although this is not a good estimator of the photosynthetic production since photobiological responses depend on the absorbed light. The quantitative filter technique (QFT) is commonly used to measure the absorbed quanta of cells retained on a filter (AbQf) as estimator of the absorbed quanta of cell suspensions (AbQs) determined by using integrating spheres. In this study, light attenuation of thin-layer cell suspensions is determined by using a measuring system designed to reduce the scattering. The light attenuation is related to the absorptance as the fraction of absorbed light by both indoor and outdoor C. fusca cultures of different cell densities. A linear relation between AbQf and AbQs (R 2?=?0.9902, p?<?0.01) was observed, AbQf?=?1.98?×?AbQs, being 1.98 an amplification factor to convert AbQs values into AbQf ones. On the other hand, depending on the culture system, the convenience of the use of the absorptance, light absorption or specific light absorption coefficient expressed per area (thin-layer cascade or flat panel cultivators), volume (cylindrical and tubular photobioreactors), or chlorophyll units (any type of cultivation system) is discussed. The procedure for the measurement of light absorption presented in this study for C. fusca could be applied in other phytoplankton groups. The absorbed quanta as determined in this study can be used to express absolute ETR instead of relative ETR, since the first one provides much more relevant photobiological information of microalgae culture systems. 相似文献
264.
豚鼠冰水游泳应激3min后,用放射免疫法测定血浆、肾上腺髓质和脑内三个脑区组织的亮—脑啡肽(Leu-enkephalin,LEK,)含量和血浆的血管紧张肽Ⅱ(Angiotensin Ⅱ,ATⅡ)浓度变化。结果表明:在冰水中游泳应激组豚鼠的血浆、肾上腺髓质、下丘脑和纹状体组织的LEK含量和血浆的ATⅡ浓度,均较对照组豚鼠明显升高(P值均<0.01)。提示:豚鼠在冰水中紧张、剧烈游泳后,可能激活了中枢神经和周围组织内的脑啡肽能神经元和血管紧张肽原酶—血管紧张肽Ⅱ系统,从而促进LEK和ATⅡ的释放增加。 相似文献
265.
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
266.
Joni Esrom Lima Vagner Augusto Benedito Antonio Figueira Lázaro Eustáquio Pereira Peres 《Plant cell reports》2009,28(8):1169-1177
We analyzed the impact of ethylene and auxin disturbances on callus, shoots and Agrobacterium rhizogenes-induced hairy root formation in tomato (Solanum lycopersicum L.). The auxin low-sensitivity dgt mutation showed little hairy root initiation, whereas the ethylene low-sensitivity Nr mutation did not differ from the control Micro-Tom cultivar. Micro-Tom and dgt hairy roots containing auxin sensitivity/biosynthesis rol and aux genes formed prominent callus onto media supplemented with cytokinin. Under the same conditions, Nr hairy roots did not form callus. Double mutants combining Rg1, a mutation conferring elevated shoot formation capacity, with either dgt or Nr produced explants that formed shoots with little callus proliferation. The presence of rol + aux genes in Rg1 hairy roots prevented shoot formation. Taken together, the results suggest that although ethylene does not affect hairy root
induction, as auxin does, it may be necessary for auxin-induced callus formation in tomato. Moreover, excess auxin prevents
shoot formation in Rg1. 相似文献
267.
Tony Robinet Eric Feunteun Philippe Keith Gérard Marquet Jean-Michel Olivier Elodie Réveillac Pierre Valade 《Environmental Biology of Fishes》2007,78(2):93-105
Anguillid eels were sampled from permanent rivers in the Réunion and Mauritius islands, western Indian Ocean, with a standardized
electrofishing method. A. marmorata was very dominant, corresponding to 91.7 and 90.7% of all the eels collected in Réunion and Mauritius, respectively. Three
other species (A. mossambica, A. bicolor bicolor and A. nebulosa labiata) were also present in both islands. A. marmorata showed a strong altitudinal gradient of densities from the lower to upper zones, especially in the younger stages (TL <250 mm),
while A. mossambica was only found in the upper zones and A. bicolor bicolor occurred only in the lower zones (A. nebulosa labiata was rare). The eel species composition in freshwaters of both islands is very similar because these two adjoining islands
are located in the same trail of drifting marine larvae. Mean estimated eel biomasses were noticeably low (11.1 and 22.2 kg ha−1 in Réunion and Mauritius islands, respectively), especially when compared to those of other tropical insular systems without
any eel fishery (Comoros or Polynesia, more than 100 kg ha−1). Nevertheless, the fluvial recruitment of A. marmorata seemed to be regular during the surveyed period, staggering from October to April. The obvious lack of large eels in Mauritius
but more significantly in Réunion suggests a high pressure from traditional fishery, and the local reproductive turnover is
uncertain. Because sexual maturation seems to occur at a large body size for A. marmorata, as for temperate species, the Réunion and Mauritius rivers may only have a weak contribution to the regional production
of spawners. However, the giant mottled eel population in the western Indian Ocean is believed to be panmictic at the regional
scale, and may not rely exclusively on these islands’ contribution. A comparison is made with those of freshwater systems
in other tropical islands. 相似文献
268.
Kung YJ Lin SS Huang YL Chen TC Harish SS Chua NH Yeh SD 《Molecular Plant Pathology》2012,13(3):303-317
MicroRNAs (miRNAs) regulate the abundance of target mRNAs by guiding cleavage at sequence complementary regions. In this study, artificial miRNAs (amiRNAs) targeting conserved motifs of the L (replicase) gene of Watermelon silver mottle virus (WSMoV) were constructed using Arabidopsis pre-miRNA159a as the backbone. The constructs included six single amiRNAs targeting motifs A, B1, B2, C, D of E, and two triple amiRNAs targeting motifs AB1E or B2DC. Processing of pre-amiRNAs was confirmed by agro-infiltration, and transgenic Nicotiana benthamiana plants expressing each amiRNA were generated. Single amiRNA transgenic lines expressing amiR-LB2 or amiR-LD showed resistance to WSMoV by delaying symptom development. Triple amiRNA lines expressing amiR-LB2, amiR-LD and amiR-LC provided complete resistance against WSMoV, with no indication of infection 28 days after inoculation. Resistance levels were positively correlated with amiRNA expression levels in these single and triple amiRNA lines. The triple amiR-LAB1E line did not provide resistance to WSMoV. Similarly, the poorly expressed amiR-LC and amiR-LE lines did not provide resistance to WSMoV. The amiR-LA- and amiR-LB1-expressing lines were susceptible to WSMoV, and their additional susceptibility to the heterologous Turnip mosaic virus harbouring individual target sequences indicated that these two amiRNAs have no effect in vivo. Transgenic lines expressing amiR-LB2 exhibited delayed symptoms after challenge with Peanut bud necrosis virus having a single mismatch in the target site. Overall, our results indicate that two amiRNAs, amiR-LB2 and amiR-LD, of the six designed amiRNAs confer moderate resistance against WSMoV, and the triple construct including the two amiRNAs provides complete resistance. 相似文献
269.
Brett M. Bennett 《Journal of the history of biology》2011,44(1):125-145
Scholars studying the globalization of Australian trees have previously emphasized the rapid natural propagation of Australian trees outside of their native habitats, believing their success to be a reversal of “ecological imperialism” from the “new world” to the “old world.” This article argues that the expansion of Australian trees should not be viewed as a biological phenomenon, but as the result of a long-term attempt by powerful states and state-sponsored scientists to select and breed Australian species that could grow in a variety of climates and ecological conditions. Five non-biological factors largely determined the success of these attempts to grow Australian trees: the abundance or paucity of natural forests, state power, the amount of scientific research directed to planting Australian trees, the cost of labor, and the ability to utilize hardwood timbers and bark. This paper compares the use of Australian trees in Australia, India, and South Africa to demonstrate that biology was not the determining factor in the long-term success of many Australian genera and species. 相似文献
270.
A juvenile common marmoset (Callithrix jacchus) was attacked by a whitetail lancehead viper (Bothrops leucurus) while playing with other group members close to the ground at a site in northeastern Brazil. The attack was almost immediately fatal, but the viper was unable to ingest the body of the marmoset. After approximately 10 min, during which it attempted to swallow the marmoset a number of times, the viper moved away, abandoning the body. While raptors are the principal predators of callitrichids, this record reinforces the relative vulnerability of these primates to snakes in comparison with other platyrrhines, although the small number of recorded events precludes a more definitive analysis of the phenomenon. 相似文献