全文获取类型
收费全文 | 13955篇 |
免费 | 1153篇 |
国内免费 | 2篇 |
出版年
2023年 | 58篇 |
2022年 | 136篇 |
2021年 | 276篇 |
2020年 | 164篇 |
2019年 | 200篇 |
2018年 | 270篇 |
2017年 | 258篇 |
2016年 | 420篇 |
2015年 | 690篇 |
2014年 | 784篇 |
2013年 | 954篇 |
2012年 | 1269篇 |
2011年 | 1203篇 |
2010年 | 728篇 |
2009年 | 647篇 |
2008年 | 926篇 |
2007年 | 960篇 |
2006年 | 816篇 |
2005年 | 785篇 |
2004年 | 689篇 |
2003年 | 664篇 |
2002年 | 662篇 |
2001年 | 126篇 |
2000年 | 98篇 |
1999年 | 127篇 |
1998年 | 158篇 |
1997年 | 94篇 |
1996年 | 81篇 |
1995年 | 81篇 |
1994年 | 76篇 |
1993年 | 76篇 |
1992年 | 64篇 |
1991年 | 69篇 |
1990年 | 57篇 |
1989年 | 45篇 |
1988年 | 37篇 |
1987年 | 36篇 |
1986年 | 28篇 |
1985年 | 19篇 |
1984年 | 33篇 |
1983年 | 25篇 |
1982年 | 11篇 |
1981年 | 22篇 |
1980年 | 16篇 |
1979年 | 14篇 |
1978年 | 13篇 |
1974年 | 10篇 |
1971年 | 15篇 |
1966年 | 11篇 |
1964年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
971.
Background
After a volcano erupts, a lake may form in the cooled crater and become an isolated aquatic ecosystem. This makes fishes in crater lakes informative for understanding sympatric evolution and ecological diversification in barren environments. From a geological and limnological perspective, such research offers insight about the process of crater lake ecosystem establishment and speciation. In the present study we use genetic and coalescence approaches to infer the colonization history of Midas cichlid fishes (Amphilophus cf. citrinellus) that inhabit a very young crater lake in Nicaragua-the ca. 1800 year-old Lake Apoyeque. This lake holds two sympatric, endemic morphs of Midas cichlid: one with large, hypertrophied lips (~20% of the total population) and another with thin lips. Here we test the associated ecological, morphological and genetic diversification of these two morphs and their potential to represent incipient speciation. 相似文献972.
Localization of a specific subset of maternal mRNAs to the vegetal cortex of Xenopus oocytes is important for the regulation of germ layer formation and germ cell development. It is driven by vegetal localization complexes that are formed with the corresponding signal sequences in the untranslated regions of the mRNAs and with a number of different so-called localization proteins. In the context of the present study, we incorporated tagged variants of the known localization protein Vg1RBP into vegetal localization complexes by means of oocyte microinjection. Immunoprecipitation of the corresponding RNPs allowed for the identification of novel Vg1RBP-associated proteins, such as the embryonic poly(A) binding protein, the Y-box RNA-packaging protein 2B and the oocyte-specific version of the elongation factor 1α (42Sp50). Incorporation of 42Sp50 into localization RNPs could be confirmed by co-immunoprecipitation of Vg1RBP and Staufen1 with myc-tagged 42Sp50. Furthermore, myc-42Sp50 was found to co-sediment with the same two proteins in large, RNAse-sensitive complexes, as well as to associate specifically with several vegetally localizing mRNAs but not with nonlocalized control RNAs. Finally, oocyte microinjection experiments reveal that 42Sp50 is a protein that shuttles between the nucleus and cytoplasm. Taken together, these observations provide evidence for a novel function of 42Sp50 in the context of vegetal mRNA transport in Xenopus oocytes. 相似文献
973.
Background and Purpose
A model for carcinoma and sarcoma induction was used to study the dependence of carcinogenesis after radiotherapy on fractionation. 相似文献974.
Alexander Ziegler Malte Ogurreck Thomas Steinke Felix Beckmann Steffen Prohaska Andreas Ziegler 《Biology direct》2010,5(1):45
Abstract
Advances in digital data acquisition, analysis, and storage have revolutionized the work in many biological disciplines such as genomics, molecular phylogenetics, and structural biology, but have not yet found satisfactory acceptance in morphology. Improvements in non-invasive imaging and three-dimensional visualization techniques, however, permit high-throughput analyses also of whole biological specimens, including museum material. These developments pave the way towards a digital era in morphology. Using sea urchins (Echinodermata: Echinoidea), we provide examples illustrating the power of these techniques. However, remote visualization, the creation of a specialized database, and the implementation of standardized, world-wide accepted data deposition practices prior to publication are essential to cope with the foreseeable exponential increase in digital morphological data. 相似文献975.
Seizures or convulsions that occur during anaesthesia in veterinary patients are infrequently reported in the literature.
Consequently, the incidence of such events is unknown. Several drugs commonly used in clinical veterinary anaesthesia have
been shown to induce epileptiform activity in both human clinical patients and experimental candidates. The present case report
describes convulsions in a four-year old male Bernese mountain dog during maintenance of anaesthesia with isoflurane after
premedication with acepromazine and methadone followed by co-induction with propofol and ketamine. The dog had no history
of previous convulsions. The use of several sedative and anaesthetic drugs makes it difficult to find one single causative
pharmaceutical. 相似文献
976.
Grebe M Pröfrock D Kakuschke A Broekaert JA Prange A 《Metallomics : integrated biometal science》2010,2(10):683-693
The health status of marine mammals such as harbour seals (Phoca vitulina) represents an indirect but powerful way for the assessment of environmental changes. The present work illustrates the first investigation and characterisation of Tf isolated from blood samples of North Sea harbour seals with a view to using changes in Tf isoform patterns as an additional parameter in extended studies of their health status. Therefore, an HPLC-ICP-MS approach has been developed which allows the highly resolved separation and fractionation of up to eight different Tf isoforms, as well as their sensitive and specific detection on the basis of their characteristic iron content. Molecule-specific detection techniques such as nanoLC-ESI-QTRAP-MS or MALDI-TOF-MS were used as complementary techniques to unambiguously identify the isolated proteins as Tf via cross species protein identification and to further characterise the molecular weight as well as the sialic acid content, which is responsible for the elution behaviour of the different isoforms during their ion exchange separation. A molecular mass above 80 kDa has been measured for the different seal Tf isoforms, which is in good agreement with the known molecular mass in other mammalian species, while the estimated pI of the different isoforms indicates some differences in comparison to other species. A number of homologies to known Tf sequences have been observed, which finally allows the cross species protein identification. The combined metallomics orientated analytical approach, which includes the complementary application of element and molecule-specific detection techniques, opens up interesting possibilities for the fast and targeted isolation and identification of a diagnostically relevant metal containing protein from an un-sequenced mammalian species prior to its utilisation in extended studies. 相似文献
977.
Schulz E Fleischhaker C Hennighausen K Heiser P Haessler F Linder M Stollhoff K Warnke A Baier M Klatt J 《Attention deficit and hyperactivity disorders》2010,2(3):133-138
Several extended-release methylphenidate medications are available for treatment of children with ADHD. Pharmacokinetic investigations suggest that the serum levels of methylphenidate are partially altered when the medication is taken without breakfast. Clinical data comparing different breakfast situations are missing. In this study, different breakfast compositions and their influence on treatment with Ritalin LA are investigated. A total of 150 patients were enrolled in a rater-blinded, randomized crossover trial that compared a minimal breakfast with a standard breakfast in patients under stable treatment with Ritalin LA. Ratings for clinical efficacy were carried out after 1 week by teachers and parents (FBB-ADHS), as well as physicians (CGI). Additionally, a math test was administered to the patients. Of the total patients, 144 finished the trial with a breakfast compliance of 93%. All of the clinical rating scales showed consistently no difference between the two breakfast conditions. Non-inferiority of minimal breakfast versus standard breakfast was shown to be statistically significant (FBB-AHDS(Teacher): 0.97 with minimal breakfast, 1.01 with standard breakfast, P < 0.0001). The clinical efficacy of Ritalin LA is not influenced by breakfast and works independently of food intake. 相似文献
978.
Manfred Türke Eric Heinze Kerstin Andreas Sarah M. Svendsen Martin M. Gossner Wolfgang W. Weisser 《Oecologia》2010,163(3):681-693
In beech-dominated forests in Central Europe, many spring geophytes show adaptations to seed dispersal by ants (myrmecochory). Ants, however, can be rare in such moist forests. Motivated by observations of slug feeding on seeds we investigated the seed consumption of two plant species, Anemone nemorosa and Asarum europaeum, by slugs, in a series of experiments. In a seed predation experiment in a beech forest, we found that seed removal was strongly reduced when gastropods were excluded from the seed depots. The contribution of insects, including ants, and rodents to seed removal was relatively less but differed between May and July. In the laboratory, slug species, in particular Arion sp., consumed seeds of both plant species. Slugs either consumed the elaiosomes of seeds or swallowed seeds intact. Swallowed seeds were defecated undamaged and germinated as well as control seeds when buried overwinter, indicating the potential for seed dispersal by slugs. We also recovered seeds of myrmecochores in the faeces of several slugs caught in forests. In a slug release experiment in the forest, slugs moved up to 14.6 m (mean 4.4 m) in 15 h, which is the median gut passage time of seeds based on measurements made in the laboratory. We also found that when slug-defecated seeds were offered to rodents, these were less attractive than control seeds, suggesting that passage through the slug gut reduces seed predation risk. Our results demonstrate that slugs are significant consumers of elaiosomes or entire seeds of ant-dispersed plants and that they can function as seed dispersers of these plants. 相似文献
979.
Alternate partial root-zone irrigation induced dry/wet cycles of soils stimulate N mineralization and improve N nutrition in tomatoes 总被引:3,自引:0,他引:3
Yaosheng Wang Fulai Liu Andreas de Neergaard Lars S. Jensen Jesper Luxhøi Christian R. Jensen 《Plant and Soil》2010,337(1-2):167-177
Given the same amount of irrigation volume, applying alternate partial root-zone irrigation (PRI) has improved crop N nutrition as compared to deficit irrigation (DI), yet the mechanisms underlying this effect remain unknown. Therefore, the objective of this study was to investigate whether PRI induced soil dry/wet cycles facilitate soil organic N mineralization hereby contributing to the improvement of N nutrition in tomatoes. The plants were grown in split-root pots in a climate-controlled glasshouse and were subjected to PRI and DI treatments during early fruiting stage. 15N-labeled maize residues were incorporated into the soils. Results showed that PRI resulted in 25% higher net 15N mineralization than did DI, indicating that the enhanced mineralization of soil organic N alone could account for the 16% increase of N accumulation in the PRI than in the DI plants. The higher net N mineralization under PRI was coincided with an intensified soil microbial activity. In addition, even though soil chloroform fumigation labile carbon (CFL-C, as an index of microbial biomass) was similar for the two irrigation treatments, a significant increase of chloroform fumigation labile nitrogen (CFL-N) was found in the PRI wetting soil. Consequently, the C:N ratio of the chloroform fumigation labile pool was remarkably modified by the PRI treatment, which might indicate physiological changes of soil microbes or changes in labiality of soil organic C and N due to the dry/wet cycles of soils, altering conditions for net N mineralization. Moreover, in both soil compartments PRI caused significantly less extractable organic carbon (EOC) as compared with DI; whilst in the PRI wetting soil significantly higher extractable organic nitrogen (EON) was observed. A low EOC:EON ratio in the PRI wetting soil may indicate an increasing net mineralization of the organic N as a result of microbial metabolism. Conclusively, PRI induced greater microbial activity and higher microbial substrates availability are seemingly responsible for the enhanced net N mineralization and improved N nutrition in tomato plants. 相似文献
980.
Henrik Lütken James R. Lloyd Mikkel A. Glaring Lone Baunsgaard Kristian Holst Laursen Anna Haldrup Jens Kossmann Andreas Blennow 《Planta》2010,232(5):1127-1139
Two glucanotransferases, disproportionating enzyme 1 (StDPE1) and disproportionating enzyme 2 (StDPE2), were repressed using
RNA interference technology in potato, leading to plants repressed in either isoform individually, or both simultaneously.
This is the first detailed report of their combined repression. Plants lacking StDPE1 accumulated slightly more starch in
their leaves than control plants and high levels of maltotriose, while those lacking StDPE2 contained maltose and large amounts
of starch. Plants repressed in both isoforms accumulated similar amounts of starch to those lacking StDPE2. In addition, they
contained a range of malto-oligosaccharides from maltose to maltoheptaose. Plants repressed in both isoforms had chlorotic
leaves and did not grow as well as either the controls or lines where only one of the isoforms was repressed. Examination
of photosynthetic parameters suggested that this was most likely due to a decrease in carbon assimilation. The subcellular
localisation of StDPE2 was re-addressed in parallel with DPE2 from Arabidopsis thaliana by transient expression of yellow fluorescent protein fusions in tobacco. No translocation to the chloroplasts was observed
for any of the fusion proteins, supporting a cytosolic role of the StDPE2 enzyme in leaf starch metabolism, as has been observed
for Arabidopsis DPE2. It is concluded that StDPE1 and StDPE2 have individual essential roles in starch metabolism in potato and consequently
repression of these disables regulation of leaf malto-oligosaccharides, starch content and photosynthetic activity and thereby
plant growth possibly by a negative feedback mechanism. 相似文献