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871.
872.
Bert Hidding Bart A. Nolet Mennobart R. van Eerden Matthieu Guillemain Marcel Klaassen 《Aquatic Botany》2009,90(4):321-327
Deep burial in the sediment of tubers of fennel pondweed (Potamogeton pectinatus) has been explained in terms of avoidance by escape against consumption by Bewick's swans (Cygnus columbianus bewickii) in autumn. We therefore expected changes in foraging pressure to ultimately result in a change in the tuber distribution across sediment depth. A trade-off underlies this idea: deep tubers are less accessible to swans but must be larger to meet the higher energy demands of sprouting in spring. To test this prediction, we compared tuber burial depth over a gradient of foraging pressure both across space and across time. Tuber samples were obtained after aboveground plant senescence but before arrival of Bewick's swans. First, we compared the current tuber bank depth profile in a shallow lake with high foraging pressure, the Lauwersmeer, with that in two wetlands with moderate and low foraging pressure. Second, we compared the current tuber burial in the Lauwersmeer with that in the early 1980s when exploitation by swans had just started there. In accordance with our hypothesis, we found significantly deeper burial of tubers under high consumption risk compared to low consumption risk, both when comparing sites and comparing time periods. Since tubers in effect only survive to the next spring, the observed differences in burial depth among sites and over time cannot be a direct result of tuber losses due to consumption by swans. Rather, these observations suggest adaptive responses in tuber burial related to foraging pressure from Bewick's swans in the recent past. We thus propose that fennel pondweed exhibits flexible avoidance by escape, of a kind rarely described for plants, where both phenotypic plasticity and genotype sorting may contribute to the observed differences in tuber burial. 相似文献
873.
Elevated CO2 affects porewater chemistry in a brackish marsh 总被引:1,自引:0,他引:1
Jason K. Keller Amelia A. Wolf Pamela B. Weisenhorn Bert G. Drake J. Patrick Megonigal 《Biogeochemistry》2009,96(1-3):101-117
As atmospheric CO2 concentrations continue to rise and impact plant communities, concomitant shifts in belowground microbial processes are likely, but poorly understood. We measured monthly porewater concentrations of sulfate, sulfide, methane (CH4), dissolved inorganic carbon and dissolved organic carbon over a 5-year period in a brackish marsh. Samples were collected using porewater wells (i.e., sippers) in a Schoenoplectus americanus-dominated (C3 sedge) community, a Spartina patens-dominated (C4 grass) community and a mixed (C3 and C4) community within the marsh. Plant communities were exposed to ambient and elevated (ambient + 340 ppm) CO2 levels for 15 years prior to porewater sampling, and the treatments continued over the course of our sampling. Sulfate reduction was stimulated by elevated CO2 in the C3-dominated community, but not in the C4-dominated community. Elevated CO2 also resulted in higher porewater concentrations of CH4 and dissolved organic carbon in the C3-dominated system, though inhibition of CH4 production by sulfate reduction appears to temper the porewater CH4 response. These patterns mirror the typical divergent responses of C3 and C4 plants to elevated CO2 seen in this ecosystem. Porewater concentrations of nitrogen (as ammonium) and phosphorus did not decrease despite increased plant biomass in the C3-dominated community, suggesting nutrients do not strongly limit the sustained vegetation response to elevated CO2. Overall, our data demonstrate that elevated CO2 drives changes in porewater chemistry and suggest that increased plant productivity likely stimulates microbial decomposition through increases in dissolved organic carbon availability. 相似文献
874.
875.
Summary: We have developed a general scenario of prebiotic physicochemical evolution during the Earth''s Hadean eon and reviewed the relevant literature. We suggest that prebiotic chemical evolution started in microspaces with membranous walls, where external temperature and osmotic gradients were coupled to free-energy gradients of potential chemical reactions. The key feature of this scenario is the onset of an emergent evolutionary transition within the microspaces that is described by the model of complex vectorial chemistry. This transition occurs at average macromolecular crowding of 20 to 30% of the cell volume, when the ranges of action of stabilizing colloidal forces (screened electrostatic forces, hydration, and excluded volume forces) become commensurate. Under these conditions, the macromolecules divide the interior of microspaces into dynamically crowded macromolecular regions and topologically complementary electrolyte pools. Small ions and ionic metabolites are transported vectorially between the electrolyte pools and through the (semiconducting) electrolyte pathways of the crowded macromolecular regions from their high electrochemical potential (where they are biochemically produced) to their lower electrochemical potential (where they are consumed). We suggest a sequence of tentative transitions between major evolutionary periods during the Hadean eon as follows: (i) the early water world, (ii) the appearance of land masses, (iii) the pre-RNA world, (iv) the onset of complex vectorial chemistry, and (v) the RNA world and evolution toward Darwinian thresholds. We stress the importance of high ionic strength of the Hadean ocean (short Debye''s lengths) and screened electrostatic interactions that enabled the onset of the vectorial structure of the cytoplasm and the possibility of life''s emergence.
Architecture is what ultimately distinguishes a living cell from a soup of the chemicals of which it is composed.—Franklin Harold (40)相似文献
876.
Bert Breugelmans Gert Simonet Vincent van Hoef Sofie Van Soest Vanden Jozef Broeck 《BMC evolutionary biology》2009,9(1):1-13
Background
This study was motivated by the observation of unusual mitochondrial haplotype distributions and associated physiological differences between populations of the killifish Fundulus heteroclitus distributed along the Atlantic coast of North America. A distinct "northern" haplotype is fixed in all populations north of New Jersey, and does not appear south of New Jersey except in extreme upper-estuary fresh water habitats, and northern individuals are known to be more tolerant of hyposmotic conditions than southern individuals. Complete mitochondrial genomes were sequenced from individuals from northern coastal, southern coastal, and fresh water populations (and from out-groups). Comparative genomics approaches were used to test multiple evolutionary hypotheses proposed to explain among-population genome variation including directional selection and hybridization.Results
Structure and organization of the Fundulus mitochondrial genome is typical of animals, yet subtle differences in substitution patterns exist among populations. No signals of directional selection or hybridization were detected. Mitochondrial genes evolve at variable rates, but all genes exhibit very low dN/dS ratios across all lineages, and the southern population harbors more synonymous polymorphism than other populations.Conclusion
Evolution of mitochondrial genomes within Fundulus is primarily governed by interaction between strong purifying selection and demographic influences, including larger historical population size in the south. Though directional selection and hybridization hypotheses were not supported, adaptive processes may indirectly contribute to partitioning of variation between populations. 相似文献877.
878.
Ilya Chuykin Irina Lapidus Elena Popova Larisa Vilianovich Valentina Mosienko Natalia Alenina Bert Binas Guixuan Chai Michael Bader Alexander Krivokharchenko 《PloS one》2010,5(3)
Background
Previous attempts to isolate pluripotent cell lines from rat preimplantation embryo in mouse embryonic stem (ES) cell culture conditions (serum and LIF) were unsuccessful, however the resulting cells exhibited the expression of such traditional pluripotency markers as SSEA-1 and alkaline phosphatase. We addressed the question, which kind of cell lineages are produced from rat preimplantation embryo under “classical” mouse ES conditions.Results
We characterized two cell lines (C5 and B10) which were obtained from rat blastocysts in medium with serum and LIF. In the B10 cell line we found the expression of genes known to be expressed in trophoblast, Cdx-2, cytokeratin-7, and Hand-1. Also, B10 cells invaded the trophectodermal layer upon injection into rat blastocysts. In contrast to mouse Trophoblast Stem (TS) cells proliferation of B10 cells occurred independently of FGF4. Cells of the C5 line expressed traditional markers of extraembryonic-endoderm (XEN) cells, in particular, GATA-4, but also the pluripotency markers SSEA-1 and Oct-4. C5 cell proliferation exhibited dependence on LIF, which is not known to be required by mouse XEN cells.Conclusions
Our results confirm and extend previous findings about differences between blastocyst-derived cell lines of rat and mice. Our data show, that the B10 cell line represents a population of FGF4-independent rat TS-like cells. C5 cells show features that have recently become known as characteristic of rat XEN cells. Early passages of C5 and B10 cells contained both, TS and XEN cells. We speculate, that mechanisms maintaining self-renewal of cell lineages in rat preimplantation embryo and their in vitro counterparts, including ES, TS and XEN cells are different than in respective mouse lineages. 相似文献879.
Nguyen Thi Khanh Nhu Ha Vinh Tran Vu Thieu Nga Richard Stabler Pham Thanh Duy Le Thi Minh Vien H. Rogier van Doorn Ana Cerde?o-Tárraga Nicholas Thomson James Campbell Nguyen Van Minh Hoang Tran Thi Thu Nga Pham Van Minh Cao Thu Thuy Brendan Wren Jeremy Farrar Stephen Baker 《PLoS neglected tropical diseases》2010,4(6)
Background
Plasmid mediated antimicrobial resistance in the Enterobacteriaceae is a global problem. The rise of CTX-M class extended spectrum beta lactamases (ESBLs) has been well documented in industrialized countries. Vietnam is representative of a typical transitional middle income country where the spectrum of infectious diseases combined with the spread of drug resistance is shifting and bringing new healthcare challenges.Methodology
We collected hospital admission data from the pediatric population attending the hospital for tropical diseases in Ho Chi Minh City with Shigella infections. Organisms were cultured from all enrolled patients and subjected to antimicrobial susceptibility testing. Those that were ESBL positive were subjected to further investigation. These investigations included PCR amplification for common ESBL genes, plasmid investigation, conjugation, microarray hybridization and DNA sequencing of a bla CTX–M encoding plasmid.Principal Findings
We show that two different bla CTX-M genes are circulating in this bacterial population in this location. Sequence of one of the ESBL plasmids shows that rather than the gene being integrated into a preexisting MDR plasmid, the bla CTX-M gene is located on relatively simple conjugative plasmid. The sequenced plasmid (pEG356) carried the bla CTX-M-24 gene on an ISEcp1 element and demonstrated considerable sequence homology with other IncFI plasmids.Significance
The rapid dissemination, spread of antimicrobial resistance and changing population of Shigella spp. concurrent with economic growth are pertinent to many other countries undergoing similar development. Third generation cephalosporins are commonly used empiric antibiotics in Ho Chi Minh City. We recommend that these agents should not be considered for therapy of dysentery in this setting. 相似文献880.