全文获取类型
收费全文 | 97068篇 |
免费 | 5052篇 |
国内免费 | 5189篇 |
出版年
2023年 | 1366篇 |
2022年 | 1227篇 |
2021年 | 2932篇 |
2020年 | 2056篇 |
2019年 | 2485篇 |
2018年 | 2189篇 |
2017年 | 1891篇 |
2016年 | 2513篇 |
2015年 | 4476篇 |
2014年 | 7915篇 |
2013年 | 7856篇 |
2012年 | 6108篇 |
2011年 | 6893篇 |
2010年 | 5046篇 |
2009年 | 4995篇 |
2008年 | 5026篇 |
2007年 | 5444篇 |
2006年 | 3939篇 |
2005年 | 3542篇 |
2004年 | 2606篇 |
2003年 | 2147篇 |
2002年 | 2030篇 |
2001年 | 1562篇 |
2000年 | 1427篇 |
1999年 | 1304篇 |
1998年 | 1114篇 |
1997年 | 903篇 |
1996年 | 995篇 |
1995年 | 1063篇 |
1994年 | 981篇 |
1993年 | 933篇 |
1992年 | 842篇 |
1991年 | 856篇 |
1990年 | 704篇 |
1989年 | 677篇 |
1988年 | 591篇 |
1987年 | 525篇 |
1986年 | 475篇 |
1985年 | 724篇 |
1984年 | 1013篇 |
1983年 | 643篇 |
1982年 | 869篇 |
1981年 | 851篇 |
1980年 | 631篇 |
1979年 | 614篇 |
1978年 | 382篇 |
1977年 | 391篇 |
1976年 | 357篇 |
1974年 | 250篇 |
1973年 | 268篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Brad Stiles I. C. McDonald J. W. Gerst T. S. Adams S. M. Newman 《In vitro cellular & developmental biology. Animal》1992,28(5):355-363
Summary Five continuous cell lines were initiated from embryonic tissue of the cotton boll weevilAnthonomus grandis Boheman in a commercially available, serum-free medium (Excell 401) and have undergone in excess of 60 passages. Isoenzyme
analysis confirmed that the lines originated from boll weevil tissue. Four of the lines grew as single attached cells of either
epithelioid or fibroblastoid morphology. The fifth line, BRL-AG-2, grew primarily as cell aggregates and was found to release
ecdysteroids (primarily ecdysone) into the culture medium. Evidence was also obtained suggesting that line BRL-AG-2 synthesizes
chitin. Three lines, BRL-AG-1, BRL-AG-3A, and BRL-AG-3C, could be induced to produce an antibacterial factor(s) which was
released into the culture medium. 相似文献
992.
Short-chain fatty acids can be produced under anaerobic conditions by fermentative soil microbes and have nematicidal properties. We evaluated the effects of butyric and propionic acids on death and recovery of stunt nematodes (Tylenchorhynchus spp.), a common parasite of turfgrass. Nematodes in a sand-soil mix (80:20) were treated with butyric or propionic acid and incubated under air or N₂ for 7 days at 25 °C. Amendment of soil with 0.1 and 1.0 µmol (8.8 and 88 µg) butyric acid/g soil or 1.0 µmol (74 µg) propionic acid/g soil resulted in the death of all nematodes. The composition of the soil atmosphere had no effect on the nematicidal activity of the acids. Addition of hydrochloric acid to adjust soil pH to 4.4 and 3.5 resulted in nematode mortality relative to controls (41% to 86%) but to a lesser degree than short-chain fatty acids at the same pH. Nematodes did not recover after a 28-day period following addition of 10 µmol butyric acid/g soil under air or N₂. Carbon mineralization decreased during this period, whereas levels of inorganic N and microbial biomass-N remained constant. Short-chain fatty acids appear to be effective in killing Tylenchorhynchus spp. independent of atmospheric composition. Nematode mortality appears to be a function of the type and concentration of fatty acid and soil pH. 相似文献
993.
Relationship between molecular and enological features of Patagonian wine yeasts: relevance in selection protocols 总被引:4,自引:0,他引:4
Christian A. Lopes María E. Rodríguez Amparo Querol Sergio Bramardi Adriana C. Caballero 《World journal of microbiology & biotechnology》2006,22(8):827-833
Summary In this paper, a study of the relationship between genetic patterns, obtained by the combination of mtDNA-RFLP and PCR-amplified inter-δ sequence DNA polymorphism analysis, and relevant enological phenotypic data (fermentative power, specific productivity, volatile and total acidity) was carried out on Argentinean Saccharomyces cerevisiae isolates from north Patagonia. The use of a powerful statistical tool, Generalized Procrustes analysis, allowed us to weigh the relationship for each isolate in particular, denoting a good enough degree of agreement between molecular and physiological data for most of the population analysed. The inclusion of a physiological feature, as the killer sensitivity biotype, within identification methods resulted in a higher degree of discrimination among isolates and in better correlation between both characterizations. The combined use of methods based on molecular polymorphisms and killer biotype could be applied so as not to miss any isolate with differential enological properties in selection protocols. 相似文献
994.
We developed eight polymorphic microsatellite simple sequence repeat (SSR) loci from genomic DNA of a clonal perennial herb, Convallaria keiskei, using a dual‐suppression‐polymerase chain reaction (PCR) technique and an improved technique. These markers with four to 10 alleles per locus identified 29 genotypes in 82 samples collected from a population in Hokkaido, Japan. The observed and expected heterozygosities ranged from 0.241 to 0.862 and from 0.639 to 0.825, respectively. These SSR markers will be available to identify genets and evaluate genetic diversity of C. keiskei. 相似文献
995.
Tobias Pamminger David Treanor William O. H. Hughes 《Proceedings. Biological sciences / The Royal Society》2016,283(1822)
The ubiquitous trade-off between survival and costly reproduction is one of the most fundamental constraints governing life-history evolution. In numerous animals, gonadotropic hormones antagonistically suppressing immunocompetence cause this trade-off. The queens of many social insects defy the reproduction–survival trade-off, achieving both an extraordinarily long life and high reproductive output, but how they achieve this is unknown. Here we show experimentally, by integrating quantification of gene expression, physiology and behaviour, that the long-lived queens of the ant Lasius niger have escaped the reproduction–immunocompetence trade-off by decoupling the effects of a key endocrine regulator of fertility and immunocompetence in solitary insects, juvenile hormone (JH). This modification of the regulatory architecture enables queens to sustain a high reproductive output without elevated JH titres and suppressed immunocompetence, providing an escape from the reproduction–immunocompetence trade-off that may contribute to the extraordinary lifespan of many social insect queens. 相似文献
996.
Temperate seagrass meadows form valuable ecosystems in coastal environments and present a distinct seasonal growth. They are threatened by an increasing amount of stressors, potentially affecting their capacity to recover from disturbances. We hypothesized that their resilience to disturbances is affected by seasonal dynamics. Hence, we investigated the effect of the timing of the disturbance on seagrass Leaf Area Index (as a proxy for presence, or ‘visible’ status), recovery from disturbance (as a proxy for meadow resilience), and rhizome carbohydrates (as a proxy for longer term resilience) by a series of four disturbance-recovery field experiments spread over the growing season at two sites in Shandong Province, China. During the course of the growing season, we found the highest recovery at the start of the growing season, lowest recovery when Leaf Area Index peaked around mid-growing season, and intermediate recovery when Leaf Area Index decreased at the end of the growing season. Rhizome carbohydrates were not affected by disturbances during any of the four experimental periods and could not explain the low recovery during mid-growing season. The two sites differed in exposure and in the occurrence of incidents like a green tide and storms, which affected recovery. However, general patterns were similar; timing strongly influenced the indicator of meadow resilience and its correlation with presence during the two main seagrass growth phases. Our results emphasize the importance of carefully considering timing in the evaluation of seagrass resilience in temperate systems. Furthermore, our study implies that, to effectively protect seagrass beds, conservation management should aim at avoiding disturbances particularly during the peak of the growing season, when resilience is lowest. 相似文献
997.
A. B. Medvinsky E. A. Kriksunov A. E. Bobyrev V. A. Burmensky M. M. Gonik B. -L. Li O. P. Sterligova 《Biophysics》2006,51(2):309-316
We present a conceptual mathematical model of the dynamics of a spatially heterogeneous population system whose prototype is the fish community of Lake Syamozero. Analysis of the solutions of this model is used to demonstrate that interactions between the predator and prey populations in two neighboring biotopes (the pelagic and coastal zones) may result in either undamped oscillations or steady states of the population sizes. The model population densities are of the same order of magnitude as the values obtained in long-term observations of the Syamozero biota. It is also demonstrated that the transition to steady states may be accompanied by long-term (dozens or hundreds of years) damped oscillations of the prey and predator population densities. Under natural conditions, long transitional periods may prevent fish communities from reaching stationary modes. 相似文献
998.
Peter Clauwaert Schalla Mulenga Peter Aelterman Willy Verstraete 《Applied microbiology and biotechnology》2009,83(2):241-247
Using the anode effluent to compensate the alkalinization in a bio-cathode has recently been proposed as a way to operate
a microbial fuel cell (MFC) in a continuous and pH neutral way. In this research, we successfully demonstrated that the operation
of a MFC without any pH adjustments is possible by completing the liquid loop over cathode and anode. During the complete
loop operation, a stable current production of 23.2 ± 2.5 A m−3 MFC was obtained, even in the presence of 3.2–5.2 mg O2 L−1 in the anode. The use of current collectors and subdivided electrical circuitries for relative large 2.5-L-scale MFCs resulted
in ohmic cell resistances in the order of 1.4–1.7 mΩ m3 MFC, which were comparable to values of ten times smaller MFCs. Nevertheless, the bio-cathode activity still needs to be
improved significantly with a factor 10–50 in order achieve desirable current densities of 1,000 A m−3 MFC.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
999.
1000.
Transient absorption changes during reduction of quinone in liposomes by external dithionite, in the absence and presence of initially trapped ferricyanide, were matched with absorption spectra of semiquinone and quinone in the blue region. Plastoquinone, ubiquinone-9 and phylloquinone, each having an isoprenoid side chain were compared with trimethyl-p-benzoquinone, ubiquinone-9 and menadione, which lack a long side chain.Semiquinone transients could only be observed by our spectroscopic technique during reduction of quinones lacking the chain. If Triton X-100 was added to the liposomes preparation semiquinone transients were also observed with the isoprenoid quinones. This result is consistent with the view that isoprenoid quinones build domains in the membranes, in which the life time of the semiquinone might be decreased by fast disproportionation, and to which dithionite has limited access. 相似文献