全文获取类型
收费全文 | 7994篇 |
免费 | 836篇 |
国内免费 | 3篇 |
专业分类
8833篇 |
出版年
2024年 | 4篇 |
2023年 | 56篇 |
2022年 | 146篇 |
2021年 | 271篇 |
2020年 | 135篇 |
2019年 | 179篇 |
2018年 | 208篇 |
2017年 | 194篇 |
2016年 | 342篇 |
2015年 | 527篇 |
2014年 | 526篇 |
2013年 | 543篇 |
2012年 | 707篇 |
2011年 | 731篇 |
2010年 | 464篇 |
2009年 | 359篇 |
2008年 | 512篇 |
2007年 | 509篇 |
2006年 | 385篇 |
2005年 | 382篇 |
2004年 | 389篇 |
2003年 | 337篇 |
2002年 | 305篇 |
2001年 | 58篇 |
2000年 | 41篇 |
1999年 | 53篇 |
1998年 | 72篇 |
1997年 | 34篇 |
1996年 | 45篇 |
1995年 | 27篇 |
1994年 | 23篇 |
1993年 | 17篇 |
1992年 | 20篇 |
1991年 | 18篇 |
1990年 | 18篇 |
1989年 | 12篇 |
1988年 | 6篇 |
1987年 | 6篇 |
1986年 | 9篇 |
1985年 | 13篇 |
1984年 | 18篇 |
1983年 | 9篇 |
1982年 | 9篇 |
1981年 | 29篇 |
1980年 | 18篇 |
1979年 | 15篇 |
1978年 | 12篇 |
1977年 | 7篇 |
1976年 | 6篇 |
1972年 | 4篇 |
排序方式: 共有8833条查询结果,搜索用时 15 毫秒
71.
Hughes SR Gibbons WR Bang SS Pinkelman R Bischoff KM Slininger PJ Qureshi N Kurtzman CP Liu S Saha BC Jackson JS Cotta MA Rich JO Javers JE 《Journal of industrial microbiology & biotechnology》2012,39(1):163-173
Scheffersomyces (formerly Pichia) stipitis NRRL Y-7124 was mutagenized using UV-C irradiation to produce yeast strains for anaerobic conversion of lignocellulosic sugars to ethanol. UV-C irradiation potentially produces large numbers of random mutations broadly and uniformly over the whole genome to generate unique strains. Wild-type cultures of S. stipitis NRRL Y-7124 were subjected to UV-C (234 nm) irradiation targeted at approximately 40% cell survival. When surviving cells were selected in sufficient numbers via automated plating strategies and cultured anaerobically on xylose medium for 5 months at 28°C, five novel mutagenized S. stipitis strains were obtained. Variable number tandem repeat analysis revealed that mutations had occurred in the genome, which may have produced genes that allowed the anaerobic utilization of xylose. The mutagenized strains were capable of growing anaerobically on xylose/glucose substrate with higher ethanol production during 250- to 500-h growth than a Saccharomyces cerevisiae yeast strain that is the standard for industrial fuel ethanol production. The S. stipitis strains resulting from this intense multigene mutagenesis strategy have potential application in industrial fuel ethanol production from lignocellulosic hydrolysates. 相似文献
72.
A variety of similarities between visual and haptic object recognition suggests that the two modalities may share common representations. However, it is unclear whether such common representations preserve low-level perceptual features or whether transfer between vision and haptics is mediated by high-level, abstract representations. Two experiments used a sequential shape-matching task to examine the effects of size changes on unimodal and crossmodal visual and haptic object recognition. Participants felt or saw 3D plastic models of familiar objects. The two objects presented on a trial were either the same size or different sizes and were the same shape or different but similar shapes. Participants were told to ignore size changes and to match on shape alone. In Experiment 1, size changes on same-shape trials impaired performance similarly for both visual-to-visual and haptic-to-haptic shape matching. In Experiment 2, size changes impaired performance on both visual-to-haptic and haptic-to-visual shape matching and there was no interaction between the cost of size changes and direction of transfer. Together the unimodal and crossmodal matching results suggest that the same, size-specific perceptual representations underlie both visual and haptic object recognition, and indicate that crossmodal memory for objects must be at least partly based on common perceptual representations. 相似文献
73.
Rebecca S. Moore Dene H. Taylor Lawrence S. Sturman Michael M. Reddy G. Wolfgang Fuhs 《Applied microbiology》1981,42(6):963-975
The adsorption of radiolabeled infectious poliovirus type 2 by 34 well-defined soils and mineral substrates was analyzed in a synthetic freshwater medium containing 1 mM CaCl2 and 1.25 mM NaHCO3 at pH 7. In a model system, adsorption of poliovirus by Ottawa sand was rapid and reached equilibrium within 1 h at 4°C. Near saturation, the adsorption could be described by the Langmuir equation; the apparent surface saturation was 2.5 × 106 plaque-forming units of poliovirus per mg of Ottawa sand. At low surface coverage, adsorption was described by the Freundlich equation. The soils and minerals used ranged from acidic to basic and from high in organic content to organic free. The available negative surface charge on each substrate was measured by the adsorption of a cationic polyelectrolyte, polydiallyldimethylammonium chloride. Most of the substrates adsorbed more than 95% of the virus. In general, soils, in comparison with minerals, were weak adsorbents. Among the soils, muck and Genesee silt loam were the poorest adsorbents; among the minerals, montmorillonite, glauconite, and bituminous shale were the least effective. The most effective adsorbents were magnetite sand and hematite, which are predominantly oxides of iron. Correlation coefficients for substrate properties and virus adsorption revealed that the elemental composition of the adsorbents had little effect on poliovirus uptake. Substrate surface area and pH, by themselves, were not significantly correlated with poliovirus uptake. A strong negative correlation was found between poliovirus adsorption and both the contents of organic matter and the available negative surface charge on the substrates as determined by their capacities for adsorbing the cationic polyelectrolyte, polydiallyldimethylammonium chloride. 相似文献
74.
The distance dependence prediction of the Janzen-Connell hypothesis: a meta-analysis 总被引:7,自引:0,他引:7
Laura A. Hyatt Michael S. Rosenberg Timothy G. Howard Gregory Bole Wei Fang Jean Anastasia Kerry Brown Rebecca Grella Katharine Hinman Josepha P. Kurdziel Jessica Gurevitch 《Oikos》2003,103(3):590-602
The Janzen-Connell hypothesis explains the maintenance of tropical diversity through the interacting effects of parent-centered dispersal patterns and distance- and density-dependent propagule survival. These effects were thought to support regular spacing of species within tropical forest, enhancing diversity. One of the predictions of the hypothesis is that seed and seedling survival should improve with increased parental distance. Although there are many independent tests of this hypothesis for individual species, there are few synthetic studies that have brought these data together to test its validity across species. This paper reports the results of a meta-analysis of the effect of distance on enhancing propagule survival, employing an odds-ratio effect size metric. We found no general support for the distance-dependent prediction of the hypothesis, and conclude that further testing to explore this hypothesis as a diversity-maintaining mechanism is unnecessary. However, we did find that distance from parent slightly reduces survivorship in the temperate zone, as contrasted with the tropics, and we saw stronger evidence in support of the hypothesis for seedlings than for seeds. The phenomenon of enhanced propagule survival with distance from the parent may be important for the population biology of particular species, but it is not a general phenomenon across communities, life history stages or life forms. 相似文献
75.
Nonlinear magic: multiphoton microscopy in the biosciences 总被引:21,自引:0,他引:21
Multiphoton microscopy (MPM) has found a niche in the world of biological imaging as the best noninvasive means of fluorescence microscopy in tissue explants and living animals. Coupled with transgenic mouse models of disease and 'smart' genetically encoded fluorescent indicators, its use is now increasing exponentially. Properly applied, it is capable of measuring calcium transients 500 microm deep in a mouse brain, or quantifying blood flow by imaging shadows of blood cells as they race through capillaries. With the multitude of possibilities afforded by variations of nonlinear optics and localized photochemistry, it is possible to image collagen fibrils directly within tissue through nonlinear scattering, or release caged compounds in sub-femtoliter volumes. 相似文献
76.
A comparison of chemical pretreatment methods for improving saccharification of cotton stalks 总被引:19,自引:0,他引:19
Silverstein RA Chen Y Sharma-Shivappa RR Boyette MD Osborne J 《Bioresource technology》2007,98(16):3000-3011
The effectiveness of sulfuric acid (H(2)SO(4)), sodium hydroxide (NaOH), hydrogen peroxide (H(2)O(2)), and ozone pretreatments for conversion of cotton stalks to ethanol was investigated. Ground cotton stalks at a solid loading of 10% (w/v) were pretreated with H(2)SO(4), NaOH, and H(2)O(2) at concentrations of 0.5%, 1%, and 2% (w/v). Treatment temperatures of 90 degrees C and 121 degrees C at 15 psi were investigated for residence times of 30, 60, and 90 min. Ozone pretreatment was performed at 4 degrees C with constant sparging of stalks in water. Solids from H(2)SO(4), NaOH, and H(2)O(2) pretreatments (at 2%, 60 min, 121 degrees C/15 psi) showed significant lignin degradation and/or high sugar availability and hence were hydrolyzed by Celluclast 1.5L and Novozym 188 at 50 degrees C. Sulfuric acid pretreatment resulted in the highest xylan reduction (95.23% for 2% acid, 90 min, 121 degrees C/15 psi) but the lowest cellulose to glucose conversion during hydrolysis (23.85%). Sodium hydroxide pretreatment resulted in the highest level of delignification (65.63% for 2% NaOH, 90 min, 121 degrees C/15 psi) and cellulose conversion (60.8%). Hydrogen peroxide pretreatment resulted in significantly lower (p相似文献
77.
The sexes of dioecious species may differ in a range of vegetative and reproductive traits as well as in physiological traits. In Siparuna grandiflora, a Neotropical dioecious shrub, we examined differences in leaf-level photosynthesis of different classes of leaf age and, using simulation models, explored whether differences in leaf-level carbon gain led to sex differences in whole-plant daily carbon gain. Male plants had higher photosynthetic capacity at the leaf level. As leaves of both sexes aged their photosynthetic capacity and specific leaf area declined as expected. Simulations of daily carbon gain using the architecturally explicit model Y-Plant and a non-architectural model incorporating a wide range of realistic light environments revealed that the difference in leaf-level photosynthetic capacity did not translate into greater crown-level carbon gain for males. Rather, differences in patterns of allocation to leaf area allow females to achieve higher crown-level carbon gain. The results demonstrate that sex differences at the leaf level do not necessarily predict patterns at the whole-plant level. 相似文献
78.
Kelly A. Speer Brandi Jo Petronio Nancy B. Simmons Rebecca Richey Kristin Magrini J. Angel Soto‐Centeno David L. Reed 《Ecology and evolution》2017,7(19):7585-7598
Dispersal is a driving factor in the creation and maintenance of biodiversity, yet little is known about the effects of habitat variation and geography on dispersal and population connectivity in most mammalian groups. Bats of the family Molossidae are fast‐flying mammals thought to have potentially high dispersal ability, and recent studies have indicated gene flow across hundreds of kilometers in continental North American populations of the Brazilian free‐tailed bat, Tadarida brasiliensis. We examined the population genetics, phylogeography, and morphology of this species in Florida and across islands of The Bahamas, which are part of an island archipelago in the West Indies. Previous studies indicate that bats in the family Phyllostomidae, which are possibly less mobile than members of the family Molossidae, exhibit population structuring across The Bahamas. We hypothesized that T. brasiliensis would show high population connectivity throughout the islands and that T. brasiliensis would show higher connectivity than two species of phyllostomid bats that have been previously examined in The Bahamas. Contrary to our predictions, T. brasiliensis shows high population structure between two groups of islands in The Bahamas, similar to the structure exhibited by one species of phyllostomid bat. Phylogenetic and morphological analyses suggest that this structure may be the result of ancient divergence between two populations of T. brasiliensis that subsequently came into contact in The Bahamas. Our findings additionally suggest that there may be cryptic species within T. brasiliensis in The Bahamas and the West Indies more broadly. 相似文献
79.
Lightbody KL Renshaw PS Collins ML Wright RL Hunt DM Gordon SV Hewinson RG Buxton RS Williamson RA Carr MD 《FEMS microbiology letters》2004,238(1):255-262
We have previously shown that the secreted M. tuberculosis complex proteins CFP-10 and ESAT-6 form a tight, 1:1 complex, which may represent their functional form. In the work reported here a combination of yeast two-hybrid and biochemical analysis has been used to characterise complex formation between two other pairs of CFP-10/ESAT-6 family proteins (Rv0287/Rv0288 and Rv3019c/Rv3020c) and to determine whether complexes can be formed between non-genome paired members of the family. The results clearly demonstrate that Rv0287/Rv0288 and Rv3019c/3020c form tight complexes, as initially observed for CFP-10/ESAT-6. The closely related Rv0287/Rv0288 and Rv3019c/Rv3020c proteins are also able to form non-genome paired complexes (Rv0287/Rv3019c and Rv0288/Rv3020c), but are not capable of binding to the more distantly related CFP-10/ESAT-6 proteins. 相似文献
80.
I Feinkohl N Sattar P Welsh RM Reynolds IJ Deary MW Strachan JF Price;on behalf of the Edinburgh Type Diabetes Study 《PloS one》2012,7(9):e44569