全文获取类型
收费全文 | 3476篇 |
免费 | 307篇 |
国内免费 | 2篇 |
出版年
2022年 | 36篇 |
2021年 | 81篇 |
2020年 | 60篇 |
2019年 | 60篇 |
2018年 | 77篇 |
2017年 | 72篇 |
2016年 | 78篇 |
2015年 | 134篇 |
2014年 | 159篇 |
2013年 | 189篇 |
2012年 | 248篇 |
2011年 | 228篇 |
2010年 | 146篇 |
2009年 | 103篇 |
2008年 | 143篇 |
2007年 | 134篇 |
2006年 | 127篇 |
2005年 | 131篇 |
2004年 | 114篇 |
2003年 | 82篇 |
2002年 | 96篇 |
2001年 | 74篇 |
2000年 | 60篇 |
1999年 | 50篇 |
1998年 | 26篇 |
1997年 | 30篇 |
1996年 | 25篇 |
1993年 | 25篇 |
1992年 | 40篇 |
1991年 | 36篇 |
1990年 | 28篇 |
1989年 | 26篇 |
1988年 | 30篇 |
1987年 | 28篇 |
1986年 | 24篇 |
1985年 | 40篇 |
1984年 | 23篇 |
1983年 | 27篇 |
1982年 | 18篇 |
1979年 | 29篇 |
1978年 | 32篇 |
1977年 | 21篇 |
1976年 | 21篇 |
1974年 | 20篇 |
1973年 | 21篇 |
1972年 | 22篇 |
1971年 | 21篇 |
1969年 | 18篇 |
1968年 | 20篇 |
1966年 | 19篇 |
排序方式: 共有3785条查询结果,搜索用时 562 毫秒
91.
Charles LaroucheBilodeau Xavier GuilbeaultMayers Christopher B. Cameron 《Ecology and evolution》2020,10(23):13544
We measured gill slit fluctuating asymmetry (FA), a measure of developmental noise, in adults of three invertebrate deuterostomes with different feeding modes: the cephalochordate Branchiostoma floridae (an obligate filter feeder), the enteropneusts Protoglossus graveolens (a facultative filter feeder/deposit feeder) and Saccoglossus bromophenolosus (a deposit feeder). FA was substantially and significantly low in B. floridae and P. graveolens and high in S. bromophenolosus. Our results suggest that the gills of species that have experienced a relaxation of the filter feeding trait exhibit elevated FA. We found that the timing of development of the secondary collagenous gill bars, compared to the primary gill bars, was highly variable in P. graveolens but not the other two species, demonstrating an independence of gill FA from gill bar heterochrony. We also discovered the occasional ectopic expression of a second set of paired gills posterior to the first set of gills in the enteropneusts and that these were more common in S. bromophenolosus. Moreover, our finding that gill slits in enteropneusts exhibit bilateral symmetry suggests that the left‐sidedness of larval cephalochordate gills, and the directional asymmetry of Cambrian stylophoran echinoderm fossil gills, evolved independently from a bilaterally symmetrical ancestor. 相似文献
92.
Recent advances in artificial intelligence show tremendous promise to improve the accuracy, reproducibility, and availability of medical diagnostics across a number of medical subspecialities. This is especially true in the field of digital pathology, which has recently witnessed a surge in publications describing state-of-the-art performance for machine learning models across a wide range of diagnostic applications. Nonetheless, despite this promise, there remain significant gaps in translating applications for any of these technologies into actual clinical practice. In this review, we will first give a brief overview of the recent progress in applying AI to digitized pathology images, focusing on how these tools might be applied in clinical workflows in the near term to improve the accuracy and efficiency of pathologists. Then we define and describe in detail the various factors that need to be addressed in order to successfully close the “translation gap” for AI applications in digital pathology. 相似文献
93.
Beylergil Sinem Balta Murray Jordan Noecker Angela M. Gupta Palak Kilbane Camilla McIntyre Cameron C. Shaikh Aasef G. Ghasia Fatema F. 《Journal of computational neuroscience》2021,49(3):345-356
Journal of Computational Neuroscience - Miniature yoked eye movements, fixational saccades, are critical to counteract visual fading. Fixational saccades are followed by a return saccades forming... 相似文献
94.
95.
Miranda N. Vesy Jessa L. Watters Raymond W. Moody Eric M. Schauber Jennie M. Mook Cameron D. Siler 《The Journal of wildlife management》2021,85(6):1267-1279
Habitat fragmentation has negative consequences on threatened and endangered species by creating isolated populations. The Texas horned lizard (Phrynosoma cornutum) is experiencing population declines and localized extirpations throughout its range and has been classified as a species of greatest conservation need in Oklahoma, USA. Younger age classes have been poorly studied but may be vital to the stability of remaining populations. To address gaps in knowledge concerning subadult (hatchling and juvenile) morphometrics, survivorship, and home range sizes, we studied 2 cohorts of subadults, for 2 years each, covering their hatching and juvenile years (2016–2019). We used a combination of radio-telemetry and novel harmonic radar methodology to study a closed population of Texas horned lizards in 15 ha of native grassland at Tinker Air Force Base, Oklahoma. Population abundance for adults and juveniles was estimated as 56.5 ± 5.5 lizards and density as 7.96 lizards/ha. Our lowest estimates of survival indicated an average survival probability for the hatchling life stage of 0.285 (95% CI = 0.15–0.44), which is lower than for adults on the site. Average home range size increased from hatchling to adult life stages. Our results will have an immediate effect on the planning and assessment of ongoing headstart and management programs for Texas horned lizards. © 2021 The Authors. The Journal of Wildlife Management published by Wiley Periodicals LLC on behalf of The Wildlife Society. 相似文献
96.
Gaowen Yang Yingjun Zhang Xin Yang Nan Liu Matthias C. Rillig Stavros D. Veresoglou Cameron Wagg 《Oikos》2021,130(3):354-365
Nutrient enrichment can reduce ecosystem stability, typically measured as temporal stability of a single function, e.g. plant productivity. Moreover, nutrient enrichment can alter plant–soil interactions (e.g. mycorrhizal symbiosis) that determine plant community composition and productivity. Thus, it is likely that nutrient enrichment and interactions between plants and their soil communities co-determine the stability in plant community composition and productivity. Yet our understanding as to how nutrient enrichment affects multiple facets of ecosystem stability, such as functional and compositional stability, and the role of above–belowground interactions are still lacking. We tested how mycorrhizal suppression and phosphorus (P) addition influenced multiple facets of ecosystem stability in a three-year field study in a temperate steppe. Here we focused on the functional and compositional stability of plant community; functional stability is the temporal community variance in primary productivity; compositional stability is represented by compositional resistance, turnover, species extinction and invasion. Community variance was partitioned into population variance defined as community productivity weighted average of the species temporal variance in performance, and species synchrony defined as the degree of temporal positive covariation among species. Compared to treatments with mycorrhizal suppression, the intact AM fungal communities reduced community variance in primary productivity by reducing species synchrony at high levels of P addition. Species synchrony and population variance were linearly associated with community variance with the intact AM fungal communities, while these relationships were decoupled or weakened by mycorrhizal suppression. The intact AM fungal communities promoted the compositional resistance of plant communities by reducing compositional turnover, but this effect was suppressed by P addition. P addition increased the number of species extinctions and thus promoted compositional turnover. Our study shows P addition and AM fungal communities can jointly and independently modify the various components of ecosystem stability in terms of plant community productivity and composition. 相似文献
97.
Pooneh Saidi Mahdi Sadeghi Claudio Tenreiro 《Physica medica : PM : an international journal devoted to the applications of physics to medicine and biology : official journal of the Italian Association of Biomedical Physics (AIFB)》2013,29(3):286-294
Monte Carlo simulations and TLD dosimetry have been performed to determine the dose distributions along the central axis of the 12 mm COMS eye plaques loaded with IRA1-103Pd seeds. Several simulations and measurements have been employed to investigate the effect of Silastic insert and air in front of the eye on dosimetry results along the central axis of the plaque and at some critical ocular structures. Measurements were performed using TLD-GR200A circular chip dosimeters in a PMMA eye phantom. The central axis TLD chips locations were arranged in one central column of eye phantom, in 3 mm intervals. The off-axis TLD chips locations were arranged in three off-axis columns around the central axis column. Version 5 of the MCNP code was also used to evaluate the dose distribution around the plaque. The presence of the Silastic insert results in dose reduction of 14% at 5 mm; also about 7% dose reduction appears at the interface point, due to the air presence and lack of the scattering condition. The overall dosimetric parameters for the COMS eye plaque loaded with new palladium seeds are similar to a commercial widely used seed such as Theragenics200. As the dose calculations under TG-43 assumptions do not consider the effect of the plaque backing and Silastic insert for accurate dosimetry, it's suggested to apply the effect of the eye plaque materials and air on dosimetry results along the central axis of the plaque and at some critical ocular structures. 相似文献
98.
Leila Sadeghi Khosro Khajeh Nasrin Mollania Bahareh Dabirmanesh Bijan Ranjbar 《Molecular biotechnology》2013,53(3):270-277
It is the common feature of α-amylases that calcium ion is required for their structural integrity and thermal stability. All amylases have at least one Ca2+ per molecule; therefore amino acids involved in calcium binding are specific and conserved. In this study, sequence analysis revealed the presence of EF-hand-like motif in calcium-binding loop of Bacillus megaterium WHO (BMW)-amylase that was previously isolated from BMW. The EF-hand motif and its variants (EF-hand-like motif) are the most common calcium-binding motifs found in a large number of protein families. To investigate the effect of calcium ion on the thermal stability and activity of BMW-amylase, we used site-directed mutagenesis to replace histidine 58 with Asp (D), Ile (I), Tyr (Y), Phe (F), and Arg (R) at the seventh position of EF-hand-like motif. Upon the addition of an extra DX unit to the calcium-binding loop in H58D variant, thermal stability, catalytic activity, and chelating power of the enzyme improved due to higher affinity toward calcium. H58D variant demonstrated calcium independency compared to the wild type and other created mutants. Conformational changes in the presence and absence of Ca2+ were monitored using fluorescence technique. 相似文献
99.
Olusegun O. Osunkoya Christine Perrett Chandima Fernando Cameron Clark Sathyamurthy Raghu 《Population Ecology》2013,55(2):291-303
It is at the population level that an invasion either fails or succeeds. Lantana camara L. (Verbenaceae) is a weed of great significance in Queensland Australia and globally but its whole life-history ecology is poorly known. Here we used 3 years of field data across four land use types (farm, hoop pine plantation and two open eucalyptus forests, including one with a triennial fire regime) to parameterise the weed’s vital rates and develop size-structured matrix models. Lantana camara in its re-colonization phase, as observed in the recently cleared hoop pine plantation, was projected to increase more rapidly (annual growth rate, λ = 3.80) than at the other three sites (λ 1.88–2.71). Elasticity analyses indicated that growth contributed more (64.6 %) to λ than fecundity (18.5 %) or survival (15.5 %), while across size groups, the contribution was of the order: juvenile (19–27 %) ≥ seed (17–28 %) ≥ seedling (16–25 %) > small adult (4–26 %) ≥ medium adult (7–20 %) > large adult (0–20 %). From a control perspective it is difficult to determine a single weak point in the life cycle of lantana that might be exploited to reduce growth below a sustaining rate. The triennial fire regime applied did not alter the population elasticity structure nor resulted in local control of the weed. However, simulations showed that, except for the farm population, periodic burning could work within 4–10 years for control of the weed, but fire frequency should increase to at least once every 2 years. For the farm, site-specific control may be achieved by 15 years if the biennial fire frequency is tempered with increased burning intensity. 相似文献
100.
Rajesh Panigrahi Sidhartha Hazari Sruti Chandra Partha K. Chandra Sibnarayan Datta Ramazan Kurt Craig E. Cameron Zhuhui Huang Haitao Zhang Robert F. Garry Luis A. Balart Srikanta Dash 《PloS one》2013,8(8)