全文获取类型
收费全文 | 7375篇 |
免费 | 549篇 |
国内免费 | 1篇 |
出版年
2023年 | 47篇 |
2022年 | 74篇 |
2021年 | 151篇 |
2020年 | 148篇 |
2019年 | 152篇 |
2018年 | 194篇 |
2017年 | 192篇 |
2016年 | 271篇 |
2015年 | 423篇 |
2014年 | 402篇 |
2013年 | 521篇 |
2012年 | 584篇 |
2011年 | 563篇 |
2010年 | 408篇 |
2009年 | 357篇 |
2008年 | 424篇 |
2007年 | 426篇 |
2006年 | 437篇 |
2005年 | 365篇 |
2004年 | 351篇 |
2003年 | 301篇 |
2002年 | 279篇 |
2001年 | 66篇 |
2000年 | 60篇 |
1999年 | 75篇 |
1998年 | 68篇 |
1997年 | 56篇 |
1996年 | 40篇 |
1995年 | 48篇 |
1994年 | 32篇 |
1993年 | 38篇 |
1992年 | 21篇 |
1991年 | 27篇 |
1990年 | 30篇 |
1989年 | 17篇 |
1988年 | 18篇 |
1987年 | 22篇 |
1986年 | 19篇 |
1985年 | 19篇 |
1984年 | 22篇 |
1983年 | 24篇 |
1982年 | 19篇 |
1981年 | 19篇 |
1980年 | 11篇 |
1977年 | 15篇 |
1976年 | 9篇 |
1975年 | 11篇 |
1974年 | 13篇 |
1973年 | 11篇 |
1971年 | 8篇 |
排序方式: 共有7925条查询结果,搜索用时 15 毫秒
931.
Valencia C Arévalo J Dujardin JC Llanos-Cuentas A Chappuis F Zimic M 《PLoS neglected tropical diseases》2012,6(6):e1656
Background
Increased rates for failure in leishmaniasis antimony treatment have been recently recognized worldwide. Although several risk factors have been identified there is no clinical score to predict antimony therapy failure of cutaneous leishmaniasis.Methods
A case control study was conducted in Peru from 2001 to 2004. 171 patients were treated with pentavalent antimony and followed up to at least 6 months to determine cure or failure. Only patients with ulcerative cutaneous leishmaniasis (N = 87) were considered for data analysis. Epidemiological, demographical, clinical and laboratory data were analyzed to identify risk factors for treatment failure. Two prognostic scores for antimonial treatment failure were tested for sensitivity and specificity to predict antimony therapy failure by comparison with treatment outcome.Results
Among 87 antimony-treated patients, 18 (21%) failed the treatment and 69 (79%) were cured. A novel risk factor for treatment failure was identified: presence of concomitant distant lesions. Patients presenting concomitant-distant lesions showed a 30.5-fold increase in the risk of treatment failure compared to other patients. The best prognostic score for antimonial treatment failure showed a sensitivity of 77.78% and specificity of 95.52% to predict antimony therapy failure.Conclusions
A prognostic score including a novel risk factor was able to predict antimonial treatment failure in cutaneous leishmaniasis with high specificity and sensitivity. This prognostic score presents practical advantages as it relies on clinical and epidemiological characteristics, easily obtained by physicians or health workers, and makes it a promising clinical tool that needs to be validated before their use for developing countries. 相似文献932.
Melittobia acasta and Melittobia australica are newly recorded from Sicily, Italy, and the second species is reported in Europe for the first time. A short historical background about Melittobia parasitoid wasps, their hosts, and distribution, with emphasis in those two species is presented together with illustrations to facilitate their identification. Brief discussion about the presence and possible distribution of the species in Sicily is also included. 相似文献
933.
Julieta Maya-Morales Guillermo Ibarra-Nú?ez Jorge L. León-Cortés Francisco Infante 《Journal of Insect Conservation》2012,16(1):25-38
We evaluated the spider diversity of a tropical montane cloud forest understory in two nearby sites with different degree
of human disturbance at the Biosphere Reserve Volcán Tacaná, Chiapas, Mexico. The study was conducted over a 24 days period
distributed in 6 months in 2009, covering dry and rainy seasons. A total of 8,370 spiders (1,208 adults and 7,162 juveniles)
were collected. Determined specimens (7,747) represented 112 species and morphospecies, 71 genera and 22 families. The results
showed that human disturbance has an influence on spider communities: species richness was significantly higher in the preserved
site as regards to the disturbed site. Despite their proximity, the composition of spider communities showed only a moderate
similarity between the two sites. No differences in abundance were found among sites when considering the whole sample, but
sites differed clearly when seasons were analyzed separately. The rainy season had a negative effect on the abundance of spiders
in the preserved site. Although the spider community structure was very similar between sites, there was a trend towards a
greater species evenness in the preserved site for the whole sampling period and for the dry season. 相似文献
934.
935.
Jorge A. Martínez-Villegas Alma Orozco-Segovia María Esther Sánchez-Coronado Irene Pisanty 《Plant Ecology》2012,213(5):871-881
Sedum oxypetalum is one of the dominant species in the xerophilous shrublands in the lava fields of the Basin of Mexico. Germination of this species was evaluated to understand its ecological response. We tested the effects of different pre-germination treatments (cold, and dry and wet heat) and storage time, as well as those of natural priming in two microhabitats with different disturbance levels. Experiments were performed in laboratory conditions under constant (25 °C) and fluctuating (20/30 °C) temperatures. Seeds did not germinate during burial and proved to be positively photoblastic. Under pre-germination treatments, final germination percentage was higher at 20/30 °C in seeds after four or more months of storage. None of the pre-germination treatments favored germination. Seeds can survive for more than 1 year and form a seed bank. Thus, seeds underwent natural priming that favored final germination percentage; however, germination rate and lag time were not affected by this process. In natural conditions, germination is delayed until the rainy season, improving the success of seedling establishment and growth. We discuss the role of fluctuating temperature in germination processes and the adaptations of seeds to their seasonal environment. 相似文献
936.
Marco A. Molina-Montenegro Jorge Gallardo-Cerda T. S. M. Flores Cristian Atala 《Plant Ecology》2012,213(1):133-142
The upper and poleward limit of tree distribution are usually determined by abiotic factors such as low temperature and strong
winds. Thus, cold resistance is a key element for survival in high altitudes and latitudes where conditions can reduce plant
growth. A trade-off between resource allocation to cold resistance and growth could emerge in populations frequently exposed
to low temperatures like those in the treeline zone. We studied annual height growth and ice nucleation temperature in Nothofagus pumilio (Nothofagaceae) populations growing in its extremes of altitudinal distribution and in 3 sites situated on a latitudinal
gradient in the Chilean Andes. Additionally, gas exchange, water and nitrogen use efficiency and total soluble sugar (TSS)
were also measured as possible mechanisms for survival in high altitudes. Individuals from the treeline populations showed
lower annual height growth and lower ice nucleation temperatures compared with those from lower populations. In the same way,
individuals from more poleward populations showed lower annual height growth and lower ice nucleation temperatures. Gas exchange,
water and nitrogen use efficiency and TSS were also higher in the high altitude populations. The results obtained support
the hypothesis of trade-off, because the upper and poleward populations showed more cold resistance but a lower height growth.
Additionally, we show that cold resistance mechanisms do not impact the physiological performance, suggesting possible adaptation
of the high altitude populations. Low temperatures may be affecting cellular growth instead of photosynthesis, creating a
pool of carbohydrates that could participate in cold tolerance. Other abiotic and biotic factors should be also assessed to
fully understand the distributional range of Nothofagus species. 相似文献
937.
Jorge Morales Pedraza Astrid Lobo Gajiwala María Esther Martinez Pardo 《Cell and tissue banking》2012,13(1):15-25
The IAEA International Standards for Tissue Banks published in 2003 were based on the Standards then currently in use in the
USA and the European Union, among others, and reflect the best practices associated with the operation of a tissue bank. They
cover legal, ethical and regulatory controls as well as requirements and procedures from donor selection and tissue retrieval
to processing and distribution of finished tissue for clinical use. The application of these standards allows tissue banks
to operate with the current good tissue practice, thereby providing grafts of high quality that satisfy the national and international
demand for safe and biologically useful grafts. The objective of this article is to review the IAEA Standards and recommend
new topics that could improve the current version. 相似文献
938.
Jorge Parodi Lenin Ochoa-de la Paz Ricardo Miledi Ataúlfo Martínez-Torres 《Molecules and cells》2012,34(4):349-355
Xenopus laevis oocytes exposed to amyloid-β aggregate generated oscillatory electric activity (blips) that was recorded by two-microelectrode voltage-clamp. The cells exhibited a series of “spontaneous” blips ranging in amplitude from 3.8 ± 0.9 nA at the beginning of the recordings to 6.8 ± 1.7 nA after 15 min of exposure to 1 μM aggregate. These blips were similar in amplitude to those induced by the channel-forming antimicrobial agents amphotericin B (7.8 ± 1.2 nA) and gramicidin (6.3 ± 1.1 nA). The amyloid aggregate-induced currents were abolished when extracellular Ca2+ was removed from the bathing solution, suggesting a central role for this cation in generating the spontaneous electric activity. The amyloid aggregate also affected the Ca2+-dependent Cl− currents of oocytes, as shown by increased amplitude of the transient-outward chloride current (Tout) and the serum-activated, oscillatory Cl− currents. Electron microcopy revealed that amyloid aggregate induced the dissociation of the follicular cells that surround the oocyte, thus leading to a failure in the electro-chemical communication between these cells. This was also evidenced by the suppression of the oscillatory Ca2+-dependent ATP-currents, which require proper coupling between oocytes and the follicular cell layer. These observations, made using the X. laevis oocytes as a versatile experimental model, may help to understand the effects of amyloid aggregate on cellular communication. 相似文献
939.
940.
Francisco J. Román‐Dañobeytia Samuel I. Levy‐Tacher James Aronson Ricardo Ribeiro Rodrigues Jorge Castellanos‐Albores 《Restoration Ecology》2012,20(3):378-386
The rainforest of Mexico has been degraded and severely fragmented, and urgently require restoration. However, the practice of restoration has been limited by the lack of species‐specific data on survival and growth responses to local environmental variation. This study explores the differential performance of 14 wet tropical early‐, mid‐ or late‐successional tree species that were grown in two abandoned pastures with contrasting land‐use histories. After 18 months, seedling survival and growth of at least 7 of the 14 tree species studied were significantly higher in the site with a much longer history of land use (site 2). Saplings of the three early‐successional species showed exceptional growth rates. However, differences in performance were noted in relation to the differential soil properties between the experimental sites. Mid‐successional species generally showed slow growth rates but high seedling survival, whereas late‐successional species exhibited poor seedling survival at both the study sites. Stepwise linear regressions revealed that the species integrated response index combining survivorship and growth measurements, was influenced mostly by differences in soil pH between the two abandoned pastures. Our results suggest that local environmental variation among abandoned pastures of contrasting land‐use histories influences sapling survival and growth. Furthermore, the similarity of responses among species with the same successional status allowed us to make some preliminary site and species‐specific silvicultural recommendations. Future field experiments should extend the number of species and the range of environmental conditions to identify site “generalists” or more narrowly adapted species, that we would call “sensitive.” 相似文献