全文获取类型
收费全文 | 557篇 |
免费 | 33篇 |
国内免费 | 9篇 |
出版年
2023年 | 14篇 |
2022年 | 11篇 |
2021年 | 15篇 |
2020年 | 18篇 |
2019年 | 49篇 |
2018年 | 34篇 |
2017年 | 22篇 |
2016年 | 14篇 |
2015年 | 15篇 |
2014年 | 25篇 |
2013年 | 70篇 |
2012年 | 24篇 |
2011年 | 23篇 |
2010年 | 15篇 |
2009年 | 18篇 |
2008年 | 21篇 |
2007年 | 15篇 |
2006年 | 22篇 |
2005年 | 19篇 |
2004年 | 13篇 |
2003年 | 13篇 |
2002年 | 8篇 |
2001年 | 10篇 |
2000年 | 9篇 |
1999年 | 11篇 |
1998年 | 10篇 |
1997年 | 4篇 |
1996年 | 5篇 |
1995年 | 5篇 |
1994年 | 3篇 |
1993年 | 8篇 |
1992年 | 1篇 |
1991年 | 7篇 |
1990年 | 3篇 |
1989年 | 5篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 6篇 |
1980年 | 4篇 |
1978年 | 1篇 |
1977年 | 4篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有599条查询结果,搜索用时 31 毫秒
81.
Abdulbari Bener John Brebner Mohammed N. S. Atta James Gomes Fatih Ozkaragoz Mohammed Y. Cheema 《Aerobiologia》1997,13(1):11-15
The aim of this study, was to determine the prevalence of some respiratory symptoms and possible diseases among taxi drivers
and manual workers. This prospective study was performed on 165 Pakistani male drivers, (mean age: 34.5±7.8 years) and 165
Pakistani male manual workers not exposed to dust or fumes, without occupational exposure to driving employed in the Water
and Electricity Department and recruited as controls (mean age: 34.6±7.6 years and mean height and weight 169.8±6.0 cm and
71.9±10.9 kg). The data on chronic respiratory symptoms showed that taxi drivers had higher prevalence of symptoms than manual
workers, being significantly greater for asthma (RR=1.72; 95% CI=1.00–2.88,P=0.037); allergic rhinitis (RR=2.41; 95% CI=1.46–3.94,P=0.0006); dyspnea (RR=2.13; 95% CI=1.22–3.71,P=0.009); and nasal catarrh (RR=2.19; 95% CI=1.22–3.91,P=0.0106). Thirty percent of taxi drivers and 27% of manual workers were smokers, there was no significant differences in the
prevalence of chronic respiratory symptoms between smokers and non-smokers. Lung function parameters in the taxi drivers were
significantly lower than in manual workers group (P<0.0001) except PEF parameter. When comparing the measured mean values of lung function parameters in the drivers among smokers
and nonsmokers, there was no significant differences between smokers and nonsmokers. Also, a comparison of ventilatory capacity
of paired predicted values with measured normal values showed statistically significant differences between predicted and
measured values for taxi drivers and manual workers for FVC, FEV1, FEF25–75 and PEF parameters except for FEV1/FVC test in manual workers. In conclusion, the results of the present study provide evidence regarding effects of such as
carbon monoxide, nitrogen dioxide, sulfur dioxide and gases exposures on taxi drivers and long-term driving, which may be
associated with the development of chronic respiratory symptoms and lung function impairment. 相似文献
82.
David Goldblum 《植被学杂志》1997,8(1):125-132
Abstract. Treefalls are a common form of disturbance in northeastern United States forests. The resultant gaps contribute to a high degree of environmental heterogeneity in the understory of these forests. Plant density, plant cover, and species richness in understory plant communities were monitored for three years during the growing season, May - September. Differences between treefall gap and closed canopy vegetation were less pronounced early in the growing season for plant density and leaf cover. Species richness was significantly greater within treefall gaps during the entire growing season. Eight species were found in greater abundance within treefall gaps (i.e., gap-phase species), while one species was found more commonly under closed canopy. Ordination results suggest that time since gap creation and treefall gap size marginally affect the species composition of vegetation found within treefall gaps. 相似文献
83.
Thomas Kitzberger Thomas W. Swetnam† Thomas T. Veblen‡ 《Global Ecology and Biogeography》2001,10(3):315-326
Fire histories were compared between the south-western United States and northern Patagonia, Argentina using both documentary records (1914–87 and 1938–96, respectively) and tree-ring reconstructions over the past several centuries. The two regions share similar fire–climate relationships and similar relationships of climatic anomalies to the El Niño–Southern Oscillation (ENSO). In both regions, El Niño events coincide with above-average cool season precipitation and increased moisture availability to plants during the growing season. Conversely, La Niña events correspond with drought conditions. Monthly patterns of ENSO indicators (southern oscillation indices and tropical Pacific sea surface temperatures) preceding years of exceptionally widespread fires are highly similar in both regions during the 20th century. Major fire years tend to follow the switching from El Niño to La Niña conditions. El Niño conditions enhance the production of fine fuels, which when desiccated by La Niña conditions create conditions for widespread wildfires. Decadal-scale patterns of fire occurrence since the mid-17th century are highly similar in both regions. A period of decreased fire occurrence in both regions from c. 1780–1830 coincides with decreased amplitude and/or frequency of ENSO events. The interhemispheric synchrony of fire regimes in these two distant regions is tentatively interpreted to be a response to decadal-scale changes in ENSO activity. The ENSO–fire relationships of the south-western USA and northern Patagonia document the importance of high-frequency climatic variation to fire hazard. Thus, in addition to long-term trends in mean climatic conditions, multi-decadal scale changes in year-to-year variability need to be considered in assessments of the potential influence of climatic change on fire regimes. 相似文献
84.
A crop growth model developed in Canterbury, New Zealand was used to assess the potential of lentil (Lens culinaris) as a grain legume crop in the UK. The model was validated using five sowing dates at Durham (54.77°N, 1.58°W) in 1999. Predicted time to flowering was within 7 days of actual time to flowering and predicted seed yields were within 9% of actual yields. Actual yields ranged from 1.40 to 1.65 t ha‐1. Seed was of high quality. The model was used to predict rate of development and yields of spring and autumn sown lentils at eight sites along a transect from NW Scotland (Stornoway, 58.22°N, 6.32°W) to SE England (East Mailing, 51.28°N, 0.45°E) chosen to encompass important environmental gradients in the UK. In general, for a 1 May sowing with 150 or 250 mm plant available water (PAW) and a 1 October sowing with 150 mm PAW, predicted mean values over the period 1987–95 for maximum crop growth rate, maximum leaf area index, radiation intercepted, total dry matter produced and seed yield were closely positively related to monthly mean values for mean daily air temperature and increased along the transect from NW to SE UK. Time to flowering generally decreased along the transect from NW to SE UK ranging from 28 June to 9 July and from 20 May to 14 June with the May and October sowings respectively. For the 1 May sowing with 250 mm PAW, predicted mean seed yield ranged from 1.00 to 1.90 t ha‐1. For all sites, yield was very stable over the 9 yr period. For the 1 May sowing with 150 mm PAW, predicted mean seed yield ranged from 0.97–1.23 t ha‐1. Yields for the four more southerly sites were more variable at the lower PAW and, in exceptionally dry years, were substantially lower than average. For these sites, autumn sowing increased seed yields in exceptionally dry years and gave similar or greater mean seed yields to spring sowing with 250 mm PAW. For East Mailing, predicted yields for autumn sowing were on average 2.78 t ha‐1. Also, for Stornoway, because of its relatively high overwinter temperatures, the model predicted substantial increases in yield with autumn sowing. It is concluded that lentil has considerable potential as a grain legume crop in the UK. 相似文献
85.
SYDEL SILVERMAN 《American anthropologist》2007,109(3):519-528
North American anthropology had an earlier interest in studies of the United States and in critical approaches than is often recognized. Such interests were pursued before World War II but were set aside during the war and in anthropology's postwar expansion. This perspective on anthropological history was inspired by the work of Hortense Powdermaker, specifically the disjunction between her 1930s research in segregated Mississippi and her pioneering study of Hollywood in the late 1940s. Reexamining that study highlights the theoretical framework that led to omissions in her account of Hollywood, while her explanation of movie content invites a more diachronic approach. Parallels between the history of the movies and that of cultural anthropology from the 1930s through the 1960s suggest how both were shaped by the Depression, World War II, and the Cold War. 相似文献
86.
Question: Can the geographic patterning of endemic plant species inform reserve selection in a region of high endemism? Location: The Southeastern Coastal Plain of North America, focusing primarily on the imperiled longleaf pine (Pinus palustris P. Miller) ecosystem. Methods: We documented the high level of plant endemism in the region, and characterized the endemic taxa into distributional subregions. Results: A total of 1630 plant taxa are endemic to the Coastal Plain, a large proportion of which are endemic to phytogeographical subregions within the Coastal Plain, with particularly large numbers of narrow endemics occurring in the East Gulf Coastal Plain and Florida Peninsula. Conclusions: This pattern of local endemism presents challenges in conserving the full biota of the region: a reserve system focusing on few and large conservation areas has theoretical benefits for long‐term management and viability, but will fail to capture many local endemics. We propose that the dispersed distribution of endemic species will require a mixture of large core reserves and smaller satellite reserves. 相似文献
87.
88.
We herein report the design, synthesis and molecular docking studies of 2,4-thiazolidinedione derivatives containing benzene sulphonyl group which are docked against the Peroxisome Proliferator Activated Receptor (PPARγ) target. Compound 7p was most effective in lowering the blood glucose level as compared to standard drugs pioglitazone and rosiglitazone. Compound 7p exhibited potent PPAR-γ transactivation of 61.2% with 1.9 folds increase in gene expression. In molecular docking studies 7p showed excellent interactions with amino acids TYR 473, SER 289, HIE 449, TYR 327, ARG 288, MET 329 and LEU 228. Compound 7p did not cause any damage to the liver without any noteworthy weight gain and may be considered as promising candidates for the development of new antidiabetic agents. 相似文献
89.
Nigel Bankes 《Ocean Development & International Law》2016,47(2):141-164
This article assesses the treaty practice of the five Arctic Ocean coastal states and Iceland in dealing with the issue of transboundary hydrocarbon deposits as part of the conclusion of maritime delimitation and other related agreements. That practice suggests a number of different ways in which states deal with the issue of hydrocarbon deposits bisected by a maritime delimitation: (1) silence, (2) a standard unity of deposit clause, (3) more complex variations on the standard clause including framework agreements for the development of transboundary deposits, and (4) a delimitation line with some form of joint development zone. This article assesses the Arctic state practice within this typology seeking to supplement the existing literature which tends to focus on the North Sea and Southeast Asia. 相似文献
90.
Three new species of Swartzieae are described and illustrated:Swartzia alternifoliolata, S. capixabensis, andZollernia cowanii. Swartzia apetala var.blanchetii and var.subcordata are considered to be synonyms ofS. apetala var.apetala, andS. grazielana a synonym ofS. macrostachya var.macrostachya. Keys to southeastern Brazil members ofSwartzia andZollernia are provided. 相似文献