首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3226篇
  免费   212篇
  国内免费   259篇
  2024年   2篇
  2023年   47篇
  2022年   70篇
  2021年   67篇
  2020年   117篇
  2019年   124篇
  2018年   138篇
  2017年   101篇
  2016年   127篇
  2015年   115篇
  2014年   162篇
  2013年   243篇
  2012年   125篇
  2011年   134篇
  2010年   111篇
  2009年   125篇
  2008年   118篇
  2007年   178篇
  2006年   157篇
  2005年   129篇
  2004年   110篇
  2003年   106篇
  2002年   105篇
  2001年   84篇
  2000年   53篇
  1999年   51篇
  1998年   78篇
  1997年   58篇
  1996年   61篇
  1995年   84篇
  1994年   53篇
  1993年   40篇
  1992年   53篇
  1991年   27篇
  1990年   38篇
  1989年   36篇
  1988年   27篇
  1987年   21篇
  1986年   21篇
  1985年   27篇
  1984年   34篇
  1983年   26篇
  1982年   27篇
  1981年   30篇
  1980年   18篇
  1979年   15篇
  1978年   10篇
  1976年   7篇
  1975年   3篇
  1973年   3篇
排序方式: 共有3697条查询结果,搜索用时 265 毫秒
121.
Catherine S. Dolan 《Ethnos》2013,78(2):239-261
This paper explores commodity exchange as a morally inflected practice, one that mediates competing tensions of greed and generosity, the sacred and profane, and affection and estrangement through the fairtrade flower. Using the UK–Kenya fairtrade flower commodity chain to examine the cultural economy of fairtrade, I suggest that like the charity business and the international development industry, fairtrade complicates the distinction between the sacred and secular and the gift and commodity as Northern consumers and NGOs weave webs of obligation through the medium of the market. Further, I argue that while fairtrade is predicated on values of partnership and interdependence, it also operates within commodity chains that advance liberal ethics as a mode of ‘governmentality’ over African producers, translating consumers' sympathy-based humanism into new technologies of regulation and surveillance.  相似文献   
122.
Alison Leitch 《Ethnos》2013,78(4):437-462
This paper explores the emergence of the Slow Food Movement, an international consumer movement dedicated to the protection of ‘endangered foods.’ The history of one of these ‘endangered foods’, lardo di Colonnata, provides the ethnographic window through which I examine Slow Food's cultural politics. The paper seeks to understand the politics of ‘slowness’ within current debates over European identity, critiques of neo-liberal models of rationality, and the significant ideological shift towards market-driven politics in advanced capitalist societies.  相似文献   
123.
Forest cover change directly affects biodiversity, the global carbon budget, and ecosystem function. Within Latin American and the Caribbean region (LAC), many studies have documented extensive deforestation, but there are also many local studies reporting forest recovery. These contrasting dynamics have been largely attributed to demographic and socio‐economic change. For example, local population change due to migration can stimulate forest recovery, while the increasing global demand for food can drive agriculture expansion. However, as no analysis has simultaneously evaluated deforestation and reforestation from the municipal to continental scale, we lack a comprehensive assessment of the spatial distribution of these processes. We overcame this limitation by producing wall‐to‐wall, annual maps of change in woody vegetation and other land‐cover classes between 2001 and 2010 for each of the 16,050 municipalities in LAC, and we used nonparametric Random Forest regression analyses to determine which environmental or population variables best explained the variation in woody vegetation change. Woody vegetation change was dominated by deforestation (?541,835 km2), particularly in the moist forest, dry forest, and savannas/shrublands biomes in South America. Extensive areas also recovered woody vegetation (+362,430 km2), particularly in regions too dry or too steep for modern agriculture. Deforestation in moist forests tended to occur in lowland areas with low population density, but woody cover change was not related to municipality‐scale population change. These results emphasize the importance of quantitating deforestation and reforestation at multiple spatial scales and linking these changes with global drivers such as the global demand for food.  相似文献   
124.
Dandelion is a common Asteraceae species that populates disturbed sites and gaps within swards where it becomes an important competitor of grasses. The natural control against dandelion includes seedling predation, with slugs, particularly Arion lusitanicus, being the most important in the Czech Republic. However, the study of slug seedling consumption is difficult because naturally established seedlings are not always available. Therefore, we developed a method of exposing laboratory‐grown seedlings as bait for slug predation. Dandelion seeds were sown in plastic cups containing a bottom layer of moist substrate. The emerged seedlings were thinned to 20 and displayed in an open area. During 2008–2010, the seedling baits were placed at 15 sites at 1 month intervals throughout the dandelion vegetative season, in parallel with plasticine baits that monitored slug feeding activity. For each 1 month interval, seedling survival was observed for a period of 8 days, and the estimated time to death was calculated; the percentage of surviving seedlings was then recorded. This method of seedling presentation demonstrated that local and temporal variation in seedling survival is correlated with slug feeding activity. The advantage of this technique is that the seedlings in baits may be presented at any place and time, as required by the experimental design; however, we found that the estimated time to seedling death was shorter for the exposed baits than for the naturally established seedlings. The method is suitable for the study of plant species other than dandelion, and also for aims other than the study of spatiotemporal trends in seedling consumption by slugs.  相似文献   
125.
Hyperglycemia-induced oxidative stress leads to diabetes-associated damage to the microvasculature of the brain. Pericytes in close proximity to endothelial cells in the brain microvessels are vital to the integrity of the blood–brain barrier and are especially susceptible to oxidative stress. According to our recently published results, streptozotocin-diabetic mouse brain exhibits oxidative stress and loose pericytes by twelve weeks of diabetes, and cerebral pericytes cultured in high glucose media suffer intracellular oxidative stress and apoptosis. Oxidative stress in diabetes is hypothesized to be caused by reactive oxygen species (ROS) produced during hyperglycemia-induced enhanced oxidative metabolism of glucose (respiration). To test this hypothesis, we investigated the effect of high glucose on respiration rate and ROS production in mouse cerebral pericytes. Previously, we showed that pharmacological inhibition of mitochondrial carbonic anhydrases protects the brain from oxidative stress and pericyte loss. The high glucose-induced intracellular oxidative stress and apoptosis of pericytes in culture were also reversed by inhibition of mitochondrial carbonic anhydrases. Therefore, we extended our current study to determine the effect of these inhibitors on high glucose-induced increases in pericyte respiration and ROS. We now report that both the respiration and ROS are significantly increased in pericytes challenged with high glucose. Furthermore, inhibition of mitochondrial carbonic anhydrases significantly slowed down both the rate of respiration and ROS production. These data provide new evidence that pharmacological inhibitors of mitochondrial carbonic anhydrases, already in clinical use, may prove beneficial in protecting the brain from oxidative stress caused by ROS produced as a consequence of hyperglycemia-induced enhanced respiration.  相似文献   
126.
Knowledge of the rate, distance and direction of dispersal within and among breeding areas is required to understand and predict demographic and genetic connectivity and resulting population and evolutionary dynamics. However dispersal rates, and the full distributions of dispersal distances and directions, are rarely comprehensively estimated across all spatial scales relevant to wild populations. We used re‐sightings of European Shags Phalacrocorax aristotelis colour‐ringed as chicks on the Isle of May (IoM), UK, to quantify rates, distances and directions of dispersal from natal to subsequent breeding sites both within IoM (within‐colony dispersal) and across 27 other breeding colonies covering 1045 km of coastline (among‐colony dispersal). Additionally, we used non‐breeding season surveys covering 895 km of coastline to estimate breeding season detection probability and hence potential bias in estimated dispersal parameters. Within IoM, 99.6% of individuals dispersed between their natal and observed breeding nest‐site. The distribution of within‐colony dispersal distances was right‐skewed; mean distance was shorter than expected given random settlement within IoM, yet some individuals dispersed long distances within the colony. The distribution of within‐colony dispersal directions was non‐uniform but did not differ from expectation given the spatial arrangement of nest‐sites. However, 10% of all 460 colour‐ringed adults that were located breeding had dispersed to a different colony. The maximum observed dispersal distance (170 km) was much smaller than the maximum distance surveyed (690 km). The distribution of among‐colony dispersal distances was again right‐skewed. Among‐colony dispersal was directional, and differed from random expectation and from the distribution of within‐colony dispersal directions. Non‐breeding season surveys suggested that the probability of detecting a colour‐ringed adult at its breeding location was high in the northeastern UK (98%). Estimated dispersal rates and distributions were therefore robust to incomplete detection. Overall, these data demonstrate skewed and directionally divergent dispersal distributions across small (within‐colony) and large (among‐colony) scales, indicating that dispersal could create genetic and demographic connectivity within the study area.  相似文献   
127.
From August to December, thousands of Black‐necked Grebes Podiceps nigricollis concentrate during the flightless moult period in salt ponds in the Odiel Marshes, southern Spain, where they feed on the brine shrimp Artemia parthenogenetica. We predicted that because Black‐necked Grebes moulted in a food‐rich, predator‐free environment, there would be no net loss of body mass caused by the use of fat stored to meet energy needs during remigial feather replacement (as is the case for some other diving waterbirds). However, because the food resource disappears in winter, we predicted that grebes moulting later in the season would put on more body mass prior to moult because of the increasing risk of an Artemia population crash before the moult period is completed. Body mass determinations of thousands of birds captured during 2000–2010 showed that grebes in active wing‐moult showed greater mass with date of capture. Early‐moulting grebes were significantly lighter at all stages than late‐moulting birds. Grebes captured with new feathers post‐moult were significantly lighter than those in moult. This is the first study to support the hypothesis that individual waterbirds adopt different strategies in body mass accumulation according to timing of moult: early‐season grebes were able to acquire an excess of energy over expenditure and accumulate fat stores while moulting. Delayed moulters acquired greater fat stores in advance of moult to contribute to energy expenditure for feather replacement and retained extra stores later, most likely as a bet hedge against the increasing probability of failing food supply and higher thermoregulatory demands late in the season. An alternative hypothesis, that mass change is affected by a trophically transmitted cestode using brine shrimps as an intermediate host and Black‐necked Grebes as final host, was not supported by the data.  相似文献   
128.
The interactions between bacteria and fungi, the main actors of the soil microbiome, remain poorly studied. Here, we show that the saprotrophic and ectomycorrhizal soil fungus Morchella crassipes acts as a bacterial farmer of Pseudomonas putida, which serves as a model soil bacterium. Farming by M. crassipes consists of bacterial dispersal, bacterial rearing with fungal exudates, as well as harvesting and translocation of bacterial carbon. The different phases were confirmed experimentally using cell counting and 13C probing. Common criteria met by other non-human farming systems are also valid for M. crassipes farming, including habitual planting, cultivation and harvesting. Specific traits include delocalization of food production and consumption and separation of roles in the colony (source versus sink areas), which are also found in human agriculture. Our study evidences a hitherto unknown mutualistic association in which bacteria gain through dispersal and rearing, while the fungus gains through the harvesting of an additional carbon source and increased stress resistance of the mycelium. This type of interaction between fungi and bacteria may play a key role in soils.  相似文献   
129.
130.

OBJECTIVE:

This cross-sectional study was undertaken to construct the new body fat % curve and provide body composition reference data for adolescent girls with Turner syndrome (TS). They diagnosed cytogenetically by blood karyotyping and not treated with growth hormone (GH).

MATERIALS AND METHODS:

The study included 70 TS girls from age 13 years to age 17 years. Body composition was measured by bioelectrical impedance. Smoothed centile charts were derived by using the least mean square (LMS) method.

RESULTS:

The new body fat curves reflect the increase of body fat mass (FM) from age 13 years to age 17 years. Body FM % of Egyptian TS girls was lower when compared with age-matched American untreated TS girls.

CONCLUSION:

This study presents the new body fat curves and reference values of body composition for untreated Egyptian TS adolescent girls. The present charts can be used for direct assessment of body FM % for Egyptian TS girls and evaluation for cases on GH treatment or other growth promoting therapy.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号