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   98901篇
  免费   706篇
  国内免费   812篇
  2023年   35篇
  2022年   23篇
  2021年   194篇
  2020年   112篇
  2019年   151篇
  2018年   11968篇
  2017年   10764篇
  2016年   7670篇
  2015年   944篇
  2014年   663篇
  2013年   760篇
  2012年   4732篇
  2011年   13241篇
  2010年   12271篇
  2009年   8507篇
  2008年   10107篇
  2007年   11704篇
  2006年   542篇
  2005年   759篇
  2004年   1229篇
  2003年   1242篇
  2002年   1008篇
  2001年   325篇
  2000年   196篇
  1999年   77篇
  1998年   77篇
  1997年   76篇
  1996年   33篇
  1995年   29篇
  1994年   42篇
  1993年   49篇
  1992年   45篇
  1991年   56篇
  1990年   22篇
  1989年   17篇
  1988年   29篇
  1987年   20篇
  1986年   7篇
  1985年   12篇
  1984年   17篇
  1983年   26篇
  1982年   13篇
  1975年   6篇
  1972年   246篇
  1971年   274篇
  1965年   13篇
  1962年   24篇
  1956年   5篇
  1944年   12篇
  1940年   10篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
991.
Subspecies were traditionally defined by identifying gaps between phenotypes across the geographic range of a species, and may represent important units in the development of conservation strategies focused on preserving genetic diversity. Previous taxonomic research proposed that phenotypic variation between scattered Indri indri populations warranted the naming of two distinct subspecies, I. i. indri and I. i. variegatus. We tested these subspecific designations using mitochondrial sequence data generated from the control region or D-loop (569 bp) and a large section (2362 bp) of multiple genes and tRNAs known as Pastorini’s fragment and nuclear microsatellite markers. This study used 114 samples of I. indri from 12 rainforest sites in eastern Madagascar, encompassing the entire range of the species. These genetic samples represent multiple populations from low- and high-elevation forests from both putative subspecies. Molecular analyses of the mitochondrial sequence data did not support the two proposed subspecies. Furthermore, the microsatellite analyses showed no significant differences across the range beyond population level differentiation. This study demonstrates the utility of incorporating multiple lines of evidence in addition to phenotypic traits to define species or subspecies.  相似文献   
992.
“Air (aire, also aigre) in the body” is a frequent explanation of illness according to the traditional medical beliefs in Mexico. Anthropologists have generally scrutinized aire in the context of other common folk illnesses treated by traditional healers (curanderas). However, drawing on my research in the communities of Northern Oaxaca I suggest that aire occupies a more distinct position in the folk medical cosmology than it has usually been credited with. This distinction rests on the notion’s exceptional ambivalence and openness to multiple interpretations. “Air” is recurred to as the cause of illness mainly in situations where every other explanation, either “traditional” or “biomedical,” seems to be inadequate. The physical properties of air—its transparency, invisibility, apparent immateriality, near omnipresence, and virtual “nothingness”—render it a suitable explanation of the last resort. Local understandings of what aire “is” are often vague and elusive, and in many respects the term functions in folk medical discourse as an “empty signifier.”  相似文献   
993.
Arboreal primates actively navigate a complex thermal environment that exhibits spatial, daily, and seasonal temperature changes. Thus, temperature measurements from stationary recording devices in or near a forest likely do not reflect the thermal microenvironments that primates actually experience. To better understand the thermal variation primates encounter, we attached automated temperature loggers to anklets worn by free-ranging mantled howling monkeys (Alouatta palliata) to record near-animal ambient temperatures. We compared these measures to conventional, stationary temperature measurements taken from within the forest, in nearby open fields, and at a remote weather station 38.6 km from the field site. We also measured temperatures across vertical forest heights and assessed the effects of wind speed, solar radiation, rain, and vapor pressure on primate subcutaneous temperatures (collected via implanted loggers). Ambient temperatures at measurement sites commonly used by researchers differed from those experienced by animals. Moreover, these differences changed between seasons, indicating dynamic shifts in thermal environment occur through space and time. Temperatures increased with height in the forest, with statistically significant, albeit low magnitude, differences between vertical distances of one meter. Near-animal temperatures showed that monkeys selected relatively warmer microhabitats during nighttime temperature lows and relatively cooler microhabitats during the day. Lastly, the thermal variables wind speed, solar radiation, vapor pressure, and rain were statistically associated with primate subcutaneous temperatures. Our data indicate that the temperatures arboreal primates experience are not well reflected by stationary devices. Attaching automated temperature loggers to animals provides a useful tool for more directly assessing primate microhabitat use.  相似文献   
994.
Primate activity budgets are dictated by food availability and distribution; thus primates living in seasonal environments must adapt their behaviors to accommodate fluctuations in resources. Cat Ba langurs (Trachypithecus poliocephalus), a Critically Endangered Asian colobine and a member of the limestone langur group (francoisi superspecies group within genus Trachypithecus), live only in fragmented and disturbed habitats on Cat Ba Island, northeastern Vietnam. This study aimed to assess the behaviors and diet of Cat Ba langurs by group, age, sex, and season. We predicted they would have high rates of inactivity and foraging, low rates of social behaviors, with seasonal variation that reflects an energy-maximizing strategy. We conducted behavioral observations through scan sampling over an 11-month period and found that Cat Ba langurs spent a significant portion of their day inactive (57 %) followed by foraging (18 %), socializing (13 %), locomoting (10 %), and engaging in “other” behaviors (2 %). Their diet was made up primarily of leaves (83 %) followed by flowers (8 %), fruit (6 %), and stems (3 %). We found groups to differ in diet and activity, which is likely owing to differences in demographics and home range between groups. Seasonally, the animals ate more leaves and spent more time foraging in the dry season than the wet season, suggesting that they are energy maximizers. Cat Ba langurs have activity and dietary budgets similar to those of other limestone langurs, and respond to a presumed seasonal fluctuation in food availability similarly.  相似文献   
995.
Recent studies in African contexts have revealed a strong association between spirit possession and severe trauma, with inclusion into a possession cult serving at times a therapeutic function. Research on spirit possession in the Dominican Republic has so far not included quantitative studies of trauma and dissociation. This study evaluated demographic variables, somatoform dissociative symptoms, and potentially traumatizing events in the Dominican Republic with a group of Vodou practitioners that either do or do not experience spirit possession. Inter-group comparisons revealed that in contrast to non-possessed participants (n = 38), those experiencing spirit possession (n = 47) reported greater somatoform dissociation, more problems with sleep, and previous exposure to mortal danger such as assaults, accidents, or diseases. The two groups did not differ significantly in other types of trauma. The best predictor variable for group classification was somatoform dissociation, although those items could also reflect the experience of followers during a possession episode. A factor analysis across variables resulted in three factors: having to take responsibility early on in life and taking on a professional spiritual role; traumatic events and pain; and distress/dissociation. In comparison with the non-possessed individuals, the possessed ones did not seem to overall have a remarkably more severe story of trauma and seemed to derive economic gains from possession practice.  相似文献   
996.
“Thinking too much”, and variations such as “thinking a lot”, are common idioms of distress across the world. The contextual meaning of this idiom of distress in particular localities remains largely unknown. This paper reports on a systematic study of the content and cause, consequences, and social response and coping related to the local terms |x’an n|a te and |eu-ca n|a te, both translated as “thinking a lot”, and was part of a larger ethnographic study among the Khwe of South Africa. Semi-structured exploratory interviews with community members revealed that “thinking a lot” refers to a common experience of reflecting on personal and interpersonal problems. Consequences were described in emotional, psychological, social, behavioral, and physical effects. Coping strategies included social support, distraction, and religious practices. Our contextualized approach revealed meanings and experiences of “thinking a lot” that go beyond a psychological state or psychopathology. The common experience of “thinking a lot” is situated in socio-political, economic, and social context that reflect the marginalized and displaced position of the Khwe. We argue that “thinking a lot” and associated local meanings may vary across settings, may not necessarily indicate psychopathology, and should be understood in individual, interpersonal, community, and socio-political dimensions.  相似文献   
997.
In the aftermath of war and armed conflict, individuals and communities face the challenge of dealing with recollections of violence and atrocity. This article aims to contribute to a better understanding of processes of remembering and forgetting histories of violence in post-conflict communities and to reflect on related implications for trauma rehabilitation in post-conflict settings. Starting from the observation that memory operates at the core of PTSD symptomatology, we more closely explore how this notion of traumatic memory is conceptualized within PTSD-centered research and interventions. Subsequently, we aim to broaden this understanding of traumatic memory and post-trauma care by connecting to findings from social memory studies and transcultural trauma research. Drawing on an analysis of scholarly literature, this analysis develops into a perspective on memory that moves beyond a symptomatic framing toward an understanding of memory that emphasizes its relational, political, moral, and cultural nature. Post-conflict memory is presented as inextricably embedded in communal relations, involving ongoing trade-offs between individual and collective responses to trauma and a complex negotiation of speech and silence. In a concluding discussion, we develop implications of this broadened understanding for post-conflict trauma-focused rehabilitation.  相似文献   
998.
Recent evidence has proved that caspase protease activities are detected in both mammals and plants during programmed cell death (PCD). The characteristics and functions of caspase-like proteases play important roles in understanding the mechanisms of PCD in plants. In this work, we report firstly the involvement of caspase-like protease activities and effects in aluminum (Al) stress in two contrasting peanut genotypes. Caspase-like activities in the root tip cells of ‘Zhonghua 2’ (Al-sensitive) and ‘99-1507’ (Al-tolerant) were detected using synthetic caspase substrates during Al-triggered PCD. Caspase-1-, -2-, -3-, -4-, -5-, -6-, -8- and -9-like proteases were found in peanut root tip cells. VDQQDase (caspase-2-like) and WEHD (caspase-5-like) were the first detected in the plants, and almost all of the caspase-like proteases were activated during Al-induced PCD, especially caspase-3-like and caspase-1-like, which was higher in ‘Zhonghua 2’ than in ‘99-1507’. The highest activity levels of caspase-3- and caspase-1-like proteases occurred 8 and 4 h after 100 µM Al treatment, respectively. Compared with 100 µM AlCl3 treatment alone, specific caspase-3 protease inhibitor Ac-DEVD-CHO inhibited the increase of caspase-3-like protease activity, Al content, Hsr203j expression, cell death and DNA fragmentation, and the decrease in root growth induced by 100 µM AlCl3 treatment, but it was more obvious in ‘Zhonghua 2’ than in ‘99-1507’. In conclusion, there were different caspase-like proteases in root tips of peanut, and caspase-3-like protease was a crucial executioner in Al-induced PCD. Its effects in the ‘Zhonghua 2’ genotype were higher than in ‘99-1507’. An improved model of the mechanism of Al-induced PCD and Al tolerance differences in different genotypes is proposed.  相似文献   
999.
1000.
Clay minerals and K feldspars were evaluated as sources of K in a Norway spruce stand (Picea abies (L.) Karst.) from the Skogaby experimental forest in southwest Sweden. The soil, developed in a Quaternary glacial till, has only 3–5% clay, and more than 95% of its K resides in feldspars. Ratios of K/Rb were assessed in interlayers of 2:1 clay minerals (extracted with hot (100 °C) 2 M HCl), biomass and the forest floor. These compartments had similarly low K/Rb ratios, whereas K feldspars were significantly poorer in Rb. A fractionation model indicated preferential retention of Rb in the biomass and forest floor, due to stronger adsorption of Rb than K in the humus, as well as preferential uptake of K from the exchange complex in the mineral soil. Preferential uptake of K may result from weaker adsorption of K by the cation exchanger, or preference for dissolved K over Rb by the roots. A quantitative mineralogical analysis revealed that loss from micas may account for half of the Holocene loss of K from the soil, which was approximately 22 Mg ha?1. Exceptionally low K/Rb ratios in HCl extracts of the upper 60 cm of the profile indicated extensive loss of K from mica in the parent material and re-fixation of K and Rb at lower ratios. The results indicate that fixation in and release from clay minerals may be prominent in the cycling of K, even in a soil that is poor in clay minerals.  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