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   2974篇
  免费   190篇
  国内免费   71篇
  3235篇
  2024年   6篇
  2023年   33篇
  2022年   56篇
  2021年   86篇
  2020年   80篇
  2019年   90篇
  2018年   83篇
  2017年   95篇
  2016年   111篇
  2015年   88篇
  2014年   142篇
  2013年   218篇
  2012年   117篇
  2011年   155篇
  2010年   105篇
  2009年   184篇
  2008年   194篇
  2007年   162篇
  2006年   156篇
  2005年   122篇
  2004年   92篇
  2003年   87篇
  2002年   67篇
  2001年   45篇
  2000年   56篇
  1999年   56篇
  1998年   40篇
  1997年   35篇
  1996年   35篇
  1995年   41篇
  1994年   22篇
  1993年   28篇
  1992年   38篇
  1991年   21篇
  1990年   24篇
  1989年   21篇
  1988年   20篇
  1987年   13篇
  1986年   24篇
  1985年   40篇
  1984年   28篇
  1983年   21篇
  1982年   23篇
  1981年   16篇
  1980年   14篇
  1979年   9篇
  1977年   6篇
  1976年   6篇
  1975年   5篇
  1971年   5篇
排序方式: 共有3235条查询结果,搜索用时 0 毫秒
1.
There is a lack of data on fatigue changes within 24 h among patients with multiple sclerosis. The purpose of this study was to evaluate the effect of time of day on central and peripheral fatigue during a continuous 2-min maximal voluntary contraction of the quadriceps muscle in women and men with multiple sclerosis (MS). We studied age-matched MS patients (range, 40–50 years). The inclusion criteria for patients were: a Kurtzke Expanded Disability Status score and a Fatigue Severity Scale score. We found a significant gender difference in central activation ratio (CAR) in the evening. At the end of the 2-min maximal voluntary contraction (MVC), the voluntary torque decreased by about 65% in men and women with MS in both the morning and evening. We also observed that, in women, CAR decreased markedly during the first 30 s in the evening test. The most interesting finding of our study is that central fatigue increased, whereas peripheral fatigue decreased markedly in the evening only in women. It remains unclear why women’s central fatigue is greater in the evening than in the morning.  相似文献   
2.
Koet  Bart J. 《Dreaming》2008,18(4):267
Dreams are often used to promote religion, or to propagate an interpretation of religion, or to bring political ideas to the fore. A dream can be used as an indication of divine help and thus it is a perfect tool for promoting ideas. Dreams figure in religious propaganda in Christian, Jewish, and Muslim traditions (e.g., in Osama bin Laden's tapes). In classical literature, we find many kings, emperors, or generals who had visions or dreams, especially in association with their accession to power. It is the thesis of this article that in the encounter between Jewish (-Christian) cultures and the Hellenistic world dreams were shared as possible means of divine communication. An important example is the dream in Acts 16,9?10, the most Hellenistic dream of the New Testament. This dream is part of a larger unit, Paul's new move from Asia to Europe. I will try to show that the author of Acts combines here the story of Aeneas (e.g., as told by Virgil) and the story of Jesus, an interesting merging of two cultures. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
3.
The pattern of feeding of Eastern spruce budworm Choristoneura fumiferana (Clem.) (Lepidoptera, Tortricidae) is compared on foliage from white spruce Picea glauca (Moench) Voss. (Pinaceae) trees previously determined to be susceptible and resistant to defoliation by budworm. No differences are observed in electrophysiological responses from taste sensilla to aqueous extracts of the two foliage types, nor is there a preference for either extract type in a choice test. Acetone extracts from the two foliage types are both preferred to a control sucrose solution, although neither elicits a preference relative to the other. These results suggest that there is no difference in phagostimulatory power of internal leaf contents of the two foliage types. Longer‐term observation of feeding behaviour in a no‐choice situation shows no difference in meal duration, confirming the lack of difference in phagostimulatory power. However, on average, intermeal intervals are twice as long on the resistant foliage, leading to an overall lower food consumption during the assay. This result suggests an anti‐digestive or toxic effect of the resistant foliage that slows behaviour and limits food intake. Previous research has shown that waxes of the resistant foliage deter initiation of feeding by the spruce budworm and that this foliage contains higher levels of tannins and monoterpenes. The data suggest that the resistant foliage contains a post‐ingestive second line of defence against the spruce budworm.  相似文献   
4.
We present a synthesis of the palaeolimnological and palaeoclimatic reconstructions of four sites in Colombia. The record from Lake El Caimito, the westernmost site on the Pacific Coast, dates from the Late Holocene and shows lacustrine sedimentation frequently interrupted by fluvial pulses. These pulses probably reflect periods of increased precipitation related to La Niña phases. East of El Caimito site is the Patía swamp, situated between the Western and Central Andean Cordilleras. The Patía records the dynamics of forest expansion/reduction and changes in water levels. Although the climatic signal of the Patia core is difficult to reconstruct, there is a clear increase in humidity in the Mid-Holocene. The Fúquene Lake record, on the Eastern Andean Cordillera, records dry and cold conditions during the Late Pleistocene, very humid conditions for the early Mid-Holocene, and dry conditions during the mid-Late Holocene. Las Margaritas site, on the eastern savannas, records dry conditions during the Early Holocene and overall humid conditions for the Mid- and Late Holocene. Climate conditions from the Fuquene and Las Margaritas sites seem to reflect the Holocene movements of the Intertropical Convergence Zone (ITCZ); the latter site being more affected by humidity coming from the Amazon region.  相似文献   
5.
The first record of Misgurnus anguillicaudatus in Germany   总被引:1,自引:0,他引:1  
Oriental weatherfish Misgurnus anguillicaudatus , from southern China, have been imported to Germany in large numbers for the garden pond trade. The first record of M. anguillicaudatus from Central Europe, a population that has been established for at least 14 years in a small nature reserve in Germany, is detailed.  相似文献   
6.
Didier Merle 《Geobios》2005,38(4):505
Jsowerbya, nov. gen. (Gastropoda: Muricidae) includes three species from the Eocene of the Paris and Hampshire basins. It increases the number of extinct muricid genera, which curiously represent a very small fraction of the described genera. The species of Jsowerbya, often mistaken for the muricopsine genus Muricopsis, possess a unique combination of characters shared with the subfamilies Ocenebrinae and Ergalataxinae. A cladistic analysis, based on structural homologies of the spiral sculpture, however, suggests that Jsowerbya is closely related to the Ocenebrinae. Thus, Jsowerbya is here regarded as one the most basal Ocenebrinae.  相似文献   
7.
Pollen data from 18,000 14C yr bp were compiled in order to reconstruct biome distributions at the last glacial maximum in southern Europe and Africa. Biome reconstructions were made using the objective biomization method applied to pollen counts using a complete list of dryland taxa wherever possible. Consistent and major differences from present‐day biomes are shown. Forest and xerophytic woods/scrub were replaced by steppe, both in the Mediterranean region and in southern Africa, except in south‐western Cape Province where fynbos (xerophytic scrub) persisted. Sites in the tropical highlands, characterized today by evergreen forest, were dominated by steppe and/or xerophytic vegetation (cf. today’s Ericaceous belt and Afroalpine grassland) at the last glacial maximum. Available data from the tropical lowlands are sparse but suggest that the modern tropical rain forest was largely replaced by tropical seasonal forest while the modern seasonal or dry forests were encroached on by savanna or steppe. Montane forest elements descended to lower elevations than today.  相似文献   
8.
Aim During recent and future climate change, shifts in large‐scale species ranges are expected due to the hypothesized major role of climatic factors in regulating species distributions. The stress‐gradient hypothesis suggests that biotic interactions may act as major constraints on species distributions under more favourable growing conditions, while climatic constraints may dominate under unfavourable conditions. We tested this hypothesis for one focal tree species having three major competitors using broad‐scale environmental data. We evaluated the variation of species co‐occurrence patterns in climate space and estimated the influence of these patterns on the distribution of the focal species for current and projected future climates. Location Europe. Methods We used ICP Forest Level 1 data as well as climatic, topographic and edaphic variables. First, correlations between the relative abundance of European beech (Fagus sylvatica) and three major competitor species (Picea abies, Pinus sylvestris and Quercus robur) were analysed in environmental space, and then projected to geographic space. Second, a sensitivity analysis was performed using generalized additive models (GAM) to evaluate where and how much the predicted F. sylvatica distribution varied under current and future climates if potential competitor species were included or excluded. We evaluated if these areas coincide with current species co‐occurrence patterns. Results Correlation analyses supported the stress‐gradient hypothesis: towards favourable growing conditions of F. sylvatica, its abundance was strongly linked to the abundance of its competitors, while this link weakened towards unfavourable growing conditions, with stronger correlations in the south and at low elevations than in the north and at high elevations. The sensitivity analysis showed a potential spatial segregation of species with changing climate and a pronounced shift of zones where co‐occurrence patterns may play a major role. Main conclusions Our results demonstrate the importance of species co‐occurrence patterns for calibrating improved species distribution models for use in projections of climate effects. The correlation approach is able to localize European areas where inclusion of biotic predictors is effective. The climate‐induced spatial segregation of the major tree species could have ecological and economic consequences.  相似文献   
9.
10.
Aim Central America is a biogeographically interesting area because of its location between the rich and very different biota of North and South America. We aim to assess phytogeographical patterns in the bryophyte floras of oak forests and páramo of the Cordillera de Talamanca, Costa Rica. Location Tropical America, in particular the montane area of Cordillera de Talamanca, Costa Rica. Methods The analysis is based on a new critical inventory of the montane bryophyte flora of Cordillera de Talamanca. All species were assigned to phytogeographical elements on the basis of their currently known distribution. Absolute and percentage similarities were employed to evaluate floristic affinities. Results A total of 401 species [191 hepatics (liverworts), one hornwort, 209 mosses] are recorded; of these, 251 species (128 hepatics, one hornwort, 122 mosses) occur in oak forests. Ninety‐three per cent of all oak forest species are tropical in distribution, the remaining 7% are temperate (4%) and cosmopolitan (3%) species. The neotropical element includes almost 74% of the species, the wide tropical element (pantropical, amphi‐atlantic, amphi‐pacific) only 19%. A significant part of the neotropical species from oak forests are species with tropical Andean‐centred ranges (27%). As compared with bryophyte species, vascular plant genera in the study region are represented by fewer neotropical, more temperate and more amphi‐pacific taxa. Bryophyte floras of different microhabitats within the oak forest and epiphytic bryophyte floras on Quercus copeyensis in primary, early secondary and late secondary oak forest show a similar phytogeographical make‐up to the total oak forest bryophyte flora. Comparison of oak forest and páramo reveals a greater affinity of the páramo bryophyte flora to temperate regions and the great importance of the páramo element in páramo. Surprisingly, oak forests have more Central American endemics than páramo. Main conclusions (1) Providing first insights into the phytogeographical patterns of the bryophyte flora of oak forests and páramo, we are able to confirm general phytogeographical trends recorded from vascular plant genera of the study area although the latter were more rich in temperate taxa. (2) Andean‐centred species are a conspicuous element in the bryophyte flora of Cordillera de Talamanca, reflecting the close historical connection between the montane bryophyte floras of Costa Rica and South America. (3) High percentages of Central American endemics in the bryophyte flora of the oak forests suggest the importance of climatic changes associated with Pleistocene glaciations for allopatric speciation.  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