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   2591篇
  免费   275篇
  国内免费   1篇
  2021年   29篇
  2019年   17篇
  2018年   34篇
  2017年   29篇
  2016年   46篇
  2015年   65篇
  2014年   90篇
  2013年   94篇
  2012年   121篇
  2011年   103篇
  2010年   66篇
  2009年   61篇
  2008年   101篇
  2007年   85篇
  2006年   99篇
  2005年   94篇
  2004年   79篇
  2003年   78篇
  2002年   78篇
  2001年   85篇
  2000年   86篇
  1999年   57篇
  1998年   30篇
  1997年   27篇
  1996年   25篇
  1995年   19篇
  1994年   28篇
  1993年   27篇
  1992年   53篇
  1991年   57篇
  1990年   55篇
  1989年   71篇
  1988年   64篇
  1987年   68篇
  1986年   46篇
  1985年   45篇
  1984年   43篇
  1983年   26篇
  1982年   38篇
  1981年   31篇
  1980年   28篇
  1979年   37篇
  1978年   40篇
  1977年   27篇
  1976年   29篇
  1975年   29篇
  1974年   35篇
  1973年   30篇
  1971年   21篇
  1966年   16篇
排序方式: 共有2867条查询结果,搜索用时 624 毫秒
991.
Background Social and financial environment has an influence on the incidence of depression. We studied perceived financial strain as a risk factor for development of depression among a large cohort of young women in Southampton, UK.Methods We recruited a large number of young women in Southampton in the Southampton Women's Survey, a longitudinal study looking at factors influencing the health of women and their offspring. Women were asked to complete a baseline questionnaire, which included the GHQ-12 (an assessment of mental health), as well as questions on perceived financial strain and past history of depression. They were followed up two years later through their general practitioner (GP) records for evidence of incident mental illness.Results A total of 7020 women completed the baseline questionnaire including the GHQ-12. Of these, 5237 (74.6%) had records available for follow-up. Among those developing depression, there was a higher proportion receiving benefits, and a higher level of perceived financial strain. There were also modest elevations in perceived stress, and poorer levels of educational attainment. Among women not depressed at baseline, and with no previous history of depression, those in receipt of state benefits at baseline had a significantly elevated risk of developing the disorder - hazard ratio 1.61 (95% confidence interval (CI) 1.13-2.3). The risk associated with perceived financial strain was 2.16 (95% CI 1.14-4.11), but this did not remain statistically significant after adjustment was made for receipt of benefits, educational qualification, and perceived stress.Conclusion Financial hardship as evidenced by receipt of benefits is a strong independent predictor for the development of depression. Although perception of financial strain is also a predictor for incident depression, the risk associated with this subjective characteristic does not remain significantly elevated after adjustment. Future studies of the aetiology of depression should incorporate ascertainment of actual financial status.  相似文献   
992.
Embryonic stem cells (ESCs) are prone to differentiation in culture, suggesting that maintenance of the pluripotent state must be actively induced. In a recent issue of Nature, Ying et al. (2008) use soluble small molecules to inhibit pro-differentiation signals and reveal ESC self-renewal as a default cell fate.  相似文献   
993.
The major histocompatibility complex (MHC) plays a key role in pathogen recognition as a part of the vertebrate adaptive immune system. The great diversity of MHC genes in natural populations is maintained by different forms of balancing selection and its strength should correlate with the diversity of pathogens to which a population is exposed and the rate of exposure. Despite this prediction, little is known about how life‐history characteristics affect selection at the MHC. Here, we examined whether the strength of balancing selection on MHC class II genes in birds (as measured with nonsynonymous nucleotide substitutions, dN) was related to their social or migratory behavior, two life‐history characteristics correlated with pathogen exposure. Our comparative analysis indicated that the rate of nonsynonymous substitutions was higher in colonial and migratory species than solitary and resident species, suggesting that the strength of balancing selection increases with coloniality and migratory status. These patterns could be attributed to: (1) elevated transmission rates of pathogens in species that breed in dense aggregations, or (2) exposure to a more diverse fauna of pathogens and parasites in migratory species. Our study suggests that differences in social structure and basic ecological traits influence MHC diversity in natural vertebrate populations.  相似文献   
994.
Despite the prevalence and costs of allergic diseases caused by pollen, we know little about the distributions of allergenic and non-allergenic pollen inside and outside homes at the continental scale. To better understand patterns in potential pollen diversity across the United States, we used DNA sequencing of a chloroplast marker gene to identify the plant DNA found in settled dust collected on indoor and outdoor surfaces across 459 homes. House location was the best predictor of the relative abundance of plant taxa found in outdoor dust samples. Urban, southern houses in hotter climates that were further from the coast were more likely to have more DNA from grass and moss species, while rural houses in northern, cooler climates closer to the coast were more likely to have higher relative abundances of DNA from Pinus and Cedrus species. In general, those plant taxa that were more abundant outdoors were also more abundant indoors, but indoor dust had uniquely high abundances of DNA from food plants and plants associated with lawns. Approximately 14 % of the plant DNA sequences found outside were from plant taxa that are known to have allergenic pollen compared to just 8 % inside. There was little geographic pattern in the total relative abundance of these allergens highlighting the difficulties associated with trying to predict allergen exposures based on geographic location alone. Together, this work demonstrates the utility of using environmental DNA sequencing to reconstruct the distributions of plant DNA inside and outside buildings, an approach that could prove useful for better understanding and predicting plant allergen exposures.  相似文献   
995.
White mould, caused by the fungus Sclerotinia sclerotiorum, is one of the most destructive diseases of beans globally. In New York State, USA, white mould causes substantial losses in soybean, snap, dry and succulent baby lima beans, which are grown successively in intensive crop rotations. Management strategies for white mould in these crops are reliant upon the prophylactic use of fungicides. No complementary information on the genetic structure of the populations of S. sclerotiorum in New York State, USA is available. Twenty isolates of S. sclerotiorum were collected from symptomatic bean plants within each of 10 fields across New York State, USA in 2014. Eight microsatellite (SSR) markers were used to characterise the genotypic diversity of the hyphal‐tipped isolates. Twenty‐four multilocus genotypes (MLGs) were detected within the population but one MLG was most prevalent. Although STRUCTURE analysis identified two subpopulations, these subpopulations were not associated with geographic location, suggesting no spatial structure to the population. In addition, the pathogen populations were predominantly clonal, with some evidence of infrequent outcrossing. These findings may assist in understanding the durability of management strategies for white mould and support the selection of representative isolates for host resistance screening for pathogen populations in the sampling area.  相似文献   
996.
997.
Brandon Dunn 《Fly》2017,11(4):271-276
The Hippo signaling pathway regulates organ size and tissue homeostasis. Given this role it is unsurprising that dysregulation of this pathway has implications for cancer progression. A convincing body of literature shows that the Hippo pathway serves a tumor suppressive function with its inactivation leading to massive overgrowth. However, additional studies have also shown that activation of Hippo signaling can promote tumor progression. It remains unknown how a single pathway can produce such diametrically opposed effects. This lack of knowledge is in part due to our inability to make meaningful comparisons from studies which have taken place in a variety of cell types, tissues, and organisms. Recently however, we have published 2 studies using the Drosophila wing disk to study the Hippo pathway and have found that Hippo pathway activation can promote cell migration and invasion while Hippo pathway inactivation leads to overgrowth. Thus we propose here that Drosophila can provide a research platform with which to begin addressing how the Hippo pathway can both enhance and suppress tumor progression due to published pro- and anti-tumor functionalities of the Hippo pathway in the same tissue.  相似文献   
998.
999.
Plaice Pleuronectes platessa populations on the west coast of England and Wales are currently managed as two stocks: in ICES Division VIIa (Irish Sea, Cardigan Bay and St George's Channel), and ICES Divisions VIIf&g (Bristol Channel and Celtic Sea). A total of 13,784 plaice were fitted with Petersen tags and released in these areas during 1979–1980 and 1993–1996. Analysis of the 2788 recaptures received by June 2000 confirmed known spawning and feeding grounds in the region. It showed also that plaice >25 cm L T tended to undertake extensive spatial movements. At this size, female plaice were likely to be mature or maturing for the first time, whilst males were probably mature. Tag recaptures indicated resident sub-stocks of plaice in the north-east Irish Sea, the south-east Irish Sea, Cardigan Bay and Bristol Channel, a contingent of plaice in all areas that undertook permanent dispersal to other areas, and a contingent which originated in the south-east Irish Sea and migrated to spawn in the Bristol Channel. Plaice originating in the Bristol Channel rarely moved north into ICES Vila. A general hypothesis of plaice population structure in the region is presented and discussed in relation to stock assessment.  相似文献   
1000.
When lactate was removed from sucrose fermentation in situ, using the anionic-exchange resin Amberlite IRA-67, by Lactococcus lactis growing in batch culture, nisin production increased by two-fold when compared to the alkali pH-controlled fermentation. In comparison to sucrose, lactate removal increased nisin production 1.5-fold and 0.3-fold when galactose and glucose were used as carbon sources respectively.  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