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   350篇
  免费   28篇
  国内免费   30篇
  2023年   5篇
  2022年   7篇
  2021年   14篇
  2020年   8篇
  2019年   8篇
  2018年   5篇
  2017年   8篇
  2016年   7篇
  2015年   6篇
  2014年   13篇
  2013年   19篇
  2012年   9篇
  2011年   23篇
  2010年   11篇
  2009年   28篇
  2008年   18篇
  2007年   15篇
  2006年   14篇
  2005年   9篇
  2004年   14篇
  2003年   15篇
  2002年   7篇
  2001年   10篇
  2000年   16篇
  1999年   9篇
  1998年   5篇
  1997年   7篇
  1996年   9篇
  1995年   9篇
  1994年   2篇
  1993年   8篇
  1992年   10篇
  1991年   6篇
  1990年   8篇
  1989年   7篇
  1988年   2篇
  1987年   6篇
  1986年   3篇
  1985年   2篇
  1983年   4篇
  1979年   2篇
  1978年   2篇
  1977年   2篇
  1976年   4篇
  1974年   2篇
  1973年   1篇
  1972年   3篇
  1970年   2篇
  1966年   1篇
  1964年   1篇
排序方式: 共有408条查询结果,搜索用时 46 毫秒
1.
The wolfberry (Lycium barbarumL.) fruit has been used in traditional Chinese medicine and food, but fruits grown in various geographic areas are significantly different in quality and content of functional components. In this study, we tested the internal nutritional ingredients and related environmental factors using forty-two wolfberry fruits collected from seven wolfberry producing regions in China (Yinchuan and Zhongning in Ningxia, Jingyuan in Gansu, Nuomuhong Farm in Qinghai,Hangjinhouqi and Wulateqianqi in Inner Mongolia, and Jinghe in Xinjiang). The nutritional ingredients of the forty-two wolfberry fruits are dependent on the environment. Correlation analysis revealed that the amount of nutrients was highly related to each other. Based on the analysis ofratio of total sugar to betaine (sugar-alkali ratio), we have successfully established quality standards for the wolfberry based on the levels of betaine and sugar. The standards were as follows: high betaine low sugar (sugar-alkali ratio≤15.1), high betaine medium sugar (15<sugar-alkali ratio≤60), medium betaine medium sugar (60<sugar-alkali ratio≤121), medium betaine high sugar (121<sugar-alkali ratio≤165), and low betaine high sugar (sugar-alkali ratio>165). Zhongning of Ningxia is the geo-authentic producing region of the wolfberry, whereby the wolfberries are rich in main pharmacological ingredients Lycium barbarum polysaccharide (LBP) and three trace elements (iron, manganese and selenium) and medium sugar-alkali ratio. In addition, the correlation analysis showed that both the altitude and average temperature difference between day and night had positive effects on the wolfberry flavonoids content but the annual sunshine duration negatively affected the betaine contents of the wolfberry fruit.  相似文献   
2.
2009年11月份在西安举行由中国实验动物学会发起的实验动物医药技术培训研讨会上,本人做了关于实验小鼠种群管理的演讲。其主题涵盖了小鼠繁殖性能的维护和基因工程种群的管理。本篇文章的目的在于强调其所涵盖的材料并为进一步研究提供参考。如若对本文所讨论的内容希望有更深刻的理解,这篇文章所引用的参考资料值得进一步的阅读。另外本文未涉及远交系小鼠的繁殖培育和基因管理,但如有需要可以联系本文作者,可推荐需阅读的材料清单。  相似文献   
3.
4.
The issue of growing water scarcity has been increasingly perceived as a global systemic risk. To solve it, an integrated approach considering different perspectives of water scarcity is at a premise. In this study, we developed an approach to calculate the blue water scarcity (BWS) and integrated the production, consumption, and water transfer perspectives into a single framework. The results are as follows: The average BWS in the Hetao irrigation district was 0.491 during the 2001–2010 year period, which was much larger than the threshold of 0.30, indicating a high water stress level. From the production perspective, the agricultural sector was the largest contributor to regional water scarcity and the average BWS was as high as 0.479. From the consumption perspective, BWS related to virtual water export was much larger than that related to water consumption for making products to be consumed locally and the values were 0.422 and 0.069, respectively. Under the influence of physical and virtual water transfer, BWS changed from 0.242 (medium to high water stress level) to 0.491 (high water stress level). Strategies for reducing agricultural water consumption, such as increasing crop water productivity, improving irrigation efficiency, and promoting more reasonable irrigation water price, could be adopted in the Hetao irrigation district to alleviate regional BWS. Compared with physical and virtual water import, the virtual water export played a more important role in influencing the regional water scarcity, and the increase in crop water productivity, decrease in crop export volume, or adjustment of trade pattern from water-intensive crops to water-extensive ones could be feasible measures to decrease virtual water export for lower water stress, while the trade-offs in the product-consuming regions should be considered.  相似文献   
5.
Summary Paleozoic carbonate ramp sedimentation has generally been described in terms of downlapping clinoforms composed of allochthonous sediment derived from shallower environments. However, during transgressive episodes when carbonate sediment production is low and down slope sediment transport by gravity becomes inactive, autochthonous carbonate sediment accumulates in vertical stacks of essentially in situ sediment. Autochthonous outer ramp deposition is probably a part of many Paleozoic ramp strata, but has heretofore not been recognized because of the general absence of adequate exposures. Evidence of autochthonous, in situ deposition and preservation of sediments in a starved setting is well displayed in the Alamogordo Member of the Lake Valley Formation in south central New Mexico. This evidence includes: 1) beds and bed sets that are individually continuous and traceable along ramp slope for 32 km, 2) down-ramp sequential distributions of depth-sensitive organisms and assemblages but patchy distribution of rock types, 3) lack of sedimentary structures indicative of transport, 4) well preserved, unabraded fossils, 5) the common occurrence of fossils in life position, 6) beds traceable into and through mounds 7) bed thickness trends ascribed to biotic productivity, and 8) geopetal structures in original position. Integrated paleontologic, sedimentologic, and stratigraphic data provide information about depositional processes and setting. The depositional slope was approximately 0.5o based on the distribution of fossil algae; this is comparable to dips reported for other Mississippian homoclinal ramps. An oxygen minimum zone may have impinged on the ramp during a major flooding event. Shifts in biotic gradients from bed to bed reveal transgressive-regressive patterns that would not be resolvable without detailed paleontological evidence. The Alamogordo Member formed as a result of transgressive and early highstand starved carbonate sedimentation along a narrow, homoclinal outer ramp. The surface of maximum flooding and the boundary between the TST and HST are within the Alamogordo Member.  相似文献   
6.
7.
Lowland rainforests on Borneo are being degraded and lost at an alarming rate. Studies on mammals report species responding in various ways to habitat changes that occur in commercial forestry concessions. Here we draw together information on the relationship between the ecological, evolutionary, and biogeographic characteristics of selected Bornean non-volant mammals, and their response to timber harvesting and related impacts. Only a minority of species show markedly reduced densities after timber harvesting. Nonetheless there are many grounds for concern as various processes can, and often do, reduce the viability of wildlife populations. Our review of what we know, and of current understanding, helps predict mammalian dynamics and subsequent mammal-induced ecosystem changes in logged forests. We identify groups of mammal species that, although largely unstudied, are unlikely to tolerate the impacts associated with timber harvesting. On a positive note we find and suggest many relatively simple and low-cost ways in which concession management practices might be modified so as to improve the value of managed forests for wildlife conservation. Improving forest management can play a vital role in maintaining the rich biodiversity of Borneo’s tropical rain forests.  相似文献   
8.
Genetic selection focused purely on production traits has proven very successful in improving the productive performance of livestock. However, heightened environmental and infectious disease challenges have raised the need to also improve the resilience of animals to such external stressors, as well as their efficiency in utilising available resources. A better understanding of the relationship between efficiency and production and health traits is needed to properly account for it in breeding programmes and to produce animals that can maintain high production performance in a range of environmental conditions with minimal environmental footprint. The aim of this study was to perform a meta-analysis of genetic parameters for production, efficiency and health traits in sheep and goats. The dataset comprised 963 estimates of heritability and 572 genetic correlations collated from 162 published studies. A threelevel meta-analysis model was fitted. Pooled heritability estimates for milk production traits ranged between 0.27 ± 0.03 and 0.48 ± 0.13 in dairy goats and between 0.21 ± 0.06 and 0.33 ± 0.07 in dairy sheep. In meat sheep, the heritability of efficiency traits ranged from 0.09 ± 0.02 (prolificacy) up to 0.32 ± 0.14 (residual feed intake). For health traits, pooled heritability was 0.07 ± 0.01 (faecal egg count) and 0.21 ± 0.01 (somatic cell score) in dairy goats and 0.14 ± 0.04 (faecal egg count) and 0.13 ± 0.02 (somatic cell score) in dairy sheep. In meat sheep, the heritability of disease resistance and survival traits ranged between 0.07 ± 0.02 (mastitis) and 0.50 ± 0.10 (breech strike). Pooled estimates of genetic correlations between resilience and efficiency traits in dairy goats were not significantly different from zero with the exception of somatic cell score and fat content (?0.19 ± 0.01). In dairy sheep, only the unfavourable genetic correlation between somatic cell score and protein content (0.12 ± 0.03) was statistically significant. In meat sheep only, the correlations between growth and faecal egg count (?0.28 ± 0.11) as well as between growth and dagginess (?0.33 ± 0.13) were statistically significant and favourable. Results of this meta-analysis provide evidence of genetic antagonism between production and health in dairy sheep and goats. This was not observed in meat sheep where most of the pooled estimates had high standard errors and were non-significant. Based on the obtained results, it seems feasible to simultaneously improve efficiency and health in addition to production by including the different types of traits in the breeding goal. However, a better understanding of potential trade-offs between these traits would be beneficial. Particularly, more studies focused on reproduction and resilience traits linked to the animal’s multi-trait response to challenges are required.  相似文献   
9.
Lepidopteran heat-tolerant(ht)cell lines have been obtained with sf-9,sf-21 and several Bombyx cells.They have a distinct karyotype,membrane lipid composition,morphology and growth kinetics from the parental cell lines.In this paper,we report the development of ht cell lines from other insect species and examination of their growth characteristics and virus susceptibility.Adaptation of cell lines sf-9,BTI-TN-5131-4(High5)and BTI-TN-MG1(MG 1)to 33℃ and 35℃ was carried out by shifting the culture temperature between 28℃ and higher temperatures by a gradual stepwise increase in temperature.The process of adaption to a higher culture temperature was accomplished over a period of 2 months.The cell lines with the temperature adaption were designated as sf9-ht33,sf9-ht35,High5-ht33,High5-ht35,MG1-ht33,MG1-ht35.These cell lines have been subcultured over 70 passages.Adaption to high temperatures was confirmed by a constant population doubling time with individual cell lines.The population doubling time of heat adapted cell lines were 1-4 h less than these of parental cell lines.Cell shapes did not show obvious change,however,the cell size of sf9-ht cells was enlarged and those of High5 and MG1 ht cells were reduced after heat adaption.When the cell lines were infected with Autographa californica nuclear polyhedrosis virus(AcMNPV)at 28℃,33℃,35℃ and 37℃,production of budded virus and occlusion bodies in each cell line was optimum at its own adapted temperature.  相似文献   
10.
The article is mainly devoted to such representatives of gut microbiota as lactic acid bacteria and bifidobacteria, with minor accent on less frequently used or new probiotic microorganisms. Positive effects in treatment and prevention of diseases by different microbial groups, their metabolites and mechanisms of action, management and market of probiotic products are considered.  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