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   15523篇
  免费   1738篇
  2021年   204篇
  2020年   138篇
  2019年   180篇
  2018年   178篇
  2017年   187篇
  2016年   314篇
  2015年   456篇
  2014年   509篇
  2013年   589篇
  2012年   788篇
  2011年   740篇
  2010年   472篇
  2009年   440篇
  2008年   615篇
  2007年   629篇
  2006年   651篇
  2005年   546篇
  2004年   589篇
  2003年   584篇
  2002年   537篇
  2001年   457篇
  2000年   427篇
  1999年   376篇
  1998年   215篇
  1997年   189篇
  1996年   209篇
  1995年   169篇
  1994年   155篇
  1993年   146篇
  1992年   311篇
  1991年   285篇
  1990年   311篇
  1989年   301篇
  1988年   261篇
  1987年   238篇
  1986年   251篇
  1985年   242篇
  1984年   186篇
  1983年   184篇
  1982年   175篇
  1981年   144篇
  1979年   188篇
  1978年   153篇
  1977年   138篇
  1976年   134篇
  1975年   127篇
  1974年   125篇
  1973年   140篇
  1972年   124篇
  1971年   114篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
1.
2.
The distribution of grooming and touching behaviours was recorded in a group of captive ring-tailed lemurs. Grooming was found to be performed chiefly by older, higher ranking animals; touching (i.e., “reach out and touch” behaviour) was directed primarily by younger, low ranking animals to older, high ranking individuals. It is suggested that such touching is a submissive gesture in this species.  相似文献   
3.
4.
Illicit trade carries the potential to magnify existing tobacco-related health care costs through increased availability of untaxed and inexpensive cigarettes. What is known with respect to the magnitude of illicit trade for Vietnam is produced primarily by the industry, and methodologies are typically opaque. Independent assessment of the illicit cigarette trade in Vietnam is vital to tobacco control policy. This paper measures the magnitude of illicit cigarette trade for Vietnam between 1998 and 2010 using two methods, discrepancies between legitimate domestic cigarette sales and domestic tobacco consumption estimated from surveys, and trade discrepancies as recorded by Vietnam and trade partners. The results indicate that Vietnam likely experienced net smuggling in during the period studied. With the inclusion of adjustments for survey respondent under-reporting, inward illicit trade likely occurred in three of the four years for which surveys were available. Discrepancies in trade records indicate that the value of smuggled cigarettes into Vietnam ranges from $100 million to $300 million between 2000 and 2010 and that these cigarettes primarily originate in Singapore, Hong Kong, Macao, Malaysia, and Australia. Notable differences in trends over time exist between the two methods, but by comparison, the industry estimates consistently place the magnitude of illicit trade at the upper bounds of what this study shows. The unavailability of annual, survey-based estimates of consumption may obscure the true, annual trend over time. Second, as surveys changed over time, estimates relying on them may be inconsistent with one another. Finally, these two methods measure different components of illicit trade, specifically consumption of illicit cigarettes regardless of origin and smuggling of cigarettes into a particular market. However, absent a gold standard, comparisons of different approaches to illicit trade measurement serve efforts to refine and improve measurement approaches and estimates.  相似文献   
5.
J. Martin  J. B. Ross 《CMAJ》1984,130(4):421-422
A newborn child was noted to have an ulcerated lesion on the vertex of her scalp, which was diagnosed as aplasia cutis congenita. As this disorder is relatively rare and frequently misdiagnosed, this case is reported and the relevant literature reviewed.  相似文献   
6.
Despite considerable progress in recent years in the promotion of flowering in conifers, relatively little is known about the endogenous control of sex expression or its practical manipulation to aid pollen and crop management in seed orchards. Well defined patterns of sexual zonation exist within the tree crown and shoot, and these appear to be associated with hormonal and possibly nutritional gradients. No firm conclusions, however, are possible regarding the specific role(s) of different PGRs in sex expression, this being as much as a problem of lack of critical study as the complexity of the process itself. Whether a PGR such as gibberellins or auxins, or a cultural treatment that itself may influence endogenous hormone levels, primarily promotes female or male flowering often depends on the position within the crown of the shoot being treated, and its timing in relation to stage of primordia development. Photoperiod also may be involved- short days favoring females and long days males, at least in some species- although its role in controlling sex expression is by no means clear. Practical manipulation of sex expression may be possible through selective crown pruning, different PGR combinations, treatment timing and/or photoperiod control, but the development of such techniques for operational use will require a better basic understanding of the endogenous regulatory mechanisms involved.  相似文献   
7.
Manitoba     
Ross Mitchell 《CMAJ》1944,51(5):475-476
  相似文献   
8.
Sodium bisulfite treatment of single-stranded DNA deaminates exposed cytosine residues to form uracil, resulting in cytosine-to-thymidine transition mutations following DNA replication. We have used this reaction in vitro to destroy the recognition sequences for the restriction endonucleases HindIII and XmaI in the aminoglycoside 3'-phosphotransferase I coding region of plasmid pUC4K. This procedure should be applicable to the mutation of any recognition sequence of restriction endonucleases which generate cytosine-containing single-stranded ends. The possibility of mutagenesis of restriction sites to generate stop codons in coding regions is discussed.  相似文献   
9.
10.
To avoid negative impacts on food production, novel non-food biofuel feedstocks need to be identified and utilised. One option is to utilise marine biomass, notably fast-growing, large marine ‘plants’ such as the macroalgal kelps. This paper reports on the changing composition of Laminaria digitata throughout it growth cycle as determined by new technologies. The potential of Laminaria sp. as a feedstock for biofuel production and future biorefining possibilities was assessed through proximate and ultimate analysis, initial pyrolysis rates using thermo-gravimetric analysis (TGA), metals content and pyrolysis gas chromatography-mass spectrometry.Samples harvested in March contained the lowest proportion of carbohydrate and the highest ash and alkali metal content, whereas samples harvested in July contained the highest proportions of carbohydrate, lowest alkali metals and ash content. July was therefore considered the most suitable month for harvesting kelp biomass for thermochemical conversion to biofuels.  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