首页 | 官方网站   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   13318篇
  免费   1411篇
  国内免费   3篇
生物科学   14732篇
  2021年   198篇
  2020年   106篇
  2019年   159篇
  2018年   147篇
  2017年   160篇
  2016年   269篇
  2015年   403篇
  2014年   462篇
  2013年   544篇
  2012年   735篇
  2011年   675篇
  2010年   427篇
  2009年   381篇
  2008年   572篇
  2007年   537篇
  2006年   539篇
  2005年   495篇
  2004年   504篇
  2003年   472篇
  2002年   502篇
  2001年   380篇
  2000年   355篇
  1999年   327篇
  1998年   180篇
  1997年   154篇
  1996年   151篇
  1995年   130篇
  1994年   117篇
  1993年   136篇
  1992年   234篇
  1991年   223篇
  1990年   212篇
  1989年   196篇
  1988年   198篇
  1987年   184篇
  1986年   197篇
  1985年   179篇
  1984年   125篇
  1983年   130篇
  1982年   129篇
  1981年   117篇
  1980年   113篇
  1979年   126篇
  1978年   122篇
  1977年   102篇
  1975年   111篇
  1974年   106篇
  1973年   121篇
  1972年   112篇
  1969年   94篇
排序方式: 共有10000条查询结果,搜索用时 671 毫秒
1.
2.
3.
4.
5.
A new fern-like fossil plant is described from the lower Upper Devonian of southern Ellesmere Island, Canadian Arctic Archipelago. The plant occurs in an Archaeopteris-dominated flora preserved in the Nordstrand Point Formation (Mid-Late Frasnian) near Bird Fiord. The plant has a pinnate vegetative system with three branch orders and laminate sphenopteroid pinnules. Primary pinnae usually diverge from the main axis in distichous pairs (quadriseriate), but can depart singly (biseriate). Each primary pinna bears a basal catadromic aphlebia. Anatomically, the plant exhibits a mesarch, bipolar protostele that is ribbon- to clepsydropsoid-shaped in the main axis. Primary pinna traces are also initially bipolar and crescent-shaped, but may become four-ribbed before dividing into a pair of bipolar traces. The morphology and anatomy of this plant are nongymnospermous and are most similar to Zygopteridales (particularly Rhacophytaceae and Zygopteridaceae). The Frasnian age of Ellesmeris shows that laminated foliage had evolved in some zygopterid ferns much earlier than previously recognized. The Sphenopteris-like pinnules of Ellesmeris indicate the need for caution when attributing such a convergent foliar design to other plant groups, such as the Devonian gymnosperms.  相似文献   
6.
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   
7.
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   
8.
9.
Saccharomyces cerevisiae CDC8 gene and its product.   总被引:14,自引:6,他引:8  
  相似文献   
10.
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-23

京公网安备 11010802026262号