全文获取类型
收费全文 | 13318篇 |
免费 | 1411篇 |
国内免费 | 3篇 |
学科分类
生物科学 | 14732篇 |
出版年
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.
Stewart A. Hill Stephen E. Scheckler James F. Basinger 《American journal of botany》1997,84(1):85-103
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.
Minh Thac Nguyen Ryan Denniston Hien Thi Thu Nguyen Tuan Anh Hoang Hana Ross Anthony D. So 《PloS one》2014,9(1)
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.
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
10.