全文获取类型
收费全文 | 7453篇 |
免费 | 860篇 |
国内免费 | 3篇 |
出版年
2021年 | 150篇 |
2020年 | 88篇 |
2019年 | 120篇 |
2018年 | 131篇 |
2017年 | 119篇 |
2016年 | 184篇 |
2015年 | 300篇 |
2014年 | 319篇 |
2013年 | 397篇 |
2012年 | 502篇 |
2011年 | 481篇 |
2010年 | 291篇 |
2009年 | 260篇 |
2008年 | 403篇 |
2007年 | 417篇 |
2006年 | 380篇 |
2005年 | 328篇 |
2004年 | 311篇 |
2003年 | 318篇 |
2002年 | 328篇 |
2001年 | 121篇 |
2000年 | 120篇 |
1999年 | 130篇 |
1998年 | 101篇 |
1997年 | 60篇 |
1996年 | 66篇 |
1995年 | 72篇 |
1994年 | 67篇 |
1993年 | 51篇 |
1992年 | 77篇 |
1991年 | 83篇 |
1990年 | 89篇 |
1989年 | 71篇 |
1988年 | 85篇 |
1987年 | 72篇 |
1986年 | 66篇 |
1985年 | 72篇 |
1984年 | 66篇 |
1983年 | 70篇 |
1982年 | 51篇 |
1981年 | 41篇 |
1980年 | 59篇 |
1979年 | 59篇 |
1978年 | 51篇 |
1977年 | 50篇 |
1974年 | 41篇 |
1973年 | 53篇 |
1972年 | 49篇 |
1971年 | 46篇 |
1969年 | 47篇 |
排序方式: 共有8316条查询结果,搜索用时 31 毫秒
21.
22.
23.
Cleavage of phycocyanobilin from C-phycocyanin 总被引:1,自引:0,他引:1
24.
The pmr spectra of fully acetylated 2S: 3R-2 amino-trans-4 octadecene-1, 3 diol (sphingosine) (lb), 2S : 3R-2 aminooctadecane-1, 3 diol (dihydrosphingosine) (2b), 1-O-β-D-galactopyranosyl-2S : 3R-2 tetracosanoylamido-trans-4 octadecene-1, 3 diol (cerasine) (3b), and 1-O-β-D-galactopyranosyl-2S : 3R-2 (2′ hydroxy) tetracosanoylamido-trans-4 octadecene-1, 3 diol (phrenosine) (4b), were determined in chloroform-d, acetone-d6, and benzene-d6 at 220 MHz. The relative chemical shifts of the various protons in the three solvents were different enough as to permit configurational and conformational information to be derived by partial first order analysis. 相似文献
25.
26.
The use of plantations to manage extensive tracks of deforested lands in the tropics is a conservation strategy that has recently received considerable attention. Plantation trees can promote seed dispersal by attracting dispersers and creating favorable site conditions, leading to increased germination and establishment of indigenous trees. Subsequently, plantation trees can be harvested for profit or left to senesce, leaving a native tree community. We evaluated the effect of vine, grass, and shrub cutting (weeding) over a 3‐year period on regeneration of indigenous trees subsequent to the removal of plantation softwoods in Kibale National Park, Uganda. Counter to what would be expected if weeding released trees from competition, we found no difference in the total number of stems or in the stems greater than 10 cm diameter at breast height between control and weeded plots; there were more stems greater than 1 cm diameter at breast height in the control plots. For species found in both control and weeded plots, the maximum size of individuals did not differ. At the end of the study, 61 species were found in the control plots and 43 species were found in the weeded plots, and in both types of plots the three most abundant species were the same. The number of species and stems classified as early or middle successional species did not differ between weeded and control plots. The fact that weeding did not promote regeneration of indigenous trees after the removal of plantation trees illustrates the importance of evaluating and field‐testing potential management options. 相似文献
27.
R. Myles Riner Kathryn Maino Collins Garrett E. Foulke George V. Moorhead Phillip Terry 《The Western journal of medicine》1987,147(5):602-608
In a pilot project sponsored by the California State Emergency Medical Services Authority, validated, verifiable criteria were developed for the vertical categorization of hospital emergency services in 11 different groupings of medical and surgical emergencies. We describe the development of an assessment process and categorization criteria to identify the most appropriate receiving facility for interfacility transfer and, in selected instances, field triage of patients with different levels of severity of illness or injury. We propose that this facility assessment project be used in the critical care planning process for the eventual vertical categorization of hospital emergency services in California and as a template for similar projects in other states. 相似文献
28.
Hybrid formation within the genus Leishmania? 总被引:4,自引:0,他引:4
Leishmanial organisms isolated from a desert rodent (Psammomys obesus) and a feral dog (Canis familiaris) in the Eastern Province of Saudi Arabia were isoenzymically distinct from Leishmania major and L. arabica, organisms usually associated with human and wild animal cutaneous leishmaniasis in this area. Further examination of isoenzyme banding patterns of cloned populations of these organisms, together with karyotyping using orthogonal field alternation gel electrophoresis and the use of highly specific kinetoplast DNA probes, has produced evidence suggesting that these organisms isolated from the dog and the rodent are hybrids of L. major and L. arabica. 相似文献
29.
Penicillium citrinum cultures have been germinated on an H2O-based medium, resuspended on a D2O-based medium and treated with [l,2-13C2] acetate. The resulting citrinin (1) has been analysed by2H and13C nuclear magnetic resonance spectroscopy and information about the metabolism of hydrogen in citrinin biosynthesis has been deduced. 相似文献
30.