全文获取类型
收费全文 | 46963篇 |
免费 | 17406篇 |
国内免费 | 24篇 |
专业分类
64393篇 |
出版年
2022年 | 250篇 |
2021年 | 719篇 |
2020年 | 2346篇 |
2019年 | 3916篇 |
2018年 | 4077篇 |
2017年 | 4313篇 |
2016年 | 4479篇 |
2015年 | 4746篇 |
2014年 | 4469篇 |
2013年 | 5084篇 |
2012年 | 3071篇 |
2011年 | 2845篇 |
2010年 | 3834篇 |
2009年 | 2506篇 |
2008年 | 1724篇 |
2007年 | 1394篇 |
2006年 | 1315篇 |
2005年 | 1355篇 |
2004年 | 1297篇 |
2003年 | 1281篇 |
2002年 | 1236篇 |
2001年 | 486篇 |
2000年 | 346篇 |
1999年 | 354篇 |
1998年 | 296篇 |
1997年 | 209篇 |
1996年 | 187篇 |
1995年 | 226篇 |
1994年 | 198篇 |
1993年 | 179篇 |
1992年 | 190篇 |
1991年 | 167篇 |
1990年 | 149篇 |
1989年 | 142篇 |
1988年 | 146篇 |
1987年 | 150篇 |
1986年 | 147篇 |
1985年 | 168篇 |
1984年 | 196篇 |
1983年 | 155篇 |
1982年 | 221篇 |
1981年 | 233篇 |
1980年 | 234篇 |
1979年 | 147篇 |
1978年 | 159篇 |
1977年 | 137篇 |
1976年 | 167篇 |
1975年 | 127篇 |
1974年 | 145篇 |
1973年 | 155篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
The RAINFOR database: monitoring forest biomass and dynamics 总被引:1,自引:0,他引:1
Problem: Data from over 100 permanent sample plots which have been studied for 10–20 years need a suitable system for storage which allows simple data manipulation and retrieval for analysis. Methods: A relational database linking tree records, taxonomic nomenclature and corresponding environmental data has been built in MS Access as part of the RAINFOR project. Conclusion: The database allows flexible and long‐term use of a large amount of data: more than 100 tree plots across Amazonia, incorporating over 80 000 records of individual trees and over 300 000 total records of tree diameter measurements from successive censuses. The database is designed to enable linkages to existing soil, floristic or plant‐trait databases. This database will be a useful tool for exploring the impact of environmental factors on forest structure and dynamics at local to continental scales, and long term changes in forest ecology. As an early example of its potential, we explore the impact of different methodological assumptions on estimates of tropical forest biomass and carbon storage. 相似文献
2.
3.
Thirteen polymorphic microsatellite loci were developed for blowflies for use in studies of genetic differentiation in wild populations of Lucilia illustris, to detect the possible occurrence of bottlenecks and to study changes in genetic variation in laboratory populations of Lucilia sericata following artificial bottlenecks. In this preliminary study it was revealed that heterozygosity was lower than expected in wild populations and genetic variation had been lost in the laboratory population despite being kept at a large size. 相似文献
4.
5.
Pierre Beuchet Laïla El kihel Michel Dherbomez Georges Charles Yves Letourneux 《Bioorganic & medicinal chemistry letters》1998,8(24):931
Δ7-5-Desaturase catalyses one of the last steps in ergosterol biosynthesis in fungi. Moreover Δ5-unsaturation is necessary for the sparking function. Synthesis of three pairs of C-6 epimeric cholestanol derivatives are described as potential growth inhibitors. Preliminary results suggest that 6β-aminocholestanol is a potent antifungal agent. 相似文献
6.
Francisco Gutirrez‐Corchero M. Victoria Arruga Lourdes Sanz Cristina García M. Angeles Hernndez Francisco Campos 《Molecular ecology resources》2002,2(1):75-77
FTA® cards were used for long‐term storage of avian blood samples. Blood DNA was extracted by a simple method and used in PCR for sex identification of adult and nestling Great Grey Shrikes Lanius excubitor. 相似文献
7.
Marianne E. Walsh Charles M. Collins Thomas F. Jenkins Alan D. Hewitt Jeff Stark Karen Myers 《Soil & Sediment Contamination》2003,12(5):631-645
Fort Greely, Alaska has an extensive complex of weapon training and testing areas located on lands withdrawn from the public domain under the Military Lands Withdrawal Act (PL106-65). The Army has pledged to implement a program to identify possible munitions contamination. Because of the large size (344,165,000 m2) of the high hazard impact areas, characterization of these constituents will be difficult. We used an authoritative sampling design to find locations most likely to contain explosives-residues on three impact areas. We focused our sampling on surface soils and collected multi-increment and discrete samples at locations of known firing events and from areas on the range that had craters, pieces of munitions, targets, or a designation as a firing point. In the two impact areas used primarily by the Army, RDX was the most frequently detected explosive. In the impact area that was also used by the Air Force, TNT was the most frequently detected explosive. Where detected, the explosives concentrations generally were low (<0.05 mg/kg) except in soils near low-order detonations, where the explosive-filler was in contact with the soil surface. These low-order detonations potentially can serve as localized sources for groundwater contamination if positioned in recharge areas. 相似文献
8.
Ruth C. Paul B. Rainey Brian J. Sheehan Orla M. Keane Charles J. Dorman 《Current biology : CB》1999,9(24)
The relationship between environment and mutation is complex [1]. Claims of Lamarkian mutation [2] have proved unfounded [3], [4] and [5]; it is apparent, however, that the external environment can influence the generation of heritable variation, through either direct effects on DNA sequence [6] or DNA maintenance and copying mechanisms [7], [8], [9] and [10], or as a consequence of evolutionary processes [11], [12], [13], [14], [15] and [16]. The spectrum of mutational events subject to environmental influence is unknown [6] and precisely how environmental signals modulate mutation is unclear. Evidence from bacteria suggests that a transient recombination-dependent hypermutational state can be induced by starvation [5]. It is also apparent that chnages in the mutability of specific loci can be influenced by alterations in DNA topology [10] and [17]. Here we describe a remarkable instance of adaptive evolution in Salmonella which is caused by a mutation that occurs in intermediate-strength osmotic environments. We show that the mutation is not ‘directed’ and describe its genetic basis. We also present compelling evidence in support of the hypothesis that the mutational event is constrained by signals transmitted from the external environment via changes in the activity of DNA gyrase. 相似文献
9.
10.