全文获取类型
收费全文 | 9716篇 |
免费 | 926篇 |
国内免费 | 1篇 |
出版年
2021年 | 136篇 |
2019年 | 76篇 |
2018年 | 100篇 |
2016年 | 173篇 |
2015年 | 240篇 |
2014年 | 292篇 |
2013年 | 421篇 |
2012年 | 508篇 |
2011年 | 510篇 |
2010年 | 298篇 |
2009年 | 258篇 |
2008年 | 380篇 |
2007年 | 425篇 |
2006年 | 401篇 |
2005年 | 320篇 |
2004年 | 344篇 |
2003年 | 378篇 |
2002年 | 325篇 |
2001年 | 270篇 |
2000年 | 226篇 |
1999年 | 198篇 |
1998年 | 119篇 |
1997年 | 116篇 |
1996年 | 110篇 |
1995年 | 139篇 |
1994年 | 102篇 |
1993年 | 111篇 |
1992年 | 150篇 |
1991年 | 137篇 |
1990年 | 143篇 |
1989年 | 128篇 |
1988年 | 132篇 |
1987年 | 125篇 |
1986年 | 87篇 |
1985年 | 122篇 |
1984年 | 107篇 |
1983年 | 103篇 |
1982年 | 79篇 |
1981年 | 88篇 |
1980年 | 83篇 |
1979年 | 107篇 |
1978年 | 123篇 |
1976年 | 76篇 |
1975年 | 90篇 |
1974年 | 85篇 |
1973年 | 90篇 |
1972年 | 78篇 |
1971年 | 73篇 |
1969年 | 69篇 |
1966年 | 64篇 |
排序方式: 共有10000条查询结果,搜索用时 62 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.
Regina M. Santella Dezider Grunberger Alfred Nordheim Alexander Rich 《Biochemical and biophysical research communications》1982,106(4):1226-1232
Poly(dG-m5dC)·poly(dG-m5dC) was modified by treatment with N-acetoxy-N-2-acetylaminofluorene (N-Aco-AAF) and its conformation examined by circular dichroism (CD) and susceptibility to S1 nuclease digestion. A sample with a modification level of 10% shows a CD spectrum characteristic of the Z form and is resistant to digestion by S1 nuclease. The relative reactivity of several polymers with N-Aco-AAF was shown to follow the order of ease of formation of Z DNA: poly(dG-m5dC)·poly(dG-m5dC) > poly(dG-dC)·poly(dG-dC) > poly(dG)·poly(dC). This suggests that AAF reacts more readily with Z DNA than B DNA. 相似文献
4.
Slash pine needles and cortex oleoresin have been found to contain a new major diterpene constituent, imbricataloic acid. The closely related imbricatoloic acid, previously reported only in Araucaria imbricata, was found to be present in small amounts in slash pine needle extract. Spectral data are given for an unidentified diterpene alcohol isolated from the cortex oleoresin. 相似文献
5.
Contrary to results published recently, we observe three, rather than two, phenotypes for the enzyme glucosephosphate isomerase (EC 5.3.1.9) from sheep. The phenotypic electrophoretic patterns conform to the patterns observed for this dimeric enzyme in other species. Genotype frequencies in a flock of Southdowns do not deviate significantly from those predicted under the assumption of the Hardy-Weinberg equilibrium. A remarkable observation is that the electrophoretically distinct phenotypes of GPI are largely or entirely obliterated by the addition of 1-10 mmol/l MgCl2 to the electrophoretic buffers. Modification of the usual staining recipe for GPI result in greater resolution and shorter staining times. 相似文献
6.
7.
The kinetics of the electrostatically induced phase transition of dimyristoyl phosphatidic acid bilayers was followed using the stopped-flow technique. The phase transition was triggered by a fast change in the pH or the magnesium ion concentration and followed by recording the time dependence of the absorbance. When the phase transition was induced by a pH jump the time course of the absorbance could be described by two exponentials, their time constants displaying the for cooperative processes characteristic maximum at the transition midpoint. The time constants are in the 10 and 100 ms range for the H+ triggered transition from the fluid to the ordered state. A third slower process shows no appreciable temperature dependence and is probably caused by vesicle aggregation. For the OH--induced transition fron the ordered to the fluid state the time constants are in the 100 and 1000 ms range. The fluid-ordered transition could also be triggered by addition of magnesium ions. Of the several observed processes only the fastest in the 10–100 ms time range could definitely be assigned to the fluid-ordered transition while the others are due to aggregation phenomena. The experimental data were compared with results obtained from pressure jump experiments and could be interpreted on the basis of theories for non-equilibrium relaxation. 相似文献
8.
9.
M A Taylor K A Pratt D F Revell K C Baker I G Sumner P W Goodenough 《Protein engineering》1992,5(5):455-459
For the first time the pro-form of a recombinant cysteine proteinase has been expressed at a high level in Escherichia coli. This inactive precursor can subsequently be processed to yield active enzyme. Sufficient protein can be produced using this system for X-ray crystallographic structure studies of engineered proteinases. A cDNA clone encoding propapain, a precursor of the papaya proteinase, papain, was expressed in E. coli using a T7 polymerase expression system. Insoluble recombinant protein was solubilized in 6 M guanidine hydrochloride and 10 mM dithiothreitol, at pH 8.6. A protein-glutathione mixed disulphide was formed by dilution into oxidized glutathione and 6 M GuHCl, also at pH 8.6. Final refolding and disulphide bond formation was induced by dilution into 3 mM cysteine at pH 8.6. Renatured propapain was processed to active papain at pH 4.0 in the presence of excess cysteine. Final processing could be inhibited by the specific cysteine proteinase inhibitors E64 and leupeptin, but not by pepstatin, PMSF or EDTA. This indicates that final processing was due to a cysteine proteinase and suggests that an autocatalytic event is required for papain maturation. 相似文献
10.
Hydrogen bonding in globular proteins 总被引:41,自引:0,他引:41