全文获取类型
收费全文 | 30993篇 |
免费 | 2625篇 |
国内免费 | 3621篇 |
出版年
2024年 | 71篇 |
2023年 | 654篇 |
2022年 | 731篇 |
2021年 | 980篇 |
2020年 | 1062篇 |
2019年 | 1407篇 |
2018年 | 1096篇 |
2017年 | 1155篇 |
2016年 | 1212篇 |
2015年 | 1290篇 |
2014年 | 1649篇 |
2013年 | 2215篇 |
2012年 | 1261篇 |
2011年 | 1504篇 |
2010年 | 1233篇 |
2009年 | 1609篇 |
2008年 | 1680篇 |
2007年 | 1664篇 |
2006年 | 1547篇 |
2005年 | 1430篇 |
2004年 | 1274篇 |
2003年 | 1133篇 |
2002年 | 1028篇 |
2001年 | 811篇 |
2000年 | 693篇 |
1999年 | 746篇 |
1998年 | 586篇 |
1997年 | 546篇 |
1996年 | 500篇 |
1995年 | 499篇 |
1994年 | 483篇 |
1993年 | 373篇 |
1992年 | 346篇 |
1991年 | 337篇 |
1990年 | 269篇 |
1989年 | 247篇 |
1988年 | 227篇 |
1987年 | 210篇 |
1986年 | 168篇 |
1985年 | 195篇 |
1984年 | 204篇 |
1983年 | 140篇 |
1982年 | 167篇 |
1981年 | 117篇 |
1980年 | 119篇 |
1979年 | 103篇 |
1978年 | 81篇 |
1977年 | 45篇 |
1976年 | 40篇 |
1974年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
112.
H. Schutkowski 《International Journal of Anthropology》1987,2(4):347-352
Facing the requirements of refined paleodemographical analyses, the access to the early ontogenetic sex ratio of skeletal
populations is an important feature. Using raw data provided byFazekas & Kosa (1978) for a sample of known sex, discriminant functions are derived from hip and thigh bone dimensions that allow an almost
unbiased classification of more than 70% of fetal and neonate individuals. 相似文献
113.
Estimation of uncertainties in X-ray refinement results by use of perturbed structures 总被引:1,自引:0,他引:1
The uncertainties in the refined parameters for a 1.5-A X-ray structure of carbon-monoxy (FeII) myoglobin are estimated by combining energy minimization with least-squares refinement against the X-ray data. The energy minimizations, done without reference to the X-ray data, provide perturbed structures which are used to restart conventional X-ray refinement. The resulting refined structures have the same, or better, R-factor and stereochemical parameters as the original X-ray structure, but deviate from it by 0.13 A rms for the backbone atoms and 0.31 A rms for the sidechain atoms. Atoms interacting with a disordered sidechain, Arg 45 CD3, are observed to have larger positional uncertainties. The uncertainty in the B-factors, within the isotropic harmonic motion approximation, is estimated to be 15%. The resulting X-ray structures are more consistent with the energy parameters used in simulations. 相似文献
114.
Phylogenetic systematics of the nymphaeales 总被引:2,自引:0,他引:2
Motomi Ito 《Journal of plant research》1987,100(1):17-35
A cladistic analysis was applied to reveal the phylogenetic relationships among the Nymphaeales. Seventeen out of twenty three
characters in gross morphology, anatomy and palynology were analyzed, for their evolutionary polarities. From the results
of the present analysis, the phylogenetic status of each genus and their relationships were clarified: 1)Nelumbo is a distinct taxon and is presumed to have originated from an ancestral stock of the Nymphaeales; 2)Ceratophyllum has a close phylogenetic relationship withCabomba; and 3) in the Nymphaeaceaesensu stricto, Nuphar and the remaining genral constitute a monophyletic group.
A conclusion obtained from the present analysis was that the following three families should be recognized in the Nymphaeales;
Nelumbonaceae Nymphaeaceae, and Ceratophyllaceae. The generaBrasenia andCabomba are traditionally classified in the Nymphaeaceae or in the independent family Cabombaceae. However, they should be included
in the family Ceratophyllaceae. 相似文献
115.
MT113, a nonphotosynthetic mutant of Rhodobacter capsulatus previously characterized as lacking cytochrome c2 is shown to lack also cytochrome c1, the Rieske iron-sulfur cluster and the antimycin sensitive semiquinone Qc, all components of the cytochrome bc1 complex. Although MT113 contained b-type cytochromes and other iron-sulfur clusters at nearly wild-type level, it lacks c-type cytochromes. Based on antibody detection, c2 apoprotein was absent in MT113, however the apoproteins corresponding to the cytochromes b and c1 and the Rieske iron-sulfur cluster were present in reduced amounts. Genetic analysis indicated that the lesion appears to be due to a single mutation which is not localized in the structural genes of cytochrome c2 or the bc1 complex. These data taken together suggest that the pleiotropic mutation in MT113 might be related to the biosynthesis of c-type cytochromes. 相似文献
116.
117.
Dennis L. Welker Arturo De Lozanne James A. Spudich 《Molecular & general genetics : MGG》1989,216(2-3):498-502
Summary A mutation (mhcA1 in strain HMM) created by insertional gene inactivation was used to map the Dictyostelium discoideum myosin heavy chain gene (mhcA) to linkage group IV. Three phenotypic traits associated with this mutation (slow colony growth, inability of the mutant to develop past aggregation, and the presence of five to ten integrated vector copies) cosegregated as expected for the consequences of a single insertional event. This linkage was confirmed using a restriction fragment length polymorphism. The mhcA1 mutation was recessive to wild type and was nonallelic with mutations at the following loci on linkage group IV: aggJ, aggL, couH, minA, phgB and tsgB. This work demonstrates the ability to apply standard techniques developed for D. discoideum parasexual genetic analyses to mutants generated by transformation, which is of particular relevance to analysis of genes for which no classical mutations or restriction fragment length polymorphisms are available. 相似文献
118.
Helmut Tr?ster Wolfgang Kissel Michael F. Trendelenburg Ansgar Hofmann 《Molecular & general genetics : MGG》1989,217(2-3):533-535
Summary Eight representative recombinant background clones of λEMBL3 were analysed usingKpnI,BamHI,SalI,EcoRI andHindIII digestion. We found that λEMBL3 carries its own left arm in theBamHI cloning site. In this way, recombinant molecules were found to be generated which can grow onEscherichia coli strain NM539. In all cases analysed, the left arm DNA was inserted in a head to tail orientation. Seven clones carried a
restoredBamHI site at thecos site-BamHI site connection. In the region where the inserted left arm and the right arm were ligated,BamHI cloning produces a large palindromic sequence consisting of two polylinkers. ThisBamHI site was incompletely cleaved in all cases analysed. We assume that a part of the λ DNA molecule in this region shows a
cruciform structure prohibiting recognition or cleavage of this site by restriction endonucleaseBamHI. 相似文献
119.
Summary Detailed growth analysis in conjunction with information on leaf display and nitrogen uptake was used to interpret competition between Abutilon theophrasti, a C3 annual, and Amaranthus retroflexus, a C4 annual, under ambient (350 l l-1) and two levels of elevated (500 and 700 l l-1) CO2. Plants were grown both individually and in competition with each other. Competition caused a reduction in growth in both species, but for different reasons. In Abutilon, decreases in leaf area ratio (LAR) were responsible, whereas decreased unit leaf rate (ULR) was involved in the case of Amaranthus. Mean canopy height was lower in Amaranthus than Abutilon which may explain the low ULR of Amaranthus in competition. The decrease in LAR of Abutilon was associated with an increase in root/shoot ratio implying that Abutilon was limited by competition for below ground resources. The root/shoot ratio of Amaranthus actually decreased with competition, and Amaranthus had a much higher rate of nitrogen uptake per unit of root than did Abutilon. These latter results suggest that Amaranthus was better able to compete for below ground resources than Abutilon. Although the growth of both species was reduced by competition, generally speaking, the growth of Amaranthus was reduced to a greater extent than that of Abutilon. Regression analysis suggests that the success of Abutilon in competition was due to its larger starting capital (seed size) which gave it an early advantage over Amaranthus. Elevated CO2 had a positive effect upon biomass in Amaranthus, and to a lesser extent, Abutilon. These effects were limited to the early part of the experiment in the case of the individually grown plants, however. Only Amaranthus exhibited a significant increase in relative growth rate (RGR). In spite of the transitory effect of CO2 upon size in individually grown plants, level of CO2 did effect final biomass of competitively grown plants. Abutilon grown in competition with Amaranthus had a greater final biomass than Amaranthus at ambient CO2 levels, but this difference disappeared to a large extent at elevated CO2. The high RGR of Amaranthus at elevated CO2 levels allowed it to overcome the difference in initial size between the two species.This study was supported by a grant from the US Department of Energy 相似文献
120.
Substrate specifity of the proton-driven hexose cotransport carrier in the plasmalemma of photoautotrophic suspension cells of Chenopodium rubrum L. has been studies through the short-term perturbation of 14C-labelled efflux of 3-O-methyl-d-glucose. Efflux, occurring exclusively via carrier-mediated exchange diffusion, is trans-stimulated by the substrate and trans-inhibited by the glucose-transport inhibitors phlorizin (K 1/2=7.9 mM) and its aglucon phloretin (K 1/2=84 μM); with both inhibitors, 3-O-methyl-d-glucose efflux may be blocked completely. Trans-stimulation of efflux (up to fourfold) by a variety of the d-enantiomers of neutral hexoses, including glucose (K 1/2=48 μM), 3-O-methyl-d-glucose (K 1/2=139 μM), and fructose (K 1/2=730 μM), but not by, for instance, d-allose, and l-sorbose, shows that carrier-substrate interaction critically involves the axial position at C-1 and C-3, respectively. We suggest that substrate binding by the Chenopodium hexose carrier involves both hydrophobic interaction with the pyran-ring and hydrogen-ion bonding at C-1 and C-3 of the d-glucose conformation. 相似文献