全文获取类型
收费全文 | 1759篇 |
免费 | 118篇 |
国内免费 | 40篇 |
专业分类
1917篇 |
出版年
2023年 | 20篇 |
2022年 | 21篇 |
2021年 | 40篇 |
2020年 | 30篇 |
2019年 | 53篇 |
2018年 | 42篇 |
2017年 | 53篇 |
2016年 | 50篇 |
2015年 | 54篇 |
2014年 | 105篇 |
2013年 | 93篇 |
2012年 | 62篇 |
2011年 | 94篇 |
2010年 | 69篇 |
2009年 | 103篇 |
2008年 | 119篇 |
2007年 | 122篇 |
2006年 | 84篇 |
2005年 | 57篇 |
2004年 | 62篇 |
2003年 | 49篇 |
2002年 | 30篇 |
2001年 | 30篇 |
2000年 | 38篇 |
1999年 | 29篇 |
1998年 | 29篇 |
1997年 | 38篇 |
1996年 | 22篇 |
1995年 | 20篇 |
1994年 | 26篇 |
1993年 | 23篇 |
1992年 | 13篇 |
1991年 | 19篇 |
1990年 | 14篇 |
1989年 | 16篇 |
1988年 | 18篇 |
1987年 | 19篇 |
1986年 | 15篇 |
1985年 | 13篇 |
1984年 | 12篇 |
1983年 | 8篇 |
1982年 | 19篇 |
1981年 | 11篇 |
1980年 | 16篇 |
1979年 | 8篇 |
1978年 | 6篇 |
1977年 | 13篇 |
1974年 | 4篇 |
1973年 | 5篇 |
1972年 | 5篇 |
排序方式: 共有1917条查询结果,搜索用时 0 毫秒
11.
12.
13.
Quantitation of the Myelin-Associated Glycoprotein in Human Nervous Tissue from Controls and Multiple Sclerosis Patients 总被引:3,自引:2,他引:1
David Johnson Shuzo Sato Richard H. Quarles Takashi Inuzuka Roscoe O. Brady Wallace W. Tourtellotte 《Journal of neurochemistry》1986,46(4):1086-1093
Myelin-associated glycoprotein (MAG) was measured by radioimmunoassay in the human CNS and peripheral nervous system (PNS). The level of MAG, expressed as ng/microgram of total protein, was approximately 20-fold higher in whole homogenates of cerebral white matter (4.7 +/- 0.60) than of peripheral nerve (0.12-0.28). MAG concentrations were only slightly higher in the isolated myelin fractions from these tissues: CNS myelin, 5.6 ng/microgram; PNS myelin, 0.37 ng/microgram. The levels of MAG were measured in nine plaques, periplaque regions, and areas of macroscopically normal-appearing white matter (NAWM) from six separate multiple sclerosis brains and compared with the levels of other myelin proteins in the same samples. MAG and other myelin proteins were reduced to very low levels in plaques. The levels of MAG and basic protein (BP) and the activity of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) in periplaque areas were significantly lower than those in control white matter, and MAG and BP levels were also significantly reduced in NAWM. In a periplaque region and NAWM from the most rapidly progressing case of multiple sclerosis examined, the MAG content was between 30 and 35% of the control level, whereas BP and PLP levels and CNP activity were between 50 and 85% of control values. The reduction of MAG content in periplaque regions from all nine multiple sclerosis plaques examined was significantly greater than the reductions of BP level and CNP activity. In NAWM samples, the mean reduction of MAG content was also greater than the reductions of BP level and CNP activity, but the difference was only statistically significant in comparison to CNP.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
14.
Hideo Ikeda 《Molecular & general genetics : MGG》1986,202(3):518-520
Summary Illegitimate recombination dependent on T4 DNA topoisomerase in a cell-free system has recently been described. In that work, recombinants between two phage DNA molecules were produced by the topoisomerase alone, without an Escherichia coli extract. In this paper, it is shown that recombination between phage and circular plasmid DNA molecules can also be detected in the presence or absence of an E. coli extract but at frequencies two or three orders of magnitude lower than that observed in the phage-phage cross. The frequency is probably lower because multiple recombination is required in the case of the phage-plasmid cross. 相似文献
15.
L. P. Lefkovitch 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1985,70(6):585-589
Summary Significant genotype-environment interactions in an ANOVA can be found for a number of reasons: one is the differences in
the among-environments variances for each genotype, another is the differences in the ordering of the environments by each
genotype. Using conditional clustering, groups may be formed in which the means, variances and patterns are used simultaneously
but separately to decide on group homogeneity.
Contribution No. I-685 from the Engineering and Statistical Research Institute 相似文献
16.
Synopsis Aspects of the life history of Barbus anoplus were studied in Lake le Roux, a turbid man-made lake on the Orange River, South Africa. This minnow underwent a population
explosion and successfully colonized the shoreline of the newly-formed lake during the early phases of reservoir filling.
Male and female B. anoplus reach sexual maturity in one year at about 40 mm fork length. They have a multiple spawning habit with the first spawning
in November–January and the second in February–March. The growth of the two resulting cohorts is discussed. It is proposed
that the offspring from the second spawning not only acts as a ‘back-up’ but is capable of prolonging the life of that year-class
into an additional reproductive season. Most of the minnows die after their second summer, but more offspring from the second
spawning, especially females, live into a third summer. Females attain a larger maximum size (73 mm FL) and age (3–4 years)
than males (60 mm FL, 2–3 years). B. anoplus is small and short-lived with a high seasonal reproductive potential, which is in contrast to the larger Barbus species in the Orange River system. These life-history traits enable the species to colonize and successfully inhabit unstable
environments and probably account for its widespread distribution. 相似文献
17.
Multiple discriminant analysis is a useful multivariate technique in vegetation studies that can be employed for several purposes, even if the underlying statistical assumptions are not satisfied. An example of application of this method is discussed: multiple discriminant analysis was successfully used for evaluating the predictivity of forest types defined by numerical classification of vegetation data with respect to soil variables.Species nomenclature follows Pignatti (1982) for vascular plants and Augier (1966) for mosses.The research has been supported by the IDROSER (Bologna, Italy) and by the Italian C. N. R. (Grant no. 83.02548.04, resp. Prof. A. Pirola). We wish to thank Prof. E. Feoli, Prof. E. van der Maarel, Prof. L. Orlóci and Prof. A. Pirola for suggestions. We are also indebted to Dr N. Filippi who analyzed the soil samples. 相似文献
18.
The vegetation succession on the dunes near Oostvoorne, The Netherlands has been followed by means of a novel combination of repeated large-scale vegetation mapping and air photograph interpretation. Vegetation units have been discerned on the formation level because these could be distinguished fairly easily on the photographs and because the rates of change are appropriate to the time interval chosen. Nineteen formations were distinguished. Five 1:6250 maps were constructed, reflecting the formation pattern in 1934, 1943, 1959, 1972 and 1980. An overlay with 2736 grid points at 25 m field distance was used to quantify changes in the formation pattern.The results suggest a pronounced multiple pathway succession with nevertheless three principal trajectories of succession from pioneer to woodland vegetation. The outer dunes, which have originated since 1910, are distinct in successional characteristics from the inner dunes, which already existed but were released from heavy grazing pressure in 1910. The rate of change in the outer dunes was high in the beginning and is slowly decreasing eversince. In the inner dunes it went the other way around. Through visual extrapolation the likely formation patterns in 1910 and in 2000 were estimated.Transition frequencies proved highly variable for most formations. Moreover, strong spatial dependence was found. Limitations in the use of Markov models in cases of long-term succession in heterogeneous environments are discussed.Nomenclature follows the same sources as in van der Maarel et al. (1984).Field work carried out 1980–1981 when the authors were at the Division of Geobotany, University of Nijmegen. We thank Jos Rijntjes, Nijmegen, for his cooperation in the field. Vegetation maps were prepared and calculations performed at Uppsala. We thank the Foundation Het Zuid-Hollands Landschap, Rotterdam, for providing facilities and a grant for fieldwork as well as additional means to reproduce the vegetation maps. Two reviewers gave useful comments. 相似文献
19.
A multivariate t probability integral 总被引:2,自引:0,他引:2
20.
János Podani 《Plant Ecology》1989,83(1-2):111-128
The methodology of comparing the results of multivariate community studies (resemblance matrices, ordinations, hierarchical and nonhierarchical classifications) is reviewed from two viewpoints: basic strategy and measure employed. The basic strategy is determined by 7 choices concerning the type of results, consensus methods or resemblance measures, hypothesis testing or exploratory analysis, lack or presence of reference basis, data set congruence or algorithmic effects, number of factors responsible for differences among results, and the number of properties considered in the comparison. Included is a brief summary of methods applicable to vegetation studies. Examples from a grassland survey demonstrate the utility of comparisons in evaluating the effects of plot size, data type, standardization, taxonomic level and number of species on classifications and ordinations.Abbreviations OUC =
Operational Unit of Comparison
- PCA =
Principal Components Analysis
- PCoA =
Principal Coordinates Analysis
- SSA =
Incremental Sum of Squares Agglomeration 相似文献