全文获取类型
收费全文 | 95854篇 |
免费 | 520篇 |
国内免费 | 885篇 |
专业分类
97259篇 |
出版年
2022年 | 26篇 |
2021年 | 41篇 |
2020年 | 20篇 |
2019年 | 32篇 |
2018年 | 11859篇 |
2017年 | 10702篇 |
2016年 | 7498篇 |
2015年 | 711篇 |
2014年 | 413篇 |
2013年 | 491篇 |
2012年 | 4399篇 |
2011年 | 12993篇 |
2010年 | 12114篇 |
2009年 | 8322篇 |
2008年 | 9921篇 |
2007年 | 11488篇 |
2006年 | 417篇 |
2005年 | 669篇 |
2004年 | 1129篇 |
2003年 | 1150篇 |
2002年 | 915篇 |
2001年 | 306篇 |
2000年 | 189篇 |
1999年 | 88篇 |
1998年 | 58篇 |
1997年 | 46篇 |
1996年 | 33篇 |
1995年 | 28篇 |
1994年 | 30篇 |
1993年 | 57篇 |
1992年 | 48篇 |
1991年 | 53篇 |
1990年 | 27篇 |
1989年 | 35篇 |
1988年 | 37篇 |
1987年 | 25篇 |
1986年 | 14篇 |
1984年 | 37篇 |
1983年 | 34篇 |
1982年 | 19篇 |
1981年 | 18篇 |
1980年 | 15篇 |
1978年 | 13篇 |
1976年 | 16篇 |
1972年 | 250篇 |
1971年 | 280篇 |
1965年 | 14篇 |
1962年 | 25篇 |
1956年 | 12篇 |
1944年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
The effect of a segregating economic trait locus (ETL) can be detected with the aid of a linked genetic marker, if specific alleles of each locus are in association among the individuals genotyped for the genetic marker. For dairy cattle this can be achieved by application of the ‘granddaughter design’. If only the sires and their sons are genotyped for the genetic markers, then the allele origin of sons having the same genotypes as their sires cannot be determined. Seven sires and 101 sons were genotyped for five microsatellites. The mean frequency of heterozygous sires was 77%. The mean number of alleles per locus was 8.2. Frequency of informative sons per locus ranged from 60% to 80% with a mean of 72%. With highly polymorphic microsatellites, at least 60% more grandsire families can be included in the analysis, and the number of sons assayed can be reduced by 40%, as compared to diallelic markers. 相似文献
62.
63.
It is widely believed that environmental variability is the main cause for fluctuations in commercially exploited small pelagic
fish populations around the world. Nevertheless, density-dependent factors also can drive population dynamics. In this paper,
we analyzed thirteen years of a relative abundance index of two clupeoids fish populations coexisting in the central-south
area off Chile, namely the common sardine, Strangomera bentincki, and anchovy, Engraulis ringens. We applied the classical diagnostic tools of time series analysis to the observed time-series. Also, the realized per capita
population growth rate was studied with the aim of detecting the feedback structure that is characterizing the population
dynamics of the two species. The analysis suggests that population fluctuations of the two species have an important density-dependent
component, displaying first-order (direct density-dependent) and second-order (delayed density-dependent) simultaneously.
The density-dependent component explained 70.5 and 55.6 % of the realized per capita population growth rate of common sardine
and anchovy, respectively. The deterministic skeleton model showed an asymptotic convergence to equilibrium density. In presence
of a stochastic environment, fluctuations were reproduced for the species showing a component of fluctuation with a period
of 4 year. The intrinsic dynamics of each species is typical of interacting species resulting from trophic interactions. It
is postulated that the second-order dynamics of S. bentincki and E. ringens in central-south Chile, may be the result from interactions with a specialist predator (the fishing fleet), interacting with
exogenous environmental factors. 相似文献
64.
Jacek Kęsy Beata Maciejewska Magdalena Sowa Magdalena Szumilak Krzysztof Kawałowski Maja Borzuchowska Jan Kopcewicz 《Plant Growth Regulation》2008,55(1):43-50
It has been shown that both IAA and ethylene application inhibit flower induction in the short-day plant Pharbitis nil. However application of IAA has elevated ethylene production in this plant, as well. Strong enhancement of ethylene production
is also correlated with the night-break effect, which completely inhibits flowering. In order to determine what the role of
IAA and ethylene is in the photoperiodic flower induction in Pharbitis nil, we measured changes in their levels during inductive and non-inductive photoperiods, and the effects of ethylene biosynthesis
and action inhibitors on inhibition of flowering by IAA. Our results have shown that the inhibitory effect of IAA on Pharbitis nil flowering is not physiological but is connected with its effect on ethylene biosynthesis. 相似文献
65.
66.
Herman van der Kooij Ron Jacobs Bart Koopman Henk Grootenboer 《Biological cybernetics》1999,80(5):299-308
A model is presented to study and quantify the contribution of all available sensory information to human standing based
on optimal estimation theory. In the model, delayed sensory information is integrated in such a way that a best estimate of
body orientation is obtained. The model approach agrees with the present theory of the goal of human balance control. The
model is not based on purely inverted pendulum body dynamics, but rather on a three-link segment model of a standing human
on a movable support base. In addition, the model is non-linear and explicitly addresses the problem of multisensory integration
and neural time delays. A predictive element is included in the controller to compensate for time delays, necessary to maintain
erect body orientation. Model results of sensory perturbations on total body sway closely resemble experimental results. Despite
internal and external perturbations, the controller is able to stabilise the model of an inherently unstable standing human
with neural time delays of 100 ms. It is concluded, that the model is capable of studying and quantifying multisensory integration
in human stance control. We aim to apply the model in (1) the design and development of prostheses and orthoses and (2) the
diagnosis of neurological balance disorders.
Received: 25 August 1997 / Accepted in revised form: 8 December 1998 相似文献
67.
68.
69.
70.
Transgenic HLA-DR alpha faithfully reconstitutes IE-controlled immune functions and induces cross-tolerance to E alpha in E alpha 0 mutant mice 总被引:5,自引:0,他引:5
We have constructed transgenic mice that express the human class II MHC molecule HLA-DR alpha on a genetic background in which the equivalent endogenous gene, H-2 IE alpha, is not expressed. In these mice, DR alpha complemented the E beta chain such that tissue-specific expression of an interspecies hybrid DR alpha-E beta heterodimer was obtained. Despite 25% amino acid differences between DR alpha and E alpha, immune responsiveness to IE-controlled antigens, clonal deletion of IE-reactive T cells, and alloantigenicity were quantitatively and qualitatively indistinguishable in IE-positive mice and in mice that had integrated at least four copies of the transgene. These results demonstrate a remarkable degree of structural, regulatory, and functional conservation. They also suggest that tolerance induction involves only discrete portions of MHC molecules. 相似文献