全文获取类型
收费全文 | 384588篇 |
免费 | 25349篇 |
国内免费 | 1189篇 |
出版年
2018年 | 16282篇 |
2017年 | 15158篇 |
2016年 | 12387篇 |
2015年 | 5385篇 |
2014年 | 6028篇 |
2013年 | 8479篇 |
2012年 | 13833篇 |
2011年 | 24195篇 |
2010年 | 20154篇 |
2009年 | 15638篇 |
2008年 | 19779篇 |
2007年 | 22083篇 |
2006年 | 8251篇 |
2005年 | 8710篇 |
2004年 | 8720篇 |
2003年 | 8744篇 |
2002年 | 8373篇 |
2001年 | 13175篇 |
2000年 | 13011篇 |
1999年 | 9895篇 |
1998年 | 3046篇 |
1997年 | 3124篇 |
1996年 | 2994篇 |
1995年 | 2768篇 |
1994年 | 2648篇 |
1993年 | 2612篇 |
1992年 | 7690篇 |
1991年 | 7501篇 |
1990年 | 7494篇 |
1989年 | 7260篇 |
1988年 | 6694篇 |
1987年 | 6350篇 |
1986年 | 5667篇 |
1985年 | 6005篇 |
1984年 | 4802篇 |
1983年 | 4171篇 |
1982年 | 2897篇 |
1981年 | 2711篇 |
1980年 | 2498篇 |
1979年 | 4363篇 |
1978年 | 3357篇 |
1977年 | 3124篇 |
1976年 | 3060篇 |
1975年 | 3519篇 |
1974年 | 3725篇 |
1973年 | 3736篇 |
1972年 | 3906篇 |
1971年 | 3647篇 |
1970年 | 2689篇 |
1969年 | 2447篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
291.
The Wageningen Delivery of Functionality symposium covered all aspects involved with food structural design to arrive at high-quality
foods which meet demanding customer expectations and regulatory requirements. The symposium integrated aspects from the structural
organization of foods at molecular and supramolecular scales to dedicated techniques required to describe and visualize such
structures, the gastro-intestinal events and how to model these in a laboratory setting, and finally the impact those food
structures and ingredients have on the consumer’s physiology and on the human perception. As an interdisciplinary platform,
bringing together more than 160 researchers from academia and industry, the symposium meanwhile fulfills an important role
in the food science community. 相似文献
292.
The Kelvin-Helmholtz instability is investigated analytically by using a one-dimensional nonuniform model of the Earth’s magnetosphere
and the adjacent solar wind region. Its properties are shown to be essentially governed by the presence of an MHD cavity that
arises in the magnetosphere because of the non-uniformity of the latter and also because of the jump in the parameters of
the medium at the magnetopause (the outer boundary of the magnetosphere). System oscillations constitute a discrete spectrum
of eigenmodes, which are determined by the wave vector k
t
along the tangential discontinuity and also by the mode number n = 0, 1, 2, …, playing the role of the wavenumber along a coordinate normal to the magnetopause. Analytic expressions are
obtained for the frequency and instability growth rate of each eigenmode and for the functions describing its spatial structure.
All these quantities depend parametrically on the solar wind velocity V
W
, or more precisely, on the Doppler frequency shift ω
W
= k
t
· V
W
. For each eigenmode, there is a lower instability threshold depending on the parameter ω
W
and a sharp maximum in the growth rate at the eigenfrequency of the magnetospheric cavity. For ω
W
values below the threshold, the properties of an eigenmode are highly sensitive to the type of solar wind nonuniformity.
Three cases are considered: a uniform solar wind and solar winds in which the speed of sound increases or decreases away from
the magnetopause. 相似文献
293.
Evergreen boreal plant species express high variability in their leaf traits. It remains controversial whether this within-species
variability is constrained to the same leaf trait relationships as has been observed across species. We sampled leaves of
three boreal evergreen woody species along a latitudinal gradient (from 57o56′N to 69o55′N). Leaf longevity (LL) of Pinus sylvestris L. and Vaccinium vitis-idaea L. correlated negatively with mean annual air temperature (MAT), whereas the LL of Ledum palustre L. was not affected by MAT. V. vitis-idaea and L. palustre had a negative relationship between leaf mass per area (LMA) and MAT. In P. sylvestris, the LMA–MAT relationship was positive. A negative correlation between LL and LMA was significant only for P. sylvestris. Leaf nitrogen concentration was positively related to leaf phosphorus concentration in all three species. Leaf potassium
concentration was related to nitrogen concentration only in L. palustre, and to phosphorus concentration in P. sylvestris and L. palustre. Our results demonstrate that although within the studied species the variation in some of the leaf traits may have the same
degree as interspecific variation, there is no such intercorrelation of leaf traits within the studied species as has been
observed across species. 相似文献
294.
295.
Biophysics - The effects of the separate and combined action of lead nitrate in a wide range of doses, uranyl nitrate, and chronic low-dose gamma irradiation on the formation of oxidative stress in... 相似文献
296.
N. Nikbakht J. C. McLachlan 《Proceedings. Biological sciences / The Royal Society》1999,266(1424):1101
The precise identification of the digits of the avian wing is of importance in evolutionary studies. If the digits are numbered two, three and four, this has been taken to suggest that birds are not descended directly from dinosaurs. If the digits are numbered one, two and three, dinosaur origins become more plausible. Studies of the development of the avian wing have failed to resolve this dilemma. However, in some instances, it is possible to deduce information about evolutionary morphologies by manipulating development experimentally. We grafted beads loaded with fibroblast growth factor 4 into the distal tip of chick wing buds at times when the apical ectodermal ridge is regressing. The consequence was that the cartilage structure conventionally labelled ''element 5'' increased dramatically in size and acquired a digit-like morphology in some instances. Corresponding changes in soft tissue morphology were also observed. We conclude that it may be possible to resolve the issue of avian digit homology by the induction of experimental atavisms of this kind. 相似文献
297.
298.
The occurrence of shallow-water (0.9 to 1.3 m) rhodoliths in back reef environments in southwest Puerto Rico is reported.
The rhodoliths were generally cylindrical, discoidal or irregular in shape with an average longest dimension of 7.2 cm. They
occurred at a maximum density of 524 m−2. The rhodoliths were composed of mostly coral nuclei with concentric laminations of aragonite-producing Cruoriella armorica (Peyssonneliaceae, Rhodophyta). Maximum Cruoriella accretion around coral nuclei was 30 mm although accretions of 1 to 20 mm were more common. Based on measurements of Cruoriella accretion, these shallow water rhodoliths are estimated to have minimum ages of 12 to 24 years. It is further estimated that
approximately 2% of the rhodoliths are turned over daily.
Accepted: 1 October 1999 相似文献
299.
The discrimination of the angular velocity of ventrodorsal and dorsoventral movement of an acoustic image was studied in nine test subjects. The experiments were performed using an apparent movement produced by consecutive activation of loudspeakers located along an arc in the vertical plane. The differential thresholds were measured by the minimum increment method. As the velocity of an acoustic image movement in opposite directions increased, the values of its mean absolute differential thresholds increased monotonically. Regression lines plotted by linear approximation of these values did not differ significantly. 相似文献
300.
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