全文获取类型
收费全文 | 562篇 |
免费 | 46篇 |
专业分类
林业 | 15篇 |
农学 | 16篇 |
基础科学 | 4篇 |
64篇 | |
综合类 | 130篇 |
农作物 | 34篇 |
水产渔业 | 26篇 |
畜牧兽医 | 291篇 |
园艺 | 4篇 |
植物保护 | 24篇 |
出版年
2023年 | 5篇 |
2020年 | 8篇 |
2019年 | 9篇 |
2018年 | 9篇 |
2017年 | 20篇 |
2016年 | 15篇 |
2015年 | 8篇 |
2014年 | 13篇 |
2013年 | 49篇 |
2012年 | 11篇 |
2011年 | 21篇 |
2010年 | 13篇 |
2009年 | 19篇 |
2008年 | 24篇 |
2007年 | 19篇 |
2006年 | 19篇 |
2005年 | 9篇 |
2004年 | 19篇 |
2003年 | 15篇 |
2002年 | 13篇 |
2001年 | 11篇 |
2000年 | 9篇 |
1999年 | 12篇 |
1998年 | 8篇 |
1997年 | 13篇 |
1996年 | 8篇 |
1995年 | 5篇 |
1994年 | 5篇 |
1993年 | 5篇 |
1992年 | 13篇 |
1991年 | 9篇 |
1990年 | 6篇 |
1989年 | 12篇 |
1988年 | 7篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1985年 | 9篇 |
1984年 | 8篇 |
1982年 | 5篇 |
1980年 | 5篇 |
1978年 | 6篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1975年 | 8篇 |
1974年 | 5篇 |
1973年 | 9篇 |
1972年 | 5篇 |
1970年 | 4篇 |
1969年 | 5篇 |
1968年 | 4篇 |
排序方式: 共有608条查询结果,搜索用时 0 毫秒
51.
Although Saturn's rings are within the Roche zone, the accretion of centimeter-sized particles into large aggregates many meters in diameter occurs readily, on a time scale of weeks. These aggregates are disrupted when tidal stresses exceed their very low strengths; thus most of the mass of the ring system is continually processed through a population of large "dynamic ephemeral bodies," which are continually forming and disintegrating. These large aggregates are not at all like the idealized ice spheres often used in modeling Saturn's ring dynamics. Their coefficient of restitution is low, hence they form a monolayer in the ring plane. The optically observable characteristics of the rings are dominated by the swarm of centimeter-sized particles. 相似文献
52.
DR Link G Natale R Shao JE Maclennan NA Clark E Korblova DM Walba 《Science (New York, N.Y.)》1997,278(5345):1924-1927
A smectic liquid-crystal phase made from achiral molecules with bent cores was found to have fluid layers that exhibit two spontaneous symmetry-breaking instabilities: polar molecular orientational ordering about the layer normal and molecular tilt. These instabilities combine to form a chiral layer structure with a handedness that depends on the sign of the tilt. The bulk states are either antiferroelectric-racemic, with the layer polar direction and handedness alternating in sign from layer to layer, or antiferroelectric-chiral, which is of uniform layer handedness. Both states exhibit an electric field-induced transition from antiferroelectric to ferroelectric. 相似文献
53.
PM Lundquist R Wortmann C Geletneky RJ Twieg M Jurich VY Lee CR Moylan DM Burland 《Science (New York, N.Y.)》1996,274(5290):1182-1185
The performance of amorphous organic photorefractive (PR) materials in applications such as optical data storage is generally limited by the concentration of active molecules (chromophores) that can be incorporated into the host without forming a crystalline material with poor optical quality. In polymeric PR systems described previously, performance has been limited by the necessity of devoting a large fraction of the material to inert polymer and plasticizing components in order to ensure compositional stability. A new class of organic PR materials composed of multifunctional glass-forming organic chromophores is described that have long-term stability and greatly improved PR properties. 相似文献
54.
Chapman HN 《Science (New York, N.Y.)》2008,321(5887):352-353
55.
56.
57.
Egloff S O'Reilly D Chapman RD Taylor A Tanzhaus K Pitts L Eick D Murphy S 《Science (New York, N.Y.)》2007,318(5857):1777-1779
RNA polymerase II (Pol II) transcribes genes that encode proteins and noncoding small nuclear RNAs (snRNAs). The carboxyl-terminal repeat domain (CTD) of the largest subunit of mammalian RNA Pol II, comprising tandem repeats of the heptapeptide consensus Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7, is required for expression of both gene types. We show that mutation of serine-7 to alanine causes a specific defect in snRNA gene expression. We also present evidence that phosphorylation of serine-7 facilitates interaction with the snRNA gene-specific Integrator complex. These findings assign a biological function to this amino acid and highlight a gene type-specific requirement for a residue within the CTD heptapeptide, supporting the existence of a CTD code. 相似文献
58.
Plasma 17-OH corticosteroid levels were determined before stimulation and at various intervals after stimulation in these limbic sites in patients in whom implanted electrodes had been placed during evaluation for surgical treatment of psychomotor epilepsy. Stimulation of the amygdala was followed by an elevation of plasma 17-hydroxycorticosteroids. Hippocampal stimulation resulted in a decreased corticosteroid level, followed in two instances by a secondary elevation. 相似文献
59.
60.
The fusion pore of regulated exocytosis is a channel that connects and spans the vesicle and plasma membranes. The molecular composition of this important intermediate structure of exocytosis is unknown. Here, we found that mutations of some residues within the transmembrane segment of syntaxin (Syx), a plasma membrane protein essential for exocytosis, altered neurotransmitter flux through fusion pores and altered pore conductance. The residues that influenced fusion-pore flux lay along one face of an alpha-helical model. Thus, the fusion pore is formed at least in part by a circular arrangement of 5 to 8 Syx transmembrane segments in the plasma membrane. 相似文献