全文获取类型
收费全文 | 979篇 |
免费 | 50篇 |
国内免费 | 2篇 |
专业分类
林业 | 57篇 |
农学 | 26篇 |
基础科学 | 5篇 |
222篇 | |
综合类 | 107篇 |
农作物 | 29篇 |
水产渔业 | 50篇 |
畜牧兽医 | 400篇 |
园艺 | 21篇 |
植物保护 | 114篇 |
出版年
2023年 | 6篇 |
2022年 | 7篇 |
2021年 | 14篇 |
2020年 | 22篇 |
2019年 | 15篇 |
2018年 | 21篇 |
2017年 | 25篇 |
2016年 | 23篇 |
2015年 | 15篇 |
2014年 | 33篇 |
2013年 | 39篇 |
2012年 | 42篇 |
2011年 | 70篇 |
2010年 | 46篇 |
2009年 | 41篇 |
2008年 | 69篇 |
2007年 | 53篇 |
2006年 | 58篇 |
2005年 | 55篇 |
2004年 | 47篇 |
2003年 | 44篇 |
2002年 | 49篇 |
2001年 | 14篇 |
2000年 | 17篇 |
1999年 | 8篇 |
1998年 | 19篇 |
1997年 | 10篇 |
1996年 | 6篇 |
1995年 | 9篇 |
1994年 | 5篇 |
1993年 | 12篇 |
1991年 | 9篇 |
1990年 | 9篇 |
1989年 | 8篇 |
1988年 | 4篇 |
1987年 | 5篇 |
1986年 | 7篇 |
1985年 | 9篇 |
1984年 | 8篇 |
1979年 | 8篇 |
1978年 | 6篇 |
1977年 | 8篇 |
1976年 | 3篇 |
1975年 | 6篇 |
1974年 | 4篇 |
1973年 | 7篇 |
1971年 | 4篇 |
1970年 | 4篇 |
1969年 | 4篇 |
1968年 | 3篇 |
排序方式: 共有1031条查询结果,搜索用时 8 毫秒
71.
Taylor HA Brinton HC Bauer SJ Hartle RE Cloutier PA Michel FC Daniell RE Donahue TM Maehl RC 《Science (New York, N.Y.)》1979,203(4382):755-757
Bennett radio-frequency ion mass spectrometers have returned the first in situ measurements of the Venus dayside ion composition, including evidence of pronounced structural variability resulting from a dynamic interaction with the solar wind. The ionospheric envelope, dominated above 200 kilometers by O(+), responds dramatically to variations in the solar wind pressure, Which is observed to compress the thermal ion distributions from heights as great as 1800 kilometers inward to 280 kilometers. At the thermal ion boundary, or ionopause, the ambient ions are swept away by the solar wind, such that a zone of accelerated suprathermnal plasma is encountered. At higher altitudes, extending outward on some orbits for thousands of kilometers to the bows shock, energetic ion currents are detected, apparently originating from the shocked solar wind plasma. Within the ionosphere, observations of pass-to-pass differences in the ion scale heights are indicative of the effects of ion convection stimlulated by the solar wind interaction. 相似文献
72.
Although mechanically simple, centrifugal spreaders used for mineral fertilization involve complex physics that cannot be fully characterized at the present time. We are developing sensors to evaluate the spatial distribution of the fertilizer on the ground based on the measurement of initial flight conditions of fertilizer granules after their ejection by the spreading disk. The techniques developed are based on the analysis of images of the area around the disk showing the granule ejection. A high resolution – low cost imaging system for the analysis of high speed particle projection developed for this specific purpose is presented in this paper. The system, based on a camera and a sequence of flashes, is used to characterize the centrifugal spreading of fertilizer particles ejected at speeds of approximately 30ms–1. It automatically computes the direction of ejection and velocity of each granule observed in the image. Multi-exposure images collected with the camera installed perpendicular to the output flow of granules are analyzed to estimate the trajectories of the fertilizer granules, using different motion estimation methods. 相似文献
73.
Long-range correlations in the diffuse seismic coda 总被引:9,自引:0,他引:9
The late seismic coda may contain coherent information about the elastic response of Earth. We computed the correlations of the seismic codas of 101 distant earthquakes recorded at stations that were tens of kilometers apart. By stacking cross-correlation functions of codas, we found a low-frequency coherent part in the diffuse field. The extracted pulses have the polarization characteristics and group velocities expected for Rayleigh and Love waves. The set of cross-correlations has the symmetries of the surface-wave part of the Green tensor. This seismological example shows that diffuse waves produced by distant sources are sufficient to retrieve direct waves between two perfectly located points of observation. Because it relies on general properties of diffuse waves, this result has potential applications in other fields. 相似文献
74.
75.
76.
Orrit M 《Science (New York, N.Y.)》2002,298(5592):369-370
77.
A physical map of the 1-gigabase bread wheat chromosome 3B 总被引:2,自引:0,他引:2
Paux E Sourdille P Salse J Saintenac C Choulet F Leroy P Korol A Michalak M Kianian S Spielmeyer W Lagudah E Somers D Kilian A Alaux M Vautrin S Bergès H Eversole K Appels R Safar J Simkova H Dolezel J Bernard M Feuillet C 《Science (New York, N.Y.)》2008,322(5898):101-104
As the staple food for 35% of the world's population, wheat is one of the most important crop species. To date, sequence-based tools to accelerate wheat improvement are lacking. As part of the international effort to sequence the 17-billion-base-pair hexaploid bread wheat genome (2n = 6x = 42 chromosomes), we constructed a bacterial artificial chromosome (BAC)-based integrated physical map of the largest chromosome, 3B, that alone is 995 megabases. A chromosome-specific BAC library was used to assemble 82% of the chromosome into 1036 contigs that were anchored with 1443 molecular markers, providing a major resource for genetic and genomic studies. This physical map establishes a template for the remaining wheat chromosomes and demonstrates the feasibility of constructing physical maps in large, complex, polyploid genomes with a chromosome-based approach. 相似文献
78.
The origin and mechanisms of human interictal epileptic discharges remain unclear. Here, we describe a spontaneous, rhythmic activity initiated in the subiculum of slices from patients with temporal lobe epilepsy. Synchronous events were similar to interictal discharges of patient electroencephalograms. They were suppressed by antagonists of either glutamatergic or gamma-aminobutyric acid (GABA)-ergic signaling. The network of neurons discharging during population events comprises both subicular interneurons and a subgroup of pyramidal cells. In these pyramidal cells, GABAergic synaptic events reversed at depolarized potentials. Depolarizing GABAergic responses in neurons downstream to the sclerotic CA1 region contribute to human interictal activity. 相似文献
79.
Thomas Galewski Ben Collen Louise McRae Jonathan Loh Patrick Grillas Michel Gauthier-Clerc Vincent Devictor 《Biological conservation》2011,(5):1392-1399
Biodiversity loss is unevenly distributed in space and time. Species have reached critically low population sizes in some areas, and remain abundant in others. Similarly, some species may benefit from successful conservation plans, while others still experience severe population depletions driven by negative impacts of human activities. Although several indicators have been proposed to measure the fate of biodiversity, they are generally only implemented globally so their relevance for regional assessment is still unclear. Here, we calculated the first regional trend in the Living Planet Index for the Mediterranean wetlands (Med LPI), an indicator that summarizes the fate of global biological diversity based on the temporal trends in abundance of vertebrate populations. The Med LPI was based on 1641 vertebrate populations of 311 species recorded in Mediterranean wetlands from 1970–2008, in 27 different countries. We investigated whether trends in the Med LPI differed between eastern and western Mediterranean countries, which have different socio-economic contexts. Finally, we assessed whether and how the trend in the Med LPI was robust to changes in the number and identity of species considered. We found that, at the Mediterranean scale, the Med LPI increased steeply, which could be taken at first sight, as a general recovery of wetland biodiversity in this biogeographical region. However, we found highly contrasting spatial trends within the Mediterranean region: the average trend was positive for western and negative for eastern countries. Moreover, we showed that depending on the method used to estimate the trend in Med LPI, it can be sensitive to the number and identity of the species considered. We suggest that understanding the regional discrepancies of the trend in biodiversity indicators as well as their robustness to the species represented in the index will enhance progress assessment towards global and regional conservation strategies. 相似文献
80.
Fresh cherry tomatoes cv. 'Susanne' contain more of the two flavonoids chalconaringenin (CN) and rutin than lycopene. Therefore some properties including antioxidant behavior of the flavonoids were studied. The two flavonoids were extracted from peel and isolated by use of different chromatographic methods. Molecular absorbtivities were found to be 26907 for CN and 20328 abs M(-1) cm(-1) for rutin. Both compounds exhibited properties as antioxidants through several assays, and rutin was found to be the strongest antioxidant except in one assay. None of the assays revealed pro-oxidative effects. As naringenin rather than CN is frequently reported as a tomato constituent, the stability of CN was investigated in order to detect potential ways of isomerization during sample preparation. CN isomerized slowly both under UVB radiation and in alkaline solutions. Thus, such factors do not explain the occurrence of naringenin in tomato samples. The deficiency in reports on CN may be explained by the similarity in chromatographic behaviors of CN and naringenin, and due to the fact that they have same molecular weights. 相似文献