全文获取类型
收费全文 | 804篇 |
免费 | 41篇 |
专业分类
林业 | 138篇 |
农学 | 64篇 |
基础科学 | 4篇 |
78篇 | |
综合类 | 106篇 |
农作物 | 16篇 |
水产渔业 | 32篇 |
畜牧兽医 | 368篇 |
园艺 | 14篇 |
植物保护 | 25篇 |
出版年
2021年 | 9篇 |
2020年 | 6篇 |
2019年 | 10篇 |
2018年 | 14篇 |
2017年 | 19篇 |
2016年 | 12篇 |
2015年 | 17篇 |
2014年 | 17篇 |
2013年 | 26篇 |
2012年 | 35篇 |
2011年 | 30篇 |
2010年 | 12篇 |
2009年 | 22篇 |
2008年 | 34篇 |
2007年 | 27篇 |
2006年 | 25篇 |
2005年 | 24篇 |
2004年 | 18篇 |
2003年 | 30篇 |
2002年 | 22篇 |
2001年 | 27篇 |
2000年 | 27篇 |
1999年 | 11篇 |
1998年 | 8篇 |
1997年 | 8篇 |
1996年 | 8篇 |
1994年 | 9篇 |
1992年 | 12篇 |
1991年 | 14篇 |
1990年 | 10篇 |
1989年 | 20篇 |
1988年 | 18篇 |
1987年 | 11篇 |
1985年 | 10篇 |
1984年 | 9篇 |
1983年 | 11篇 |
1980年 | 5篇 |
1979年 | 8篇 |
1978年 | 10篇 |
1977年 | 14篇 |
1975年 | 8篇 |
1974年 | 10篇 |
1973年 | 6篇 |
1972年 | 9篇 |
1971年 | 8篇 |
1969年 | 8篇 |
1967年 | 8篇 |
1966年 | 14篇 |
1924年 | 5篇 |
1921年 | 9篇 |
排序方式: 共有845条查询结果,搜索用时 46 毫秒
81.
A persistent oxygen anomaly reveals the fate of spilled methane in the deep Gulf of Mexico 总被引:3,自引:0,他引:3
Kessler JD Valentine DL Redmond MC Du M Chan EW Mendes SD Quiroz EW Villanueva CJ Shusta SS Werra LM Yvon-Lewis SA Weber TC 《Science (New York, N.Y.)》2011,331(6015):312-315
Methane was the most abundant hydrocarbon released during the 2010 Deepwater Horizon oil spill in the Gulf of Mexico. Beyond relevancy to this anthropogenic event, this methane release simulates a rapid and relatively short-term natural release from hydrates into deep water. Based on methane and oxygen distributions measured at 207 stations throughout the affected region, we find that within ~120 days from the onset of release ~3.0 × 10(10) to 3.9 × 10(10) moles of oxygen were respired, primarily by methanotrophs, and left behind a residual microbial community containing methanotrophic bacteria. We suggest that a vigorous deepwater bacterial bloom respired nearly all the released methane within this time, and that by analogy, large-scale releases of methane from hydrate in the deep ocean are likely to be met by a similarly rapid methanotrophic response. 相似文献
82.
Weber MF Stover CA Gilbert LR Nevitt TJ Ouderkirk AJ 《Science (New York, N.Y.)》2000,287(5462):2451-2456
Multilayer mirrors that maintain or increase their reflectivity with increasing incidence angle can be constructed using polymers that exhibit large birefringence in their indices of refraction. The most important feature of these multilayer interference stacks is the index difference in the thickness direction (z axis) relative to the in-plane directions of the film. This z-axis refractive index difference provides a variable that determines the existence and value of the Brewster's angle at layer interfaces, and it controls both the interfacial Fresnel reflection coefficient and the phase relations that determine the optics of multilayer stacks. These films can yield optical results that are difficult or impossible to achieve with conventional multilayer optical designs. The materials and processes necessary to fabricate such films are amenable to large-scale manufacturing. 相似文献
83.
Alkanes in fungal spores 总被引:1,自引:0,他引:1
The chlamydospores of Ustilago maydis, U. nuda, and Sphacelotheca reiliana were analyzed by gas chromatography and mass spectrometry for their hydrocarbon contents. For the first time we observed that they contain paraffinic hydrocarbons; the average contents were 42, 58, and 146 parts per million, respectively. n-Alkanes having odd numbers of carbon atoms predom-inate, with carbon-chain lengths ranging from C(14) to C(37). The major alkanes are n-C(27) in U. maydis, n-C(27) and n-C(35) in U. nuda, and n-C(29) in S. reiliana. Each type of spore carried a distinctly characteristic population of hydrocarbons. 相似文献
84.
Moreau JW Weber PK Martin MC Gilbert B Hutcheon ID Banfield JF 《Science (New York, N.Y.)》2007,316(5831):1600-1603
High-spatial-resolution secondary ion microprobe spectrometry, synchrotron radiation-based Fourier-transform infrared spectroscopy, and polyacrylamide gel analysis demonstrated the intimate association of proteins with spheroidal aggregates of biogenic zinc sulfide nanocrystals, an example of extracellular biomineralization. Experiments involving synthetic zinc sulfide nanoparticles and representative amino acids indicated a driving role for cysteine in rapid nanoparticle aggregation. These findings suggest that microbially derived extracellular proteins can limit the dispersal of nanoparticulate metal-bearing phases, such as the mineral products of bioremediation, that may otherwise be transported away from their source by subsurface fluid flow. 相似文献
85.
A gravity cell can be used to determine the weights of particles greater than 10(8) molecular weight units. The solution is maintained at a temperature which is constant to +/- 0.001 degrees C until equilibrium between sedimentation produced by gravity is balanced by back diffusion. The weight-average molecular (particle) weight of tipula iridescent virus was (1.05 +/- 0.02) x 10(9). 相似文献
86.
Production of an epidermal growth factor receptor-related protein 总被引:24,自引:0,他引:24
Human epidermoid carcinoma A431 cells in culture produce a soluble 105-kilodalton protein which, by the criteria of epidermal growth factor (EGF) binding, recognition by monoclonal and polyclonal antibodies to the EGF receptor, amino-terminal sequence analysis and carbohydrate content, is related to the cell surface domain of the EGF receptor. The high rate of production and the finding that with biosynthetic labeling the specific activity of this 105-kilodalton protein exceeds that of the intact receptor indicate that it is not derived from membrane-bound mature receptor but is separately produced by the cell. These cells thus separately synthesize an EGF receptor that is inserted into the membrane and an EGF receptor-related protein that is secreted. 相似文献
87.
Weber NA 《Science (New York, N.Y.)》1966,153(3736):587-604
Fungus-growing ants (Attini) are in reality unique fungus-culturing insects.There are several hundred species in some dozen genera, of which Acromyrmex and Atta are the conspicuous leaf-cutters. The center of their activities is the fungus garden, which is also the site of the queen and brood. The garden, in most species, is made from fresh green leaves or other vegetal material. The ants forage for this, forming distinct trails to the vegetation that is being harvested. The cut leaves or other substrate are brought into the nest and prepared for the fungus. Fresh leaves and flowers are cut into pieces a millimeter or two in diameter; the ants form them into a pulpy mass by pinching them with the mandibles and adding saliva. Anal droplets are deposited on the pieces, which are then forced into place in the garden. Planting of the fungus is accomplished by an ant's picking up tufts of the adjacent mycelium and dotting the surface of the new substrate with it. The combination of salivary and anal secretions, together with the constant care given by the ants, facilitates the growth of the ant fungus only, despite constant possibilities for contamination. When the ants are removed, alien fungi and other organisms flourish. A mature nest of Atta Sexdens may consist of 2000 chambers, some temporarily empty, some with refuse, and the remainder with fungus gardens. Thousands of kilograms of fresh leaves will have been used. A young laboratory colony of Atta cephalotes will use 1 kilogram of fresh leaves for one garden. The attines are the chief agents for introducing organic matter into the soil in tropical rain forests; this matter becomes the nucleus for a host of other organisms, including nematodes and arthropods, after it is discarded by the ants. One ant species cultures a yeast; all others grow a mycelium. In the higher species the mycelium forms clusters of inflated hyphae. Mycologists accept as valid two names for confirmed fruiting stages: Leucocoprinus ( or Leucoagaricus) gongylophora (Moeller 1893) and Lepiota n. sp. 相似文献
88.
89.
Horneck G Bücker H Reitz G Requardt H Dose K Martens KD Mennigmann HD Weber P 《Science (New York, N.Y.)》1984,225(4658):226-228
Preliminary results of the Spacelab 1 experiment on the response of Bacillus subtilis spores to conditions of free space are presented. Exposure to the vacuum of space on the Spacelab pallet reduced viability counts about 50 percent and increased mutation frequencies by a factor of about 10. Interpretation of apparent differences in the photobiological and photochemical data between flight and ground simulation experiments will require more statistical analyses and data from actual fluence measurements. 相似文献
90.
Aulanier G Golub L Deluca EE Cirtain JW Kano R Lundquist LL Narukage N Sakao T Weber MA 《Science (New York, N.Y.)》2007,318(5856):1588-1591
Magnetic reconnection of solar coronal loops is the main process that causes solar flares and possibly coronal heating. In the standard model, magnetic field lines break and reconnect instantaneously at places where the field mapping is discontinuous. However, another mode may operate where the magnetic field mapping is continuous but shows steep gradients: The field lines may slip across each other. Soft x-ray observations of fast bidirectional motions of coronal loops, observed by the Hinode spacecraft, support the existence of this slipping magnetic reconnection regime in the Sun's corona. This basic process should be considered when interpreting reconnection, both on the Sun and in laboratory-based plasma experiments. 相似文献