全文获取类型
收费全文 | 862篇 |
免费 | 46篇 |
专业分类
林业 | 14篇 |
农学 | 11篇 |
基础科学 | 6篇 |
63篇 | |
综合类 | 174篇 |
农作物 | 19篇 |
水产渔业 | 45篇 |
畜牧兽医 | 507篇 |
园艺 | 12篇 |
植物保护 | 57篇 |
出版年
2020年 | 12篇 |
2019年 | 14篇 |
2018年 | 9篇 |
2017年 | 17篇 |
2016年 | 14篇 |
2015年 | 13篇 |
2014年 | 16篇 |
2013年 | 41篇 |
2012年 | 32篇 |
2011年 | 42篇 |
2010年 | 20篇 |
2009年 | 17篇 |
2008年 | 31篇 |
2007年 | 31篇 |
2006年 | 17篇 |
2005年 | 29篇 |
2004年 | 33篇 |
2003年 | 24篇 |
2002年 | 35篇 |
2001年 | 29篇 |
2000年 | 16篇 |
1999年 | 21篇 |
1998年 | 8篇 |
1997年 | 10篇 |
1996年 | 6篇 |
1995年 | 6篇 |
1994年 | 7篇 |
1993年 | 6篇 |
1992年 | 15篇 |
1991年 | 15篇 |
1990年 | 10篇 |
1989年 | 16篇 |
1988年 | 18篇 |
1987年 | 11篇 |
1986年 | 18篇 |
1985年 | 12篇 |
1984年 | 11篇 |
1983年 | 7篇 |
1982年 | 6篇 |
1981年 | 7篇 |
1979年 | 12篇 |
1978年 | 7篇 |
1976年 | 11篇 |
1975年 | 9篇 |
1974年 | 14篇 |
1973年 | 12篇 |
1972年 | 6篇 |
1971年 | 6篇 |
1970年 | 10篇 |
1904年 | 6篇 |
排序方式: 共有908条查询结果,搜索用时 15 毫秒
91.
A spectrum of comet Austin (1988 c(1)) has been obtained from 910 to 1180 A. Three bright emission lines were detected, including a forbidden oxygen line (1128 A), which are attributable to radiative pumping of neutral oxygen by solar Lyman beta. The relative strengths of the observed features should prove to be a useful diagnostic of the physical conditions and radiation fields in cometary comae. In addition, the absence of strong spectral features from highly volatile species such as He, Ar, or N(2) can be used to place constraints on the thermal environment under which the comet was formed and has been processed. 相似文献
92.
Three-dimensional structure of recombinant human interferon-gamma 总被引:18,自引:0,他引:18
S E Ealick W J Cook S Vijay-Kumar M Carson T L Nagabhushan P P Trotta C E Bugg 《Science (New York, N.Y.)》1991,252(5006):698-702
The x-ray crystal structure of recombinant human interferon-gamma has been determined with the use of multiple-isomorphous-replacement techniques. Interferon-gamma, which is dimeric in solution, crystallizes with two dimers related by a noncrystallographic twofold axis in the asymmetric unit. The protein is primarily alpha helical, with six helices in each subunit that comprise approximately 62 percent of the structure; there is no beta sheet. The dimeric structure of human interferon-gamma is stabilized by the intertwining of helices across the subunit interface with multiple intersubunit interactions. 相似文献
93.
94.
Cook E 《Science (New York, N.Y.)》1979,206(4423):1141-1147
Helium appears indispensable for certain energy-related uses that may be important 50 years from now, when helium-bearing natural gas, a much cheaper source than air, may be exhausted. Present demand, however, is lower than productive capacity, and much helium is being dissipated into the atmosphere as natural gas is burned for fuel. Controversy over the need for a government-directed helium-conservation program reflects fundamental differences in viewpoints on the economic future of industrial society, on the limits of substitution of labor and capital for a depleting resource, and on intergenerational equity and risk-bearing. 相似文献
95.
Adipsin: a circulating serine protease homolog secreted by adipose tissue and sciatic nerve 总被引:8,自引:0,他引:8
K S Cook H Y Min D Johnson R J Chaplinsky J S Flier C R Hunt B M Spiegelman 《Science (New York, N.Y.)》1987,237(4813):402-405
Adipsin is a serine protease homolog whose primary structure was predicted from the nucleotide sequence of a differentiation-dependent adipocyte messenger RNA. Immunoblots probed with antisera to synthetic peptides identify two forms of adipsin that are synthesized and secreted by 3T3 adipocytes. These proteins of 44 and 37 kilodaltons are converted to 25.5 kilodaltons by enzymatic deglycosylation. Although adipsin is principally synthesized in adipose tissue, it is also produced by sciatic nerve and is found in the bloodstream. Because of the apparent restriction of adipsin synthesis to tissues highly active in lipid metabolism, its presence in serum, and its modulation in altered metabolic states, this molecule may play a previously unrecognized role in systemic lipid metabolism or energy balance. 相似文献
96.
Smith BA Soderblom LA Beebe R Boyce J Briggs G Carr M Collins SA Cook AF Danielson GE Davies ME Hunt GE Ingersoll A Johnson TV Masursky H McCauley J Morrison D Owen T Sagan C Shoemaker EM Strom R Suomi VE Veverka J 《Science (New York, N.Y.)》1979,206(4421):927-950
Voyager 2, during its encounter with the Jupiter system, provided images that both complement and supplement in important ways the Voyager 1 images. While many changes have been observed in Jupiter's visual appearance, few, yet significant, changes have been detected in the principal atmospheric currents. Jupiter's ring system is strongly forward scattering at visual wavelengths and consists of a narrow annulus of highest particle density, within which is a broader region in which the density is lower. On Io, changes are observed in eruptive activity, plume structure, and surface albedo patterns. Europa's surface retains little or no record of intense meteorite bombardment, but does reveal a complex and, as yet, little-understood system of overlapping bright and dark linear features. Ganymede is found to have at least one unit of heavily cratered terrain on a surface that otherwise suggests widespread tectonism. Except for two large ringed basins, Callisto's entire surface is heavily cratered. 相似文献
97.
Elasmosaurid plesiosaurs were an important part of Cretaceous marine reptile communities and are generally considered to have been predators of small, agile, free-swimming fish and cephalopods. Two elasmosaurid specimens from Aptian and Albian deposits in Queensland, Australia, include fossilized gut contents dominated by benthic invertebrates: bivalves, gastropods, and crustaceans. Both specimens also contained large numbers of gastroliths (stomach stones). These finds point to a wider niche than has previously been supposed for these seemingly specialized predators and may also influence long-running controversy over the question of gastrolith function in plesiosaurs. 相似文献
98.
The hexagonal ferrites form an unusual group of complex, ferrimagnetic oxides embodying some 60 known crystal structures. These include phases for which the structural unit cell is larger than that in any known inorganic materials. The various hexagonal ferrite modifications fall into two distinct structural series, each formed by the ordered interlayering (stacking) of two discrete building blocks; these blocks stack along the c crystallographic axis in varying ratios and varying permutations to form strictly coherent, reproducible crystal structures. This mixed-layering aspect of the hexagonal ferrites permits direct, visual observation of the sequence of their subunit-cell stacking elements, after etching, by means of electron microscopy. The sequence of stacked blocks in such structures constitutes the only information lacking for a complete, three-dimensional structure determination. Direct access to this information provides an immediate, unique solution of the crystal structure problem in each case and thereby avoids the dilemmas of a classical diffraction approach to such large unit cells. Ferrite structures with hexagonal c dimensions of 1455 and 1577 angstroms have been uniquely solved by direct electron microscopic readout of surface etch features. One must exercise caution, however, in generalizing these findings to other materials. The method is successful in the case of the hexagonal ferrites because these are mixed-layer structures, wherein the building blocks react at different rates to a specific etchant. Mixed-layer systems are not uncommon in crystallography, and it is likely that similar techniques can be developed for other such materials. Regardless of the validity of this prognosis, however, it is quite evident that high-resolution replica electron microscopy is a most promising tool for the direct observation of surface structure on an ultramicro scale. During the studies reported here replica resolution capability was improved to about 10 angstroms; final resolution is limited by the particle size of the platinum shadowing material. Careful control of experimental conditions during replica preparation or an alternate choice of shadowing material, or both, might reasonably improve the resolution by a factor of 2. This resolution is within the range of most unit cell dimensions and approaches interatomic distances in solid-state materials. The potential of such an experimental capability needs no elaboration. 相似文献
99.
100.
L W Cook F W Zach J R Fleeker 《Journal of the Association of Official Analytical Chemists》1982,65(2):215-217
A procedure based on steam distillation is described for the determination of residues of the thiocarbamate herbicides diallate and triallate. The herbicides are steam-distilled directly from aqueous suspensions of milk and plant samples and trapped in hexane. After column cleanup on either activated Florisil or silica cartridges, samples are quantitated by gas-liquid chromatography. Recoveries of diallate and triallate from milk, lettuce, peas, corn, canarygrass seed and straw, and flax straw ranged from 77 to 96%. 相似文献