全文获取类型
收费全文 | 234篇 |
免费 | 24篇 |
专业分类
林业 | 4篇 |
农学 | 1篇 |
16篇 | |
综合类 | 40篇 |
农作物 | 3篇 |
水产渔业 | 6篇 |
畜牧兽医 | 182篇 |
园艺 | 2篇 |
植物保护 | 4篇 |
出版年
2021年 | 5篇 |
2020年 | 2篇 |
2019年 | 3篇 |
2018年 | 2篇 |
2016年 | 2篇 |
2015年 | 5篇 |
2014年 | 5篇 |
2013年 | 15篇 |
2012年 | 5篇 |
2011年 | 2篇 |
2010年 | 7篇 |
2008年 | 13篇 |
2007年 | 6篇 |
2006年 | 11篇 |
2005年 | 7篇 |
2004年 | 10篇 |
2003年 | 11篇 |
2002年 | 8篇 |
2001年 | 7篇 |
2000年 | 8篇 |
1999年 | 4篇 |
1998年 | 6篇 |
1997年 | 2篇 |
1996年 | 5篇 |
1994年 | 4篇 |
1993年 | 2篇 |
1992年 | 7篇 |
1991年 | 10篇 |
1990年 | 10篇 |
1989年 | 4篇 |
1988年 | 8篇 |
1987年 | 8篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1976年 | 4篇 |
1974年 | 7篇 |
1973年 | 3篇 |
1972年 | 2篇 |
1971年 | 4篇 |
1967年 | 3篇 |
1953年 | 1篇 |
1931年 | 1篇 |
1924年 | 1篇 |
1918年 | 1篇 |
1916年 | 1篇 |
1907年 | 1篇 |
1886年 | 1篇 |
1883年 | 1篇 |
1881年 | 3篇 |
排序方式: 共有258条查询结果,搜索用时 15 毫秒
161.
SA Bowring DH Erwin Jin M W Martin YG K Davidek W Wang 《Science (New York, N.Y.)》1998,280(5366):1039-1045
The mass extinction at the end of the Permian was the most profound in the history of life. Fundamental to understanding its cause is determining the tempo and duration of the extinction. Uranium/lead zircon data from Late Permian and Early Triassic rocks from south China place the Permian-Triassic boundary at 251.4 +/- 0.3 million years ago. Biostratigraphic controls from strata intercalated with ash beds below the boundary indicate that the Changhsingian pulse of the end-Permian extinction, corresponding to the disappearance of about 85 percent of marine species, lasted less than 1 million years. At Meishan, a negative excursion in delta13C at the boundary had a duration of 165,000 years or less, suggesting a catastrophic addition of light carbon. 相似文献
162.
W. Stuart W. M. Peacock W. S. Baird W. Saxby Blair R. H. Helmer W. H. Hicks W. R. Leslie B. F. Lutman H. H. Mann S. D. Nagpurkar G. S. Kulkarni R. S. Kasargode S. R. Paranjupe B. M. Joshi J. A. McClary W. C. Mckillican Wm. H. Martin A. M. Musser C. A. Ludwig C. F. Clark R. G. Newton T. H. Parks E. E. Clayton F. H. Reed R. N. Salaman E. M. Straight C. T. Tice W. C. Edmundson M. J. Tinline H. O. WERNER R. F. Howard C. F. Clark C. M. White T. K. Wolfe 《American Journal of Potato Research》1924,1(4):54-66
163.
164.
All the fish used in this investigation were caught by commercial side trawlers along the eastern Cape coast of South Africa. Age and growth of P. laniarius was determined from otoliths collected monthly from April 1974 to September 1975. The validity of annuli was proved by monthly edge examination of the otoliths. There is no difference in growth between the sexes. The Von Bertalanffy equation Lt = 48,1 (l-e-0,10(e + 0.22)) was found to describe the growth of the panga in length. Some fish become sexually mature at a total length of 26 cm. Fifty percent maturity is attained at a length of 28 cm, between the ages of 4 and 5 years. All fish are sexually mature at a total length of 32 cm, at an age of 6 years. The spawning season of the panga extends from mid-September to May. 相似文献
165.
Iron, calcium, aluminum, silicon, and sulfur are major elements in the first surface sample of Mars that has been analyzed by the Viking x-ray fluorescence spectrometer. Titanium is present in minor quantities. This is consistent with the sample being a mixture of fine silicate and oxide mineral grains, with a significant proportion of sulfates, possibly hydrated. Ferric oxide is regarded as the red pigmenting agent on the martian surface, but if it coats silicate grains, the coatings must be very thin (= 2 micrometers) or discontinuous. A high abundance of Fe, relatively low abundances of Al, Rb, Sr, and Zr, and a high Ca/K ratio are distinctive features of the spectra. Preliminary determinations indicate the following abundances (as percentages by weight): Fe, 14 +/- 2; Ti < 1; S, 2 to 5; the Ca/K ratio by weight is greater than 5. 相似文献
166.
The argon content of the martian atmosphere at the Viking 1 landing site is = 0.15 millibar or = 2 percent by volume (95 percent confidence level). 相似文献
167.
A comparison of the mean life-spans for populations of Drosophila melanogaster at 10 degrees , 20 degrees , 25 degrees , and 30 degrees C maintained on media prepared with distilled water and with 50 percent deuterium oxide shows that deuteration decreases longevity at all four temperatures. The magnitude of the difference between the mean survival times of populations maintained on deuterated and nondeuterated media is inversely related to temperature between 10 degrees and 30 degrees C. 相似文献
168.
169.
Baird AK Toulmin P Clark BC Rose HJ Keil K Christian RP Gooding JL 《Science (New York, N.Y.)》1976,194(4271):1288-1293
Chemical results from four samples of martian fines delivered to Viking landers 1 and 2 are remarkably similar in that they all have high iron; moderate magnesium, calcium, and sulfur; low aluminum; and apparently very low alkalies and trace elements. This composition is best interpreted as representing the weathering products of mafic igneous rocks. A mineralogic model, derived from computer mixing studies and laboratory analog preparations, suggests that Mars fines could be an intimate mixture of about 80 percent iron-rich clay, about 10 percent magnesium sulfate (kieserite?), about 5 percent carbonate (calcite), and about 5 percent iron oxides (hematite, magnetite, maghemite, goethite?). The mafic nature of the present fines (distributed globally) and their probable source rocks seems to preclude large-scale planetary differentiation of a terrestrial nature. 相似文献
170.
Human immunodeficiency virus infection of human-PBL-SCID mice 总被引:32,自引:0,他引:32
D E Mosier R J Gulizia S M Baird D B Wilson D H Spector S A Spector 《Science (New York, N.Y.)》1991,251(4995):791-794
Severe combined immunodeficient (SCID) mice reconstituted with human peripheral blood leukocytes (hu-PBL-SCID mice) have inducible human immune function and may be useful as a small animal model for acquired immunodeficiency syndrome (AIDS) research. Hu-PBL-SCID mice infected with human immunodeficiency virus-1 (HIV-1) contained virus that was recoverable by culture from the peritoneal cavity, spleen, peripheral blood, and lymph nodes for up to 16 weeks after infection; viral sequences were also detected by in situ hybridization and by amplification with the polymerase chain reaction (PCR). Mice could be infected with multiple strains of HIV-1, including LAV-1/Bru, IIIB, MN, SF2, and SF13. HIV-1 infection affected the concentration of human immunoglobulin and the number of CD4+ T cells in the mice. These results support the use of the hu-PBL-SCID mouse for studies of the pathogenesis and treatment of AIDS. 相似文献