全文获取类型
收费全文 | 1333篇 |
免费 | 96篇 |
专业分类
林业 | 46篇 |
农学 | 45篇 |
基础科学 | 11篇 |
163篇 | |
综合类 | 263篇 |
农作物 | 126篇 |
水产渔业 | 37篇 |
畜牧兽医 | 677篇 |
园艺 | 15篇 |
植物保护 | 46篇 |
出版年
2017年 | 14篇 |
2016年 | 24篇 |
2015年 | 15篇 |
2014年 | 21篇 |
2013年 | 52篇 |
2012年 | 31篇 |
2011年 | 44篇 |
2010年 | 26篇 |
2009年 | 39篇 |
2008年 | 42篇 |
2007年 | 46篇 |
2006年 | 47篇 |
2005年 | 44篇 |
2004年 | 41篇 |
2003年 | 40篇 |
2002年 | 40篇 |
2001年 | 29篇 |
2000年 | 38篇 |
1999年 | 23篇 |
1998年 | 22篇 |
1997年 | 16篇 |
1995年 | 14篇 |
1993年 | 14篇 |
1992年 | 17篇 |
1991年 | 18篇 |
1990年 | 29篇 |
1989年 | 19篇 |
1988年 | 19篇 |
1987年 | 15篇 |
1986年 | 13篇 |
1985年 | 11篇 |
1984年 | 16篇 |
1983年 | 22篇 |
1982年 | 21篇 |
1979年 | 20篇 |
1978年 | 24篇 |
1977年 | 21篇 |
1976年 | 13篇 |
1975年 | 16篇 |
1974年 | 24篇 |
1973年 | 19篇 |
1972年 | 14篇 |
1971年 | 18篇 |
1970年 | 26篇 |
1969年 | 18篇 |
1968年 | 17篇 |
1948年 | 10篇 |
1945年 | 12篇 |
1944年 | 14篇 |
1943年 | 14篇 |
排序方式: 共有1429条查询结果,搜索用时 0 毫秒
81.
alpha-Tomatine, an alkaloid in tomato plants, is toxic to an endoparasite of a major lepidopterous pest of tomatoes. The parasite acquires the alkaloid from its host after the host ingests the alkaloid. This form of interaction creates a potential dilemma to controlling herbivorous pests through chemical antibiosis in plants. 相似文献
82.
83.
Campbell RT 《Science (New York, N.Y.)》1984,224(4649):594-595
84.
Brünger AT Campbell RL Clore GM Gronenborn AM Karplus M Petsko GA Teeter MM 《Science (New York, N.Y.)》1987,235(4792):1049-1053
Model calculations were performed to test the possibility of solving crystal structures of proteins by Patterson search techniques with three-dimensional structures obtained from nuclear magnetic resonance (NMR) interproton distance restraints. Structures for crambin obtained from simulated NMR data were used as the test system; the root-mean-square deviations of the NMR structures from the x-ray structure were 1.5 to 2.2 A for backbone atoms and 2.0 to 2.8 A for side-chain atoms. Patterson searches were made to determine the orientation and position of the NMR structures in the unit cell. The correct solution was obtained by comparing the rotation function results of several of the NMR structures and the average structure derived from them. Conventional refinement techniques reduced the R factor from 0.43 at 4 A resolution to 0.27 at 2 A resolution without inclusion of water molecules. The partially refined structure has root-mean-square backbone and side-chain atom deviations from the x-ray structure of 0.5 and 1.3 A, respectively. 相似文献
85.
Observations of the Galilean satellites with the radar system at the Arecibo Observatory, Puerto Rico, show that their surfaces are highly diffuse scatterers of radio waves of length 12.6 centimeters; spectra of the radar echoes are asymmetric and broad. The geometric radar albedos for the outer three satellites-0.42 +/- 0.10, 0.20 +/- 0.05, and 0.09 +/- 0.02 for Europa, Ganymede, and Callisto, respectively-show about the same relative decreases as do the optical albedos, although the latter presumably bear only on material much nearer the surface. Radii of 1420 +/- 30, 2640 +/- 80, and 2360 +/- 70 kilometers for Europa, Ganymede, and Callisto were determined from the radar data and are in good agreement with the corresponding optically derived values. Io, observed successfully only once, appears to have an albedo comparable to Ganymede's, but no radius was estimated for it. 相似文献
86.
Genetic engineering of livestock 总被引:20,自引:0,他引:20
V G Pursel C A Pinkert K F Miller D J Bolt R G Campbell R D Palmiter R L Brinster R E Hammer 《Science (New York, N.Y.)》1989,244(4910):1281-1288
Genetic engineering of livestock is expected to have a major effect on the agricultural industry. However, accurate assessment of the consequences of transgene expression is impossible without multigenerational studies. A systematic study of the beneficial and adverse consequences of long-term elevations in the plasma levels of bovine growth hormone (bGH) was conducted on two lines of transgenic pigs. Two successive generations of pigs expressing the bGH gene showed significant improvements in both daily weight gain and feed efficiency and exhibited changes in carcass composition that included a marked reduction in subcutaneous fat. However, long-term elevation of bGH was generally detrimental to health: the pigs had a high incidence of gastric ulcers, arthritis, cardiomegaly, dermatitis, and renal disease. The ability to produce pigs exhibiting only the beneficial, growth-promoting effects of growth hormone by a transgenic approach may require better control of transgene expression, a different genetic background, or a modified husbandry regimen. 相似文献
87.
Head JW Campbell DB Elachi C Guest JE McKenzie DP Saunders RS Schaber GG Schubert G 《Science (New York, N.Y.)》1991,252(5003):276-288
Magellan images confirm that volcanism is widespread and has been fimdamentally important in the formation and evolution of the crust of Venus. High-resolution imaging data reveal evidence for intrusion (dike formation and cryptodomes) and extrusion (a wide range of lava flows). Also observed are thousands of small shield volcanoes, larger edifices up to several hundred kilometers in diameter, massive outpourings of lavas, and local pyroclastic deposits. Although most features are consistent with basaltic compositions, a number of large pancake-like domes are morphologically similar to rhyolite-dacite domes on Earth. Flows and sinuous channels with lengths of many hundreds of kilometers suggest that extremely high effusion rates or very fluid magmas (perhaps komatiites) may be present. Volcanism is evident in various tectonic settings (coronae, linear extensional and compressional zones, mountain belts, upland rises, highland plateaus, and tesserae). Volcanic resurfacing rates appear to be low (less than 2 Km(3)/yr) but the significance of dike formation and intrusions, and the mode of crustal formation and loss remain to be established. 相似文献
88.
89.
90.
Liver mitochondria from the desert tortoise Gopherus agassizii and the Texas tortoise G. berlandieri contain glutamine synthetase, the enzyme used by birds and higher reptiles to detoxify ammonia generated intramitochondrially during amino acid catabolism. Tortoise mitochondria also contain carbamyl phosphate synthetase-I and ornithine transcarbamylase, the enzymes used by mammals for this purpose. Since chelonid reptiles have changed little since their divergence from the stem reptiles, this finding suggests that both systems were present in the latter before the emergence of the ruling reptile, avian, and mammalian lines of descent. 相似文献