全文获取类型
收费全文 | 186530篇 |
免费 | 10288篇 |
国内免费 | 168篇 |
专业分类
林业 | 8181篇 |
农学 | 6190篇 |
基础科学 | 1212篇 |
23920篇 | |
综合类 | 30499篇 |
农作物 | 7472篇 |
水产渔业 | 9519篇 |
畜牧兽医 | 94732篇 |
园艺 | 2504篇 |
植物保护 | 12757篇 |
出版年
2019年 | 1947篇 |
2018年 | 2764篇 |
2017年 | 3110篇 |
2016年 | 2906篇 |
2015年 | 2422篇 |
2014年 | 3035篇 |
2013年 | 7412篇 |
2012年 | 5549篇 |
2011年 | 6772篇 |
2010年 | 4354篇 |
2009年 | 4495篇 |
2008年 | 6592篇 |
2007年 | 6247篇 |
2006年 | 5790篇 |
2005年 | 5476篇 |
2004年 | 5184篇 |
2003年 | 5368篇 |
2002年 | 4914篇 |
2001年 | 5673篇 |
2000年 | 5661篇 |
1999年 | 4529篇 |
1998年 | 1982篇 |
1997年 | 1932篇 |
1996年 | 1772篇 |
1995年 | 2123篇 |
1994年 | 1857篇 |
1993年 | 1757篇 |
1992年 | 3557篇 |
1991年 | 3796篇 |
1990年 | 3692篇 |
1989年 | 3724篇 |
1988年 | 3417篇 |
1987年 | 3347篇 |
1986年 | 3482篇 |
1985年 | 3346篇 |
1984年 | 2793篇 |
1983年 | 2457篇 |
1982年 | 1702篇 |
1979年 | 2589篇 |
1978年 | 2034篇 |
1977年 | 1783篇 |
1976年 | 1802篇 |
1975年 | 1909篇 |
1974年 | 2264篇 |
1973年 | 2321篇 |
1972年 | 2178篇 |
1971年 | 2059篇 |
1970年 | 1986篇 |
1969年 | 1955篇 |
1967年 | 1772篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
In the series of experiments with labelled urea three colostomized laying hybrids were butchered after a six-day application of 1% urea with 96.06 atom-% 15N excess (15N') in the ration and another 2 days with a supplement of 1% unlabelled urea. Out of the individual samples from crop, gizzard, small intestine, caecum and rectum, the content of the small intestine and the caecum showed the highest labelling with greater than 1 atom-% 15N'. The TCA soluble fraction of the content of the gizzard was more highly and that of the intestines less labelled than the total nitrogen. The tissue of the gizzard is distinctly less labelled than the "omasum system" and the small intestine. The atom-% 15N' of the oesophagus with crop and glandular stomach largely showed agreement in the individual hens with that of intestinal tissue and ranged between 0.71 and 0.89 atom-%. 2% of the 15N' supplemented with the urea could be recovered in the content and the tissue of the gastro-intestinal tract. 相似文献
932.
933.
934.
The earliest fossil record of African anthropoid primates (monkeys and apes) comes from the Jebel Qatrani Formation in the Fayum depression of Egypt. Reevaluation of both geologic and faunal evidence indicates that this formation was deposited in the early part of the Oligocene Epoch, more than 31 million years ago, earlier than previous estimates. The great antiquity of the fossil higher primates from Egypt accords well with their primitive morphology compared with later Old World higher primates. Thus, the anthropoid primates and hystricomorph rodents from Fayum are also considerably older than the earliest higher primates and rodents from South America. 相似文献
935.
Recent optical experiments have demonstrated cases in which mirror symmetry in stable atoms is broken during absorption of light. These results, which are in contradiction with quantum electrodynamics, support the theory of unification of the electromagnetic and weak forces. The interpretation of these experimental results is based on exchanges of weak neutral Z(0) bosons between the electrons and the nucleus of the atom. The information obtained from low-energy experiments is different from, but complementary to, the results of high-energy experiments. Sensitive measurements in a simple, reliably computable atom are in quantitative agreement with the standard electroweak theory and put stringent constraints on alternative models. Attaining sufficient accuracy in the experiments and the computations for the electroweak radiative corrections to manifest themselves is now the challenge for experimenters and theorists. 相似文献
936.
Temperature profiles measured in permafrost in northernmost Alaska usually have anomalous curvature in the upper 100 meters or so. When analyzed by heat-conduction theory, the profiles indicate a variable but widespread secular warming of the permafrost surface, generally in the range of 2 to 4 Celsius degrees during the last few decades to a century. Although details of the climatic change cannot be resolved with existing data, there is little doubt of its general magnitude and timing; alternative explanations are limited by the fact that heat transfer in cold permafrost is exclusively by conduction. Since models of greenhouse warming predict climatic change will be greatest in the Arctic and might already be in progress, it is prudent to attempt to understand the rapidly changing thermal regime in this region. 相似文献
937.
938.
939.
940.
Hildebrand JG 《Science (New York, N.Y.)》1986,233(4768):1101-1102