全文获取类型
收费全文 | 135644篇 |
免费 | 7788篇 |
国内免费 | 71篇 |
专业分类
林业 | 5577篇 |
农学 | 4644篇 |
基础科学 | 862篇 |
17487篇 | |
综合类 | 18957篇 |
农作物 | 5088篇 |
水产渔业 | 7093篇 |
畜牧兽医 | 72972篇 |
园艺 | 1798篇 |
植物保护 | 9025篇 |
出版年
2020年 | 1171篇 |
2019年 | 1410篇 |
2018年 | 2230篇 |
2017年 | 2490篇 |
2016年 | 2295篇 |
2015年 | 2000篇 |
2014年 | 2452篇 |
2013年 | 5731篇 |
2012年 | 4351篇 |
2011年 | 5163篇 |
2010年 | 3502篇 |
2009年 | 3548篇 |
2008年 | 5152篇 |
2007年 | 4970篇 |
2006年 | 4642篇 |
2005年 | 4262篇 |
2004年 | 4108篇 |
2003年 | 4176篇 |
2002年 | 3808篇 |
2001年 | 4773篇 |
2000年 | 4641篇 |
1999年 | 3747篇 |
1998年 | 1472篇 |
1997年 | 1453篇 |
1996年 | 1341篇 |
1995年 | 1599篇 |
1994年 | 1328篇 |
1993年 | 1359篇 |
1992年 | 2763篇 |
1991年 | 2901篇 |
1990年 | 2866篇 |
1989年 | 2899篇 |
1988年 | 2610篇 |
1987年 | 2592篇 |
1986年 | 2539篇 |
1985年 | 2400篇 |
1984年 | 1956篇 |
1983年 | 1715篇 |
1982年 | 1154篇 |
1979年 | 1649篇 |
1978年 | 1284篇 |
1977年 | 1135篇 |
1976年 | 1100篇 |
1975年 | 1164篇 |
1974年 | 1357篇 |
1973年 | 1448篇 |
1972年 | 1378篇 |
1971年 | 1195篇 |
1970年 | 1247篇 |
1969年 | 1146篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
271.
This paper describes the distinctive radiographic changes detected in the dorso-caudal lungfields of four racing thoroughbreds recently affected by exercise-induced epistaxis. A diffuse but localized increase in density was seen in all four cases, which demonstrated a variation from a predominantly alveolar density to an interstitial pattern and finally to increased bronchial markings. Evolution of the radiographic pattern of the pulmonary densities appeared to be related to the time that had elapsed since the bleeding incident. The implications of the changing pattern and site of the densities are discussed. 相似文献
272.
Waldrop MM 《Science (New York, N.Y.)》1982,216(4550):1086-1088
273.
274.
275.
Gas vacuolation in Oscillatoria rubescens decreased with increased nitrogen limitation and increased with transitions from nitrogen to inorganic carbon limitation. Gas vacuoles consist of protein vesicles that can accumulate in carbon- limited but not in unenriched nitrogen-limited cells. Nitrogen limitation is a factor in the formation of deep population maxima; carbon limitation can promote surface blooms. 相似文献
276.
277.
278.
279.
280.
Logsdon JM 《Science (New York, N.Y.)》1992,255(5042):294-300
The relations between Japan and the United States in space form a microcosm of the complex, multidimensional interactions between these two powerful societies. Cooperation and competition exist side by side, and the future balance between them is uncertain. The United States needs to develop a strategy with respect to future U.S.-Japanese space relations that balances national security, political, scientific, and economic interests. Crafting such a strategy is particularly difficult while both the United States and Japan debate the goals and content of their future space programs and while the two nations try to assess their broader interests and roles in the rapidly changing geopolitical environment. Essential to a productive approach to U.S.-Japanese space relations is an accurate understanding of the character and content of the Japanese space effort. 相似文献