首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   196878篇
  免费   10810篇
  国内免费   132篇
林业   7394篇
农学   5822篇
基础科学   1350篇
  22900篇
综合类   35631篇
农作物   7256篇
水产渔业   9555篇
畜牧兽医   102129篇
园艺   2373篇
植物保护   13410篇
  2018年   2431篇
  2017年   2632篇
  2016年   2535篇
  2015年   2212篇
  2014年   2694篇
  2013年   7330篇
  2012年   5042篇
  2011年   6300篇
  2010年   3990篇
  2009年   4088篇
  2008年   6204篇
  2007年   5836篇
  2006年   5705篇
  2005年   5281篇
  2004年   5126篇
  2003年   5171篇
  2002年   4952篇
  2001年   6144篇
  2000年   5978篇
  1999年   4781篇
  1998年   1928篇
  1997年   2010篇
  1996年   1887篇
  1995年   2308篇
  1994年   2117篇
  1993年   2015篇
  1992年   4236篇
  1991年   4446篇
  1990年   4398篇
  1989年   4399篇
  1988年   4089篇
  1987年   4214篇
  1986年   4346篇
  1985年   4197篇
  1984年   3452篇
  1983年   3054篇
  1982年   2149篇
  1981年   1955篇
  1979年   3170篇
  1978年   2648篇
  1977年   2269篇
  1976年   2093篇
  1975年   2208篇
  1974年   2906篇
  1973年   2931篇
  1972年   2857篇
  1971年   2570篇
  1970年   2434篇
  1969年   2253篇
  1967年   1945篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
12.
13.
14.
15.
16.
17.
Disseminated Aspergillus terreus infection was diagnosed in ten previously healthy adult dogs--nine German shepherds and one dalmatian. The disease was characterized by the presence of multiple granulomas and infarcts in a wide range of organs. The kidney, spleen, and skeletal system were most commonly and severely affected. Fungal hyphae were demonstrated in large numbers within granulomas and thrombi, and A. terreus was readily isolated by culture. This disseminated mycosis appears unique; in this series of cases there was no apparent predisposing factor, portal of entry, or primary focus for dissemination of the infection.  相似文献   
18.
19.
Chinese catfish ( Clarias fuscus ) were successfully spawned in Hawaii using human chorionic gonadotropin (HCG) at dosage rates of two and four international units (IU) per gram body weight. Fish not injected with HCG did not produce viable eggs. Successful spawns with HCG occurred between May and October. Hatch rates of up to 80% were obtained during June, July, and August for those fish given either a 2 or 4 IU per gram body weight injection of HCG. Fish spawned in either May or October yielded significantly higher hatch rates when injected with 4 rather than 2 IU per gram body weight. Fish held at elevated temperatures (28 to 30 C) prior to the normal spawning season developed significantly larger oocyte diameters, 60 days earlier than fish held under ambient temperature conditions (21.5 to 24 C). Photoperiod manipulation at ambient temperature conditions was associated with earlier oocyte maturation, but photoperiod effects were much less important than temperature.  相似文献   
20.
Current practices in many nurseries involve the germination of tropical rainforest seedlings in shaded conditions and transferral, at a later date, to environments with greater light intensities. Determination of the ability of these seedlings to acclimate to increased light intensities will allow seedling stock to be processed with maximum efficiency within the nursery. The acclimation abilities of three species, Agyrodendron actinophyllum, Cardwellia sublimis, and Flindersia brayleyana, commonly found in north Queensland's rainforests were investigated in this study. These particular species are highly valued for their cabinet timber qualities and are being reared in nurseries for use in reforestation trials and programs in north Queensland. Seedlings were initially raised in greenhouse conditions under two layers of shade cloth (16% of full sunlight) and then transferred into full sunlight at three different ages (3 weeks, 10 weeks, 14 weeks). Upon transfer, organ ratios and the direction of dry matter distribution was determined for each species and age group. Approximately 3 months after the seedlings were transferred, the acclimation ability of each species and age group was then determined. Dry matter distribution was found to change with age, irrespective of light environment. Individuals within a species with larger root systems and thicker or more dense leaves had a greater acclimation ability than those with smaller root systems and thinner or less dense leaves. Furthermore, individuals within a species whose dry matter distribution upon transfer was directed towards developing a large root system, and a small photosynthetic area and mass, had a greater acclimation ability than those whose dry matter distribution was directed away from such morphologies. Awareness of these relationships allows a better understanding of seedling response to gap formation in natural forests, and also allows plant nursery operators to make a more informed decision about when to move seedlings to environments with a higher light intensity.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号