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   267篇
  免费   15篇
  国内免费   2篇
林业   15篇
农学   9篇
  44篇
综合类   35篇
农作物   10篇
水产渔业   44篇
畜牧兽医   105篇
园艺   8篇
植物保护   14篇
  2023年   3篇
  2021年   4篇
  2020年   8篇
  2019年   9篇
  2018年   5篇
  2017年   8篇
  2016年   12篇
  2015年   2篇
  2014年   9篇
  2013年   13篇
  2012年   21篇
  2011年   16篇
  2010年   15篇
  2009年   18篇
  2008年   14篇
  2007年   18篇
  2006年   11篇
  2005年   18篇
  2004年   16篇
  2003年   16篇
  2002年   8篇
  2001年   3篇
  2000年   6篇
  1999年   2篇
  1997年   1篇
  1996年   2篇
  1995年   1篇
  1994年   1篇
  1992年   3篇
  1991年   1篇
  1986年   1篇
  1984年   1篇
  1981年   1篇
  1980年   1篇
  1979年   1篇
  1974年   2篇
  1966年   1篇
  1958年   1篇
  1954年   1篇
  1953年   1篇
  1938年   2篇
  1937年   1篇
  1936年   1篇
  1934年   1篇
  1928年   1篇
  1927年   1篇
  1924年   1篇
  1922年   1篇
排序方式: 共有284条查询结果,搜索用时 46 毫秒
1.
2.
Radiocarbon dates associated with engraved stones from sealed archeological deposits at Wonderwerk Cave in the northern Cape Province indicate that rock engraving in South Africa is at least 10,000 years old.  相似文献   
3.
Coastal Brook Trout (Salvelinus fontinalis) populations are found from northern Canada to New England. The extent of anadromy generally decreases with latitude, but the ecology and movements of more southern populations are poorly understood. We conducted a 33‐month acoustic telemetry study of Brook Trout in Red Brook, MA, and adjacent Buttermilk Bay (marine system) using 16 fixed acoustic receivers and surgically implanting acoustic transmitters in 84 individuals. Tagged Brook Trout used the stream, estuary (50% of individuals) and bay (10% of individuals). Movements into full sea water were brief when occurring. GAMM models revealed that transitions between habitat areas occurred most often in spring and fall. Environmental data suggest that use of the saline environment is limited by summer temperatures in the bay. Movements may also be related to moon phase. Compared to more northern coastal populations of Brook Trout, the Red Brook population appears to be less anadromous overall, yet the estuarine segment of the system may have considerable ecological importance as a food resource.  相似文献   
4.
Over 3 years, 32,444 age‐0 group Atlantic salmon (Salmo salar) parr tagged with passive integrated transponder tags throughout the River Frome catchment were assigned to one of three groups, nonmigrants, autumn migrants and spring smolts, depending on the detection and the timing of detection at downstream tag readers (situated 8.6 km above the tidal limit). We examined the effect of density at the time of tagging (n·m?2), distance upstream from the tidal limit (km), fish length (mm), Fulton condition index, habitat type (divided into two types, main river and carrier), days after 1 September that each fish was tagged and year (replication) on the proportions of fish in each of the migration groups. Distance upstream from tidal limit was strongly negatively related to the proportion of autumn migrants and positively related to the proportion of spring smolts. Nonmigrants had a lower average body size than migrants, although there were no differences in the sizes of autumn migrants and spring smolts in September prior to migration. Fish density had no effect on migration strategy. A lower proportion of fish migrated as autumn migrants from the smaller carrier habitats than the main river channel. There is some evidence that those parr destined to become autumn migrants underwent a lower mortality rate during September before tagging than those destined to become spring migrants indicating possible physiological or behavioural differences between these two groups of fish at that time. More research into the factors responsible for initiating the autumn migration is required.  相似文献   
5.
6.
7.
Topical vecuronium bromide (Norcuron) and combinations with atropine and phenylephrine, were evaluated as mydriatics in juvenile double-crested cormorants (Phalacrocorax auritus). Nine cormorants were treated with each of four protocols: 1% atropine; 4 mg/mL vecuronium bromide (total 0.16 mg/eye); atropine with vecuronium; and atropine, 2.5% phenylephrine, followed by vecuronium. Drugs were applied topically at 15-min intervals (0.01 mL/drop). Pupil diameter was measured manually every 15 min with a pupil gauge calibrated to the nearest 0.5 mm. No effect was observed with atropine alone. Average +/- SD peak pupil diameter for vecuronium, atropine/vecuronium, and atropine/phenylephrine/vecuronium were 5.4 +/- 1.1 mm, 5.7 +/- 0.8 mm and 6.2 +/- 0.4 mm, respectively; and duration of peak diameters were 38 +/- 28 min, 79 +/- 71 min and 103 +/- 58 min, respectively. The combined atropine, phenylephrine and vecuronium provided the most consistent dilation with larger average pupil size and longer average duration. No side-effects from vecuronium were observed in these birds.  相似文献   
8.
Urban forestry is generally defined as the art, science and technology of managing trees and forest resources in and around urban community ecosystems for the physiological, sociological, economic, and aesthetic benefits trees provide society. First mentioned in the United States as early as in 1894, the concept underwent a revival during the 1960s as a comprehensive and interdisciplinary approach to the specific challenges related to growing trees in urban environments. Later, urban forestry evoked the interest of scientists and practitioners in other parts of the world. However, harmonization of urban forestry terminology has been complicated by, for example, the involvement of different disciplines and translation difficulties. In many European languages, for example, the direct translation of ‘urban forestry’ relates more to forest ecosystems than to street and park trees. Efforts in North America and Europe defining ‘urban forest’, ‘urban forestry’ and related terms are introduced. A comparative analysis of selected urban forestry terminology in both parts of the world shows that urban forestry has a longer history in North America, based on traditions of shade tree management. Moreover, urban forestry has become more institutionalized in North America. Urban forestry in Europe has built strongly on a century-long tradition of ‘town forestry’. In both parts of the world, definitions of urban forestry and urban forest have become more comprehensive, including all tree stands and individual trees in and around urban areas. Agreement also exists on the multifunctional and multidisciplinary character of urban forestry. These similarities offer opportunities for international harmonization of terminology.  相似文献   
9.
10.
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