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   407篇
  免费   8篇
  2013年   11篇
  2012年   9篇
  2011年   12篇
  2010年   21篇
  2009年   20篇
  2008年   12篇
  2007年   30篇
  2006年   22篇
  2005年   15篇
  2004年   12篇
  2003年   5篇
  2002年   6篇
  2001年   3篇
  2000年   3篇
  1999年   3篇
  1998年   10篇
  1997年   8篇
  1996年   6篇
  1995年   4篇
  1994年   8篇
  1993年   6篇
  1992年   7篇
  1991年   4篇
  1990年   8篇
  1989年   7篇
  1988年   3篇
  1987年   3篇
  1985年   4篇
  1984年   4篇
  1983年   6篇
  1982年   3篇
  1981年   3篇
  1980年   4篇
  1979年   4篇
  1977年   5篇
  1975年   5篇
  1973年   4篇
  1972年   3篇
  1971年   4篇
  1969年   3篇
  1959年   7篇
  1958年   9篇
  1957年   13篇
  1956年   16篇
  1955年   11篇
  1954年   7篇
  1953年   8篇
  1952年   5篇
  1951年   5篇
  1950年   5篇
排序方式: 共有415条查询结果,搜索用时 0 毫秒
341.
342.
Isoperoxidase A3 from tobacco tissue culture, Nicotiana tabacam W-38, apparently does not have a subunit structure. Its molecular weight is approximately 54,000. The oxidation of scopoletin appears to occur in more than a single step since a blue intermediate is first formed. This is followed by the production of the final yellow product. A plausible explanation for this observation is the coupling of free radicals generated by the primary enzyme catalyzed reaction.  相似文献   
343.
Error and uncertainty in habitat models   总被引:9,自引:2,他引:7  
  相似文献   
344.
Abstract:  A new suite of arthropod trace fossils, attributed to a decapod crustacean, is described from the Lower Jurassic Saltford Shale Member of the Blue Lias Formation of Southam Cement Works Quarry, eastern Warwickshire, England. Solusichnium southamensis igen. et isp. nov. consists of small, isolated, bilaterally symmetrical, suboval hypichnia, comprising three regions. The concave anterior region contains imprints of chelate appendages, antennae and antennules. The elongate middle region contains abdominal appendage imprints that extend laterally, separated by a bifurcated medial imprint. The convex posterior region terminates in a globular V-shaped telson imprint. The large sample size and range of trace morphologies allows identification of five morphotypes within a taphoseries. S. southamensis is found on the base of siltstone lenses in what is otherwise a dysaerobic laminated mudstone unit, associated with epichnial Rusophycus , and the suite of trace fossils is interpreted as the resting traces (Cubichnia) and escape reactions (Fugichnia) of small decapods that were trapped below a distal storm deposit. The producer of S. southamensis was possibly an Eryon -like decapod, similar to those known from the slightly older Wilmcote Limestone Member of southwestern Warwickshire.  相似文献   
345.
346.
Lapwing Vanellus vanellus is known to be declining throughout much of its UK breeding range. The ecological requirements of the species are well known and a variety of mechanisms exist which can be implemented to help recovery. Arable options within agri-environment schemes can be used to provide breeding habitat throughout the species' UK range, and targeted recovery projects have been shown to have the potential to deliver increases in breeding pairs at the local level. The farming community need to be made aware of the requirements of Lapwing if agriculture and conservation needs are to be integrated.  相似文献   
347.
348.
Stigmas in 24 species of 11 genera, classified in three groups, Caesalpinia, Peltophorum and Dimorphandra, of the tribe Caesalpinieae have been examined by light and scanning electron microscopy. All are of the WN (wet, non-papillate) form. The receptive stigmatic surface comprises a crater, generally at the apex of the style, which in fresh flowers is full of clear fluid. The crater rim may be fringed with non-receptive hairs, and its shape and depth are variable between species. Extreme forms are found in Caesalpinia sappan where crater hairs are short and the crater funnel-shaped and very deep, and C. vesicaria and Delonix regia where crater hairs are long and the crater appears very shallow.  相似文献   
349.
Published records of cetaceans stranded on the Irish coast during the period 1901–95 are reviewed. In this review the number of stranding events has been used in the analysis and includes both live strandings and those animals washed up dead. There were 529 records involving 21 species. The Harbour Porpoise (27%) was the most frequently reported species, followed by Common Dolphins (16%) and Pilot Whales (15%). Minke Whales (8%) were the most frequently reported mysticete. The number of reported strandings has increased since the 1960s which is thought to be mainly due to increased observer effort. Cetaceans have stranded on all Irish coasts but mainly along the south coast and along the western seaboard but with no apparent overall seasonal trend. There was a peak in the strandings of Common Dolphins during 1991–92 when 27% (28 records) of all strandings were reported and of White-sided Dolphins when 60% (28 records) were reported, both of which were attributed to possible interactions with fisheries. The number of Striped Dolphins stranded on the Irish coast has increased steadily since the 1980s and may reflect increasing water temperatures.
These stranding records are considered inadequate to determine the status of most species of cetaceans in Irish waters but are sufficient to identify unusual stranding events such as high mortalities due to fisheries interactions or epizootics. More observer coverage is required before the stranding data are adequate for monitoring the status of most species but a stranding scheme is considered the most effective and efficient method of long-term monitoring of cetaceans in Irish waters.  相似文献   
350.
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