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   184119篇
  免费   8880篇
  国内免费   848篇
  2021年   1440篇
  2020年   940篇
  2019年   1242篇
  2018年   13044篇
  2017年   11744篇
  2016年   9493篇
  2015年   4448篇
  2014年   4661篇
  2013年   5527篇
  2012年   11167篇
  2011年   19280篇
  2010年   15961篇
  2009年   12092篇
  2008年   15233篇
  2007年   16774篇
  2006年   5729篇
  2005年   5796篇
  2004年   6049篇
  2003年   5909篇
  2002年   5377篇
  2001年   1199篇
  2000年   862篇
  1999年   985篇
  1998年   1228篇
  1997年   832篇
  1996年   748篇
  1995年   766篇
  1994年   661篇
  1993年   763篇
  1992年   678篇
  1991年   595篇
  1990年   570篇
  1989年   550篇
  1988年   523篇
  1987年   480篇
  1986年   434篇
  1985年   608篇
  1984年   687篇
  1983年   634篇
  1982年   721篇
  1981年   657篇
  1980年   659篇
  1979年   386篇
  1978年   465篇
  1977年   395篇
  1976年   397篇
  1974年   351篇
  1973年   343篇
  1972年   456篇
  1971年   457篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
1.
Insect and veteran trees are important parts of ecosystems and are usually included in ecological studies of forest management. The loss of veteran trees in woodlands and open landscapes would lead to the loss of saproxylic organisms—an important part of biodiversity. Hence, the persistence of many specialized insects depends on the presence of veteran trees scattered in woodlands (e.g. ancient wood pastures, game parks or protected areas), cities, towns and villages (e.g. avenues, parks or chateau parks) or open landscapes (e.g. fishpond dams, solitary trees or fruit orchards). Veteran tree conditions could be fairly well described by three components—diameter, age and microhabitats present. The problem is that diameter belongs to the most studied characteristics, while age and microhabitats, which can be quite complicated to measure, are much less studied. This paper illustrates that, due to this unbalanced use of indicators of veteran-tree conditions, we are still missing some important information on saproxylic species ecology—and sometimes only large trees might be studied, rather than real veterans. Although we already know that veteran trees are essential habitat for a range of saproxylic organisms, there are still gaps in our knowledge of the specific conditions that veteran trees provide. It is vital that these are quantified and understood so that this information can be used to conserve veteran trees and their associated species.  相似文献   
2.
Tourism accounts for 9% of global GDP and comprises 1.1 billion tourist arrivals per annum. Visits to wildlife tourist attractions (WTAs) may account for 20–40% of global tourism, but no studies have audited the diversity of WTAs and their impacts on the conservation status and welfare of subject animals. We scored these impacts for 24 types of WTA, visited by 3.6–6 million tourists per year, and compared our scores to tourists’ feedback on TripAdvisor. Six WTA types (impacting 1,500–13,000 individual animals) had net positive conservation/welfare impacts, but 14 (120,000–340,000 individuals) had negative conservation impacts and 18 (230,000–550,000 individuals) had negative welfare impacts. Despite these figures only 7.8% of all tourist feedback on these WTAs was negative due to conservation/welfare concerns. We demonstrate that WTAs have substantial negative effects that are unrecognised by the majority of tourists, suggesting an urgent need for tourist education and regulation of WTAs worldwide.  相似文献   
3.
4.
5.
6.
7.
8.
9.
10.
A nonplanar distortion of the heme ofc-type cytochromes is conserved in the proteins isolated from diverse species based upon a comprehensive analysis of available highresolution X-ray crystal structures. This distortion is induced through the cysteine thioether linkages between the porphyrin pyrrole groups and the polypeptide and results in an asymmetric pyrrole distortion. This asymmetry in the heme distortion is also conserved. For other heme proteins which lack these covalent bonds, nearly planar porphyrins are observed. Resonance Raman evidence indicates that nonplanar distortion of porphyrins containing metals, like iron, with large core sizes (?2.00 Å) is energetically unfavorable and can occur only in the presence of significant environmental perturbations. Further, energy minimization and dynamics calculations on the ferric form of yeast iso-1-cytochromec, starting from the crystallographic coordinates and using a molecular mechanics force field which accurately reproduces nonplanar distortions in metalloporphyrins, suggest that this distortion is indeed maintained by the protein tertiary structure. It is proposed that this protein-linked heme distortion modulates electron transfer function through modification of redox potentials of the porphyrin ring and the protein binding properties ofc-type cytochromes.  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