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   2235篇
  免费   294篇
  国内免费   1篇
  2022年   20篇
  2021年   42篇
  2020年   18篇
  2019年   14篇
  2018年   29篇
  2017年   31篇
  2016年   60篇
  2015年   79篇
  2014年   102篇
  2013年   119篇
  2012年   133篇
  2011年   128篇
  2010年   107篇
  2009年   81篇
  2008年   107篇
  2007年   101篇
  2006年   127篇
  2005年   116篇
  2004年   94篇
  2003年   100篇
  2002年   81篇
  2001年   61篇
  2000年   58篇
  1999年   42篇
  1998年   17篇
  1997年   29篇
  1996年   23篇
  1995年   23篇
  1994年   23篇
  1993年   22篇
  1992年   36篇
  1991年   37篇
  1990年   41篇
  1989年   27篇
  1988年   36篇
  1987年   25篇
  1986年   20篇
  1985年   27篇
  1984年   21篇
  1983年   27篇
  1981年   14篇
  1980年   15篇
  1977年   19篇
  1976年   13篇
  1975年   11篇
  1973年   19篇
  1972年   11篇
  1971年   14篇
  1970年   15篇
  1969年   18篇
排序方式: 共有2530条查询结果,搜索用时 265 毫秒
71.
Abstract

The Dutch have a long tradition of hospitality towards ethnic immigrants. In the nineties, however, quite dramatic changes have taken place among the Dutch. The central question addressed in this contribution is: to what extent do specific categories within the electorate favour ethnocentric policies? This question is answered by deducing hypotheses that are tested using recent data polled within the framework of the Dutch National Election Studies. Our crucial conclusion is that a rather widespread support for ethnocentric policies is present in contemporary Dutch society, especially among manual labourers, self‐employed and lowly educated people, but also among young cohorts and among modal income categories.  相似文献   
72.
Good lateral, Anthonsen oblique, anterior-posterior, and axial roentgenograms are necessary to define the extent of fractures of the calcaneus. Roentgenograms of the lumbar spine, including the thoracolumbar junction, are recommended whenever a fall has occurred sufficient to fracture the calcaneus.The great majority of calcaneal fractures are compression shearing fractures. Significant disruption of articular surfaces and spreading of the body beneath the fibula are major surgical problems and require careful treatment. In a series of 50 fractures treated by four methods, open reduction with stabilization by bone grafts, followed by active motion at six to eight weeks, gave the best results.  相似文献   
73.
ABSTRACT

In most mammals, adults produce relatively low frequency vocalizations compared to those of juveniles. This rule is not maintained however at least in four species of ground squirrels, whose juveniles call at the adult's fundamental frequency. These findings have been obtained however with separate sets of juveniles and adults and no data is available concerning the ontogeny linked to these differences. Here we analyze the acoustic structure of alarm calls of the same Yellow Spermophilus fulvus and Speckled S. suslicus ground squirrel individuals, recorded as pups and as adults after hibernation. We found the fundamental frequencies of adults within the same frequency ranges as those of pups, in spite of the significant difference in body mass. In ground squirrels, severing the relationship between body size and call frequency removes some vocal cues to age. We discuss some functional hypotheses advanced to explain manipulations with fundamental frequencies in ground squirrels and other animals, and suggest the lack of data for discussing the mechanisms of such vocal tuning.  相似文献   
74.
Bioprocess and Biosystems Engineering - The production of microbial protein in the form of yeast grown on lignocellulosic sugars and nitrogen-rich industrial residues is an attractive approach for...  相似文献   
75.
The Atacama Desert is the driest non‐polar desert on Earth, presenting precarious conditions for biological activity. In the arid coastal belt, life is restricted to areas with fog events that cause almost daily wet–dry cycles. In such an area, we discovered a hitherto unknown and unique ground covering biocenosis dominated by lichens, fungi, and algae attached to grit‐sized (~6 mm) quartz and granitoid stones. Comparable biocenosis forming a kind of a layer on top of soil and rock surfaces in general is summarized as cryptogamic ground covers (CGC) in literature. In contrast to known CGC from arid environments to which frequent cyclic wetting events are lethal, in the Atacama Desert every fog event is answered by photosynthetic activity of the soil community and thus considered as the desert's breath. Photosynthesis of the new CGC type is activated by the lowest amount of water known for such a community worldwide thus enabling the unique biocenosis to fulfill a variety of ecosystem services. In a considerable portion of the coastal Atacama Desert, it protects the soil from sporadically occurring splash erosion and contributes to the accumulation of soil carbon and nitrogen as well as soil formation through bio‐weathering. The structure and function of the new CGC type are discussed, and we suggest the name grit–crust. We conclude that this type of CGC can be expected in all non‐polar fog deserts of the world and may resemble the cryptogam communities that shaped ancient Earth. It may thus represent a relevant player in current and ancient biogeochemical cycling.  相似文献   
76.
Vegetation History and Archaeobotany - Laguna Santa Elena (8.9290° N, 82.9257° W, 1055 m a.s.l.) is a small lake in the Diquís archaeological sub-region of southern Pacific...  相似文献   
77.
Plant and Soil - Cyperaceae are common on nutrient-poor soils, including in western Japan. We examined the ability of the native Cyperaceae from western Japan to form dauciform roots and assessed...  相似文献   
78.
79.
80.
BioMetals - The cytotoxicity against five human tumor cell lines (THP-1, U937, Molt-4, Colo-205 and NCI-H460) of three water soluble copper(II) coordination compounds containing the ligands...  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