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   781篇
  免费   39篇
  国内免费   76篇
  2023年   12篇
  2022年   11篇
  2021年   12篇
  2020年   15篇
  2019年   23篇
  2018年   20篇
  2017年   17篇
  2016年   22篇
  2015年   19篇
  2014年   28篇
  2013年   44篇
  2012年   18篇
  2011年   28篇
  2010年   28篇
  2009年   35篇
  2008年   45篇
  2007年   44篇
  2006年   48篇
  2005年   50篇
  2004年   38篇
  2003年   38篇
  2002年   33篇
  2001年   23篇
  2000年   16篇
  1999年   27篇
  1998年   19篇
  1997年   19篇
  1996年   19篇
  1995年   24篇
  1994年   19篇
  1993年   19篇
  1992年   13篇
  1991年   11篇
  1990年   9篇
  1989年   8篇
  1988年   5篇
  1987年   4篇
  1986年   8篇
  1985年   4篇
  1984年   2篇
  1983年   1篇
  1982年   3篇
  1981年   2篇
  1980年   5篇
  1979年   1篇
  1978年   2篇
  1977年   2篇
  1976年   1篇
  1972年   1篇
  1968年   1篇
排序方式: 共有896条查询结果,搜索用时 15 毫秒
1.
Over the past two decades, the dendroclimate community has produced various annually resolved, warm season temperature reconstructions for the extratropical Northern Hemisphere. Here we compare these tree-ring based reconstructions back to 831 CE and present a set of basic metrics to provide guidance for non-specialists on their interpretation and use. We specifically draw attention to (i) the imbalance between (numerous) short and (few) long site chronologies incorporated into the hemispheric means, (ii) the beneficial effects of including maximum latewood density chronologies in the recently published reconstructions, (iii) a decrease in reconstruction covariance prior to 1400 CE, and (iv) the varying amplitudes and trends of reconstructed temperatures over the past 1100 years. Whereas the reconstructions agree on several important features, such as warmth during medieval times and cooler temperatures in the 17th and 19th centuries, they still exhibit substantial differences during 13th and 14th centuries. We caution users who might consider combining the reconstructions through simple averaging that all reconstructions share some of the same underlying tree-ring data, and provide four recommendations to guide future efforts to better understand past millennium temperature variability.  相似文献   
2.
Sardinero  Santiago 《Plant Ecology》2000,148(1):81-103
An analysis of vegetation along an altitudinal gradient on the Presidential Range, New Hampshire, USA, using the Braun–Blanquet approach followed by multivariate data analysis is presented. Twelve main plant communities have been distinguished. Floristic information is presented in twelve tables and one appendix. The relationships of the communities to complex environmental gradients are analyzed using Correspondence Analysis. Floristic composition and community structure are controlled primarily by the altitudinal gradient (temperature, precipitation), and by mesotopographic conditions (snow accumulation, exposure and cryoturbation, slope position, and soil moisture).  相似文献   
3.
4.
The mechanism of placental alkaline phosphatase (PLAP) induction by prednisolone in a uterine cervical epidermoid cancer cell line SKG-IIIa was investigated in vitro by enzyme-cytochemistry, enzyme immunoassay, Northern and Southern blot analysis, and in situ hybridization. Enzyme-cytochemical alkaline phosphatase (ALP) staining and immunoassay revealed increased levels of PLAP (heat-stable ALP) in prednisolone-treated cells. Northern blot analysis and in situ hybridization showed increased amounts of PLAP mRNA. Southern blot analysis indicated that PLAP was not a product of an amplified or rearranged gene. These findings suggest that the induction of PLAP mRNA in SKG-IIIa cells by prednisolone in turn increased the levels of PLAP.  相似文献   
5.
I. Munda 《Hydrobiologia》1977,52(1):73-87
A comparison betweenCorallina officinalis associations from the northern Adriatic and the Icelandic coastal area is given regarding their floristic composition, zonal position and physionomy, as related to the environmental parameters and the different origins of the respective floras.  相似文献   
6.
华北地区针叶林下凋落物层化学性质的研究   总被引:10,自引:0,他引:10  
森林凋落物中贮积了大量的有机、无机养分物质,它是森林土壤自然肥力的重要来源之一。在森林生态系统中,养分物质的内部循环主要是通过凋落物来实现的。在分解、转化过  相似文献   
7.
8.
Populations of Noctiluca scintillans (hereafter Noctiluca) were compared from two regions: the northeastern-central Black Sea and the northern Adriatic Sea. In both seas samples were collected in near-shore waters 2–3 times per month during 2004–2012. For analysis of feeding activities and seasonal dynamics additional cruise data on the open waters of the Black Sea were used. Comparison between the two populations shows similarity in size structure with two classes 401–500 μm and 501–600 μm being the most numerous. Seasonal changes in cell abundance in both seas demonstrated a regular annual maximum with the peak period of high abundances in May–June with additional sporadic peaks in other seasons. In spring the average number of food vacuoles in the cell (1.78) and the proportion of feeding cells in populations (79%) in the Adriatic Sea were similar to those in the Black Sea (1.58 and 76%). In September–October, these parameters were lower both in the Adriatic Sea (0.69 and 49%) and in the Black Sea (1.46 and 65%) demonstrating that Noctiluca was better provided with food in spring. Among biotic parameters (wet phytoplankton biomass, chlorophyll biomass and zooplankton species) only the concentration of the eggs of Calanus euxinus was significantly positively correlated with abundance of Noctiluca. The possible effect of a high concentration of copepod eggs on the growth of Noctiluca in the peak period is discussed. An obvious negative relationship was observed between Noctiluca cell numbers in the peak period and wind velocity in both seas. The most significant negative correlation was observed between the number of windy hours per month (velocity more than 5–6 m s−1) and cell concentrations in the Black Sea (r = −0.92) and in the northern Adriatic Sea (r = −0.67). On this basis, a new hypothesis has been proposed and discussed: in connection with features of the food behavior of Noctiluca, its outbursts during the peak period are controlled by the wind. An evident positive relationship was observed between the number of Noctiluca in the peak period and its quantity in the preceding months in both seas. Thus, we suggest that abundance data during early spring and weather forecasts (winds) may be used for medium-term prediction of Noctiluca outbursts and red tides.  相似文献   
9.
10.
This article aims to explore nationalism, generally, and Ulster Loyalism, specifically, through the use of discourse theory and the Althusserian concept of ideology as interpellation. It suggests that nationalism is best understood as a particular discursive articulation that constructs subjects as being of a particular nation and thereby having certain characteristics and political needs and interests. This entails a larger definition of nationalism which encompasses some phenomena not generally taken to be nationalistic. The method is illustrated by a study of some aspects of the ideological discourse of Ulster Loyalism showing this, contrary to some accounts, to be a kind of nationalism. In the nationalist articulation of Loyalism, religion, democracy and identity are linked together making a unified ideological discourse based around a sense of specificity and difference to Irish nationalism and Roman Catholicism. Special attention is paid to the role of democracy within Loyalist discourse, in particular, and in nationalist discourse generally. The article suggests that this way of thinking about nationalism can avoid functionalist and teleological analyses, allowing us to focus on the specific ways in which a nationalism operates and to stress the centrality of politics in the process.  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