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   1728篇
  免费   115篇
  国内免费   184篇
  2023年   25篇
  2022年   47篇
  2021年   49篇
  2020年   57篇
  2019年   68篇
  2018年   54篇
  2017年   57篇
  2016年   52篇
  2015年   59篇
  2014年   76篇
  2013年   106篇
  2012年   61篇
  2011年   72篇
  2010年   64篇
  2009年   86篇
  2008年   74篇
  2007年   103篇
  2006年   101篇
  2005年   78篇
  2004年   70篇
  2003年   58篇
  2002年   58篇
  2001年   54篇
  2000年   42篇
  1999年   36篇
  1998年   24篇
  1997年   35篇
  1996年   29篇
  1995年   32篇
  1994年   28篇
  1993年   30篇
  1992年   27篇
  1991年   14篇
  1990年   15篇
  1989年   13篇
  1988年   15篇
  1987年   10篇
  1986年   18篇
  1985年   14篇
  1984年   19篇
  1983年   10篇
  1982年   17篇
  1981年   9篇
  1980年   11篇
  1979年   9篇
  1978年   8篇
  1977年   9篇
  1976年   7篇
  1973年   5篇
  1971年   4篇
排序方式: 共有2027条查询结果,搜索用时 203 毫秒
221.
1. Alba, a white wing colour morph of butterflies of the genus Colias , makes fitness-related changes at several levels of phenotypic organization: physiology, development and behaviour. Two sympatric species of Colias are studied, which differ greatly in their frequency of alba vs its sister yellow/orange morph. How resource and time constraints on Colias interact with the morphs' different patterns of pupal resource allocation to alter the balance of the morphs' fitness components in the two species is discussed.
2. These species, C. alexandra and C. scudderi , differ in melanin-based solar energy absorption, in larval dietary richness and in local adult nectar resources. The two female morphs determine alternate thermal balance, internal resource allocation and behavioural effects in each species, thus changing the morphs' time budgets and fitness-component impacts between the species. In particular, female egg output differences between the morphs appears to reverse between species: alba fecundity is greater than yellow fecundity in C. scudderi , but alba is less than yellow in C. alexandra .
3. These differences are consistent with the large observed differences in alba frequency between the species. Some important questions about the selective regime maintaining a polymorphism here, rather than an alternation of monomorphisms, remain.  相似文献   
222.
The insulin receptor (IR) plays critical roles in metabolism and growth, directed by the binding of insulin. Decades of research to understand the mechanism of insulin binding and activation of the IR have identified a region of the receptor, the C-terminal (CT) peptide, to be crucial for insulin binding. In particular, a truncated IR consisting of the first three domains fused to the CT peptide was found to bind insulin with nanomolar affinity, with undetectable binding in the absence of fused or soluble CT peptide. Problematically, all current crystal structures of the IR indicate the fusion point of the CT peptide to the three domains is located far from the position of the CT peptide as resolved in such structures. We have attempted to address this problem using molecular modelling and dynamics simulations. The results led to the identification of a potential inter-domain interaction between the L2 domain and the CT peptide that is not observed in any of the crystal structures of the IR. Investigations into this new interaction found a conformational change that could potentially be in response to insulin binding. Additionally, further simulation work with the new conformation demonstrated its compatibility with the position and orientation of insulin from the latest insulin-bound IR crystal structure.  相似文献   
223.
Jungers and German (1981) found differences when they compared 1) coefficients of allometry from bivariate plots of log measurements versus log body weight with 2) those coefficients from the first principal component of the log measurements excluding body weight. It is argued here that an arbitrary choice of unit for “internal size” is all that separates these coefficients. When the unit is chosen to make internal size isometric with body weight the coefficients agree rather well.  相似文献   
224.
Many monitoring programs for white-tailed deer (Odocoileus virginianus) on both private and public lands across the United States have long relied on the use of road-based spotlight surveys for monitoring population size and trends. Research has suggested spotlight surveys are ineffective and that road-based surveys for deer are biased because of highly variable detection rates. To evaluate variability in detection rates relative to the assumption that repeated surveys along roads will provide reliable trend data for use in calculating deer density estimates, we collected 5 years of thermal-imager and spotlight survey data using a multiple-observer, closed-capture approach. Using a Huggin's closed capture model, data bootstrapping, and variance components analyses, our results suggest that density estimates for white-tailed deer generated from data collected during road-based spotlight surveys are likely not reflective of the standing deer population. Detection probabilities during individual spotlight surveys ranged from 0.00 to 0.80 (median = 0.45) across all surveys, and differed by observer, survey, management unit, and survey transect replicate. Mean spotlight detection probability (0.41) and process standard deviation (0.12) estimates indicated considerable variability across surveys, observers, transects, and years, which precludes the generation of a correction factor or use of spotlight data to evaluate long-term trends at any scale. Although recommended by many state, federal, and non-governmental agencies, our results suggest that the benefit of spotlight survey data for monitoring deer populations is limited and likely represents a waste of resources with no appreciable management information gained. © 2012 The Wildlife Society.  相似文献   
225.
Abstract

The efficient synthesis of Grp and Arp, minor tRNA components, has been developed.  相似文献   
226.
Components of quantitative resistance, spore production, incubation period, infection frequency and mean lesion size were measured in 17 sunflower accessions inoculated with conidia of Alternuria helianthi under controlled conditions. The same accessions were also rated for disease reaction in the field in 1994 and 1995 using a generated epidemic and varied in their disease reactions from highly susceptible to highly resistant. Spearman's ranking of accessions was highly correlated (r = 0.9) for both years; however, the ranking of components measured under controlled conditions with field severity was generally poor. Regression analysis of components with field severity ratings of the accessions showed that mean lesion size was highly correlated (r = O.74) and infection frequency was moderately correlated (r = 0.58) with the field severity ratings taken over the two years. Infection frequency was also well correlated (r = 0.75) with mean lesion size. Spore production and incubation period were poorly correlated with the field severity ratings for both years. An index based on infection frequency and mean lesion size gave a better correlation with the 1995 field severity ratings than either component alone, but in 1994 the index was not as well correlated with field severity as mean lesion size alone. It is suggested that mean lesion size, determined from plants 7–9 days after inoculation could be used to select for resistance to A. helianthi in the greenhouse. Infection frequency could also be used as a predictor of resistance, but to a lesser degree.  相似文献   
227.
This article outlines the subsector of the information and communication technology (ICT) industry concerned with reducing the economy's environmental impact, dubbed ICT‐enabled low carbon technologies (ICTeLCTs). The article is based on a study funded by United Kingdom (UK) Trade and Investment, a division of the UK Department for Business, Innovation and Skills. ICTeLCTs can be segmented into specialist and generalist operators. Specialists focus on one or two ICT applications to monitor or reduce environmental issues, while generalists supply products and services enabling a firm or a private household to reduce the environmental impact of its activities. The subsector can be further segmented into green ICT, energy management, building management, carbon accounting, waste management, intelligent transport systems (ITSs), and water management. The main factors driving ICTeLCTs include legislation, voluntary environmental standards, corporate social responsibility (CSR) activities, customer demand, and competitive market factors. Policy makers should continue to drive the growth of ICTeLCTs with the introduction and refinement of environmental legislation regulating energy use and markets.  相似文献   
228.
Incubation represents a life stage of crucial importance for the optimal development of avian embryos. For most birds, incubation poses a trade‐off between investing in self‐maintenance and offspring care. Furthermore, incubation is affected by environmental temperatures and, therefore, will be likely impacted by climate change. Despite its relevance and readily available temperature logging methods, avian incubation research is hindered by recognised limitations in available software. In this paper, a new quantitative approach to analyse incubation behaviour is presented. This new approach is embedded in a free R package, incR. The flexibility of the R environment eases the analysis, validation and visualisation of incubation temperature data. The core algorithm in incR is validated here and it is shown that the method extracts accurate metrics of incubation behaviour (e.g. number and duration of incubation bouts). This paper also presents a suggested workflow along with detailed R code to aid the practical implementation of incR.  相似文献   
229.
ABSTRACT

Sleep deprivation impairs performance on cognitive tasks, but it is unclear which cognitive processes it degrades. We administered a semantic matching task with variable stimulus onset asynchrony (SOA) and both speeded and self-paced trial blocks. The task was administered at the baseline and 24 hours later after 30.8 hours of total sleep deprivation (TSD) or matching well-rested control. After sleep deprivation, the 20% slowest response times (RTs) were significantly increased. However, the semantic encoding time component of the RTs remained at baseline level. Thus, the performance impairment induced by sleep deprivation on this task occurred in cognitive processes downstream of semantic encoding.  相似文献   
230.
Traditional Chinese Medicine (TCM) is a very important raw material source for natural medicines in China. The content and activity of active component are main indexes that evaluate the quality of TCM, however, they may vary with environmental factors. In this study, the effects of environmental factors on the active component contents and antioxidant activity of Dasiphora fruticosa collected from the five main growing areas of China were investigated. The contents of tannin, total flavonoids and rutin were determined to be 7.65 – 10.69%, 2.30 – 5.39% and 0.18 – 0.81%, respectively. Antioxidant activity was determined by DPPH assay, with the DPPH IC50 values ranged from 8.791 to 32.534 μg mL?1. In order to further explore the cause of these significant geographical variations, the chemometric methods including correlation analysis, principal component analysis, gray correlation analysis and path analysis were applied. The results showed that environmental factors had significant effect on the contents of active components and antioxidant activity. Rapidly available phosphorus (RAP) and rapidly available nitrogen (RAN) were common dominant factors, and a significant positive action existed between RAP and active components and antioxidant activity (< 0.05). Contributed by their high active components and strong antioxidant activity, Bange in Tibet and Geermu in Qinghai Province were selected as a favorable growing location, respectively.  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