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   3396篇
  免费   286篇
  2023年   14篇
  2021年   71篇
  2020年   40篇
  2019年   43篇
  2018年   78篇
  2017年   57篇
  2016年   88篇
  2015年   152篇
  2014年   158篇
  2013年   179篇
  2012年   233篇
  2011年   259篇
  2010年   157篇
  2009年   125篇
  2008年   196篇
  2007年   190篇
  2006年   201篇
  2005年   168篇
  2004年   188篇
  2003年   171篇
  2002年   166篇
  2001年   29篇
  2000年   19篇
  1999年   30篇
  1998年   38篇
  1997年   26篇
  1996年   38篇
  1995年   34篇
  1994年   25篇
  1993年   17篇
  1992年   26篇
  1991年   16篇
  1990年   17篇
  1989年   16篇
  1988年   15篇
  1987年   19篇
  1986年   16篇
  1985年   24篇
  1984年   20篇
  1983年   15篇
  1982年   29篇
  1981年   29篇
  1980年   18篇
  1979年   27篇
  1978年   26篇
  1977年   26篇
  1976年   22篇
  1975年   13篇
  1974年   15篇
  1973年   11篇
排序方式: 共有3682条查询结果,搜索用时 281 毫秒
101.
Conservation is an important organizational focus for zoos and aquariums. Organizational identity theory predicts a relationship between what is central to organizations, such as their mission statements, and their strategic activities. Based on this theory, we tested how organizational missions relate to their conservation strategies and practices. Tax forms and websites provided data from 173 zoos and 38 aquariums in the Association of Zoos and Aquariums in North America. We analyzed conservation mission strength, organizational characteristics, and project features with dependent variables representing the depth of organizational conservation commitments: amount of grants zoos funded, number of partner organizations, and number of projects per zoo. On tax forms, the average amount of total conservation grants donated to partner organizations per zoo was over $650,000, while the average number of partners reported on the tax forms was two. The content analysis of websites revealed an average of 14 conservation partners and 10 projects per zoo. Negative binomial regression models were significant. The financial analysis revealed that only the number of zoo personnel, as a surrogate for organization size, significantly predicted the amount of zoos’ conservation grants. Website analyses found increasing budgets, organizational involvement, and geographic reach of the projects predicted increasing numbers of conservation projects and partnerships. However, our findings did not support the hypothesized relationship—strength of organizational mission did not predict strategy and activities. Zoos could do more to strategically activate their organizational identities and conservation missions to achieve their conservation goals.  相似文献   
102.
Macroclimatic niches are indirect and potentially inadequate predictors of the realized environmental conditions that many species experience. Consequently, analyses of niche evolution based on macroclimatic data alone may incompletely represent the evolutionary dynamics of species niches. Yet, understanding how an organisms’ climatic (Grinnellian) niche responds to changing macroclimatic conditions is of vital importance for predicting their potential response to global change. In this study, we integrate microclimatic and macroclimatic data across 26 species of plethodontid salamanders to portray the relationship between microclimatic niche evolution in response to changing macroclimate. We demonstrate stronger phylogenetic signal in microclimatic niche variables than at the macroclimatic scale. Even so, we find that the microclimatic niche tracks climatic changes at the macroscale, but with a phylogenetic lag at million-year timescales. We hypothesize that behavioral tracking of the microclimatic niche over space and phenology generates the lag: salamanders preferentially select microclimates similar to their ancestral conditions rather than adapting with changes in physiology. We demonstrate that macroclimatic variables are weak predictors of niche evolution and that incorporating spatial scale into analyses of niche evolution is critical for predicting responses to climate change.  相似文献   
103.
104.
105.
106.
A predominant theme in the study of orchid evolution has been the importance of floral traits contributing to pollinator-mediated isolating barriers (i.e., floral isolation). However, few studies have quantified the contribution of floral isolation in sympatric orchid populations. Cypripedium parviflorum vars. makasin and pubescens are excellent taxa to test the strength of floral isolation because of their recent phylogenetic separation, overlapping flowering phenologies, and sympatric populations that can lack intermediate morphologies. In this paper, we use sympatric populations to (1) test for pre-mating and early-acting post-mating reproductive isolating barriers, (2) quantify genetic differentiation among populations using allozyme loci, and (3) document floral size differences between varieties. Pollen tracking experiments using fluorescent powders revealed minimal inter-varietal pollen movement. Across two sympatric sites, only 4 inter-varietal pollinations were observed among 52 var. makasin pollinations and 40 var. pubescens pollinations. All of these inter-varietal pollinations had var. makasin as the pollen donor. In contrast, artificial crosses within and between the varieties revealed no statistically significant differences in fruit set, seed weight, or embryo morphology. Allozyme data are consistent with restricted inter-varietal gene flow, showing strong allele frequency differences between the varieties across multiple loci. In a clustering analysis, these differences caused the varieties to group taxonomically and not by their geographic location. These data along with statistically significant morphological differences between the varieties strongly suggest the existence of a pollinator-mediated isolating barrier. The possible nature of the isolating barrier is discussed as well as the likelihood of other late acting post-zygotic barriers.  相似文献   
107.
NOX (NADPH oxidase) plays an important role during several pathologies because it produces the superoxide anion (O2•−), which reacts with NO (nitric oxide), diminishing its vasodilator effect. Although different isoforms of NOX are expressed in ECs (endothelial cells) of blood vessels, the NOX2 isoform has been considered the principal therapeutic target for vascular diseases because it can be up-regulated by inhibiting the interaction between its p47phox (cytosolic protein) and p22phox (transmembrane protein) subunits. In this research, two ethers, 4-(4-acetyl-2-methoxy-phenoxy)-acetic acid (1) and 4-(4-acetyl-2-methoxy-phenoxy)-butyric acid (2) and two esters, pentanedioic acid mono-(4-acetyl-2-methoxy-phenyl) ester (3) and heptanedioic acid mono-(4-acetyl-2-methoxy-phenyl) ester (4), which are apocynin derivatives were designed, synthesized and evaluated as NOX inhibitors by quantifying O2•− production using EPR (electron paramagnetic resonance) measurements. In addition, the antioxidant activity of apocynin and its derivatives were determined. A docking study was used to identify the interactions between the NOX2′s p47phox subunit and apocynin or its derivatives. The results showed that all of the compounds exhibit inhibitory activity on NOX, being 4 the best derivative. However, neither apocynin nor its derivatives were free radical scavengers. On the other hand, the in silico studies demonstrated that the apocynin and its derivatives were recognized by the polybasic SH3A and SH3B domains, which are regions of p47phox that interact with p22phox. Therefore this experimental and theoretical study suggests that compound 4 could prevent the formation of the complex between p47phox and p22phox without needing to be activated by MPO (myeloperoxidase), this being an advantage over apocynin.  相似文献   
108.
Monocyte chemoattractant protein-1 (MCP-1)-induced monocyte chemotaxis is a major event in inflammatory disease. Our prior studies have demonstrated that MCP-1-dependent chemotaxis requires release of arachidonic acid (AA) by activated cytosolic phospholipase A2 (cPLA2). Here we investigated the involvement of AA metabolites in chemotaxis. Neither cyclooxygenase nor lipoxygenase pathways were required, whereas pharmacologic inhibitors of both the cytochrome-P450 (CYP) and the soluble epoxide hydrolase (sEH) pathways blocked monocyte chemotaxis to MCP-1. To verify specificity, we demonstrated that the CYP and sEH products epoxyeiscosatrienoic acids (EETs) and dihydroxyeicosatrienoic acids (DHETs), respectively, restored chemotaxis in the presence of the inhibitors, indicating that sEH-derived products are essential for MCP-1-driven chemotaxis. Importantly, DHETs also rescued chemotaxis in cPLA2-deficient monocytes and monocytes with blocked Erk1/2 activity, because Erk controls cPLA2 activation. The in vitro findings regarding the involvement of CYP/sEH pathways were further validated in vivo using two complementary approaches measuring MCP-1-dependent chemotaxis in mice. These observations reveal the importance of sEH in MCP-1-regulated monocyte chemotaxis and may explain the observed therapeutic value of sEH inhibitors in treatment of inflammatory diseases, cardiovascular diseases, pain, and even carcinogenesis. Their effectiveness, often attributed to increasing EET levels, is probably influenced by the impairment of DHET formation and inhibition of chemotaxis.  相似文献   
109.
A survey was carried out between 2004 and 2005 in two ecologically different locations, Kakamega and Muhaka to assess diversity and abundance of wild host plants of lepidopteran stem borers as compared to maize plots during the cropping and non-cropping seasons. Kakamega in Western Kenya is characterized by a Guineo-Congolian rain forest mosaic and Muhaka at the Kenyan coast by a Zanzibar Inhambane mosaic with secondary grassy and woody vegetation. In Kakamega, wild host plants and maize covered 2 and 43% of the surveyed area. No variation in diversity and relative abundance of wild host plants was observed between both the cropping and non-cropping seasons. In Muhaka, the diversity and relative abundance of wild host plant species differed between seasons, with the Shannon Weaver Index (H) of 1.67 and 0.95 for cropping and non-cropping seasons, respectively. Similarly in this location, wild host plant cover varied between cropping (23%) and noncropping (17.9%). During both seasons, this was higher than the maize cover, with 10.7% and 0% for the cropping and non-cropping seasons, respectively. For both localities, the implication of the differences found in the abundance and diversity between the cropping and non-cropping seasons is discussed.  相似文献   
110.
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