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   156篇
  免费   5篇
  2017年   3篇
  2015年   2篇
  2013年   3篇
  2012年   3篇
  2011年   7篇
  2010年   7篇
  2009年   9篇
  2008年   4篇
  2007年   6篇
  2006年   4篇
  2005年   7篇
  2004年   4篇
  2003年   7篇
  2002年   2篇
  2001年   3篇
  1999年   1篇
  1998年   6篇
  1997年   5篇
  1996年   4篇
  1995年   2篇
  1994年   3篇
  1993年   2篇
  1992年   5篇
  1991年   1篇
  1990年   6篇
  1989年   2篇
  1988年   3篇
  1987年   2篇
  1986年   6篇
  1985年   3篇
  1984年   2篇
  1983年   3篇
  1982年   2篇
  1981年   4篇
  1980年   3篇
  1979年   1篇
  1978年   3篇
  1977年   2篇
  1975年   2篇
  1974年   1篇
  1973年   7篇
  1971年   1篇
  1968年   1篇
  1958年   1篇
  1956年   3篇
  1955年   1篇
  1950年   1篇
  1946年   1篇
排序方式: 共有161条查询结果,搜索用时 203 毫秒
11.
The family Ciehlidae is a large group of tropical fishes in the order Perciformes, with an estimated number of living species exceeding 1400. The modern distribution of the family Ciehlidae is predominantly in fresh waters of Central and South America, Africa, Madagascar, India and the Middle East, with fossil members known from Africa, Saudi Arabia, the Levant, Europe, South America and Haiti. Many authors have referred to the distribution as being Gondwanan and have postulated that cichlids originated over 130 million years ago, in the Early Cretaceous. However, the suggested evidence for an Early Cretaceous origin of cichlids is equally or more compatible with a much younger age of origin. Based on the biology and distribution of modern and fossil cichlids, it is more probable that they arose less than 65 million years ago, in the Early Tertiary, and crossed marine waters to attain their current distribution.  相似文献   
12.
Understanding the relative magnitudes of inbreeding and outbreeding depression in rare plant populations is increasingly important for effective management strategies. There may be positive and negative effects of crossing individuals in fragmented populations. Conservation strategies may include introducing new genetic material into rare plant populations, which may be beneficial or detrimental based on whether hybrid offspring are of increased or decreased quality. Thus, it is important to determine the effects of pollen source on offspring fitness in rare plants. We established pollen crosses (i.e. geitonogamous‐self, autonomous‐self, intrasite‐outcross, intersite‐outcross and open‐pollinated controls) to determine the effects of pollen source on fitness (seeds/fruit and seed mass) and early offspring traits (probability of germination, number of leaves, leaf area and seedling height) in the rare plant Polemonium vanbruntiae. Open‐pollinated, intrasite‐outcross and geitonogamous‐self treatments did not differ in fitness. However, plants receiving autonomous‐self pollen had the lowest fitness and the lowest probability of seed germination. Intersite‐outcross plants contained fewer seeds/fruit, but seeds germinated at higher frequencies and seedlings were more vigorous. We also detected heterosis at the seed germination stage. These data may imply that natural populations of P. vanbruntiae exhibit low genetic variation and little gene flow. Evidence suggests that deleterious alleles were not responsible for reduced germination; rather environmental factors, dichogamy, herkogamy and/or lack of competition among pollen grains may have caused low germinability in selfed offspring. Although self‐pollination may provide some reproductive assurance in P. vanbruntiae, the result is a reduction in germination and size‐related early traits for selfed offspring.  相似文献   
13.
Energy crops offer an opportunity to substantially increase bioenergy resources which can replace rapidly depleting fossil fuel reserves and mitigate the effect of climate change. Energy crops are typically established within traditional agricultural systems such as tillage land or grassland. Associated land use conversion has environmental implications. The aim of this paper is to propose a framework to examine how such environmental implications can be assessed, based on (a) a Strategic Environmental Assessment (SEA) approach which considers potential impacts at different stages of a plan across a wide range of environmental receptors and (b) a literature review. The example we used was that of Miscanthus replacing grassland farming. This scenario is particularly relevant to Ireland, where over 90% of the agricultural land is permanent pasture, but is also applicable to grassland conversion throughout Europe and the United States. Two consecutive phases of land‐use change were identified for assessment, each with a distinct set of environmental impacts. The first was a transition phase, lasting from initial livestock clearance and grassland ploughing until the Miscanthus crop became established (2–3 years). The second phase was the mature crop phase, lasting up to 25 years. Miscanthus cultivation was more likely to impact negatively on the environment during the transition phase than the mature phase, primarily due to abrupt disturbance and the time required for a new equilibrium to establish. However, a literature review of the impact on the environmental receptors revealed that replacing Irish agricultural grassland with Miscanthus had the potential to improve biodiversity, water, air and soil quality, and climatic factors once the crop became established and reached maturity. In order to confirm these findings an appropriate monitoring programme involving objectives and indicators associated with each environmental receptor would need to be developed.  相似文献   
14.
The diagnosis of rare genetic disorders involves classifying patients on the basis of clinical understandings of the relationship between visible, measurable characteristics (the phenotype) and an underlying genetic characterization (the genotype). Diagnosis is performed through the routine ritual observation and interpretation of images of patients in relation to their clinical features and test results. In this process, which is mediated by senior clinicians, the experience of the effect of a patient's image is integral to the skilful elicitation of a diagnosis. Such skill develops over time, through regular participation in diagnostic performances. I suggest that, from this perspective, genetic diagnosis is a clinical process rather than a form of genetic reductionism and that assumptions about 'geneticization' require further exploration. The article also shows that participant observation and a cross-cultural mode of analysis can contribute to documenting and interpreting these processes.  相似文献   
15.
This study compares the population genetic structure of two obligate parthenogenetic sawfly species, Aneugmenus padi (L.) Zhelochovtsev and Eurhadinoceraea ventralis (Panzer) Enslin (Hymenoptera: Tenthredinidae). Allozymes were used to detect genetic differences in larvae collected at different sites in six European countries. For A. padi , scoring six polymorphic enzymes revealed the existence of five dominant, widely distributed clones and several more with only very few individuals occurring locally. The clonal diversity and identity differed across collection sites, with up to at maximum eight clones coexisting at a single locality. In contrast, in E. ventralis , individuals from different localities were all monomorphic for at least six different enzymes and are therefore assumed to belong to the same clone. Differences in population genetic structure of these sawfly species can be related to the differing distributions of their host plants: the fern, Pteridium aquilinum , the host of A. padi , has been an invasive species for more than two hundred years. Spread of clones of A. padi is likely to have closely followed the complex invasive pattern of spread of the plant. Larvae of E. ventralis were collected from local, horticultural plantings of Clematis spp. of recent origin probably with gardeners having traded plants infested with the sawfly.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 83 , 219–227.  相似文献   
16.
17.
The rise of the early intervention paradigm in psychotic disorders represents a maturing of the therapeutic approach in psychiatry, as it embraces practical preventive strategies which are firmly established in mainstream health care. Early intervention means better access and systematic early delivery of existing and incremental improvements in knowledge rather than necessarily requiring dramatic and elusive breakthroughs. A clinical staging model has proven useful and may have wider utility in psychiatry. The earliest clinical stages of psychotic disorder are non-specific and multidimensional and overlap phenotypically with the initial stages of other disorders. This implies that treatment should proceed in a stepwise fashion depending upon safety, response and progression. Withholding treatment until severe and less reversible symptomatic and functional impairment have become entrenched represents a failure of care. While early intervention in psychosis has developed strongly in recent years, many countries have made no progress at all, and others have achieved only sparse coverage. The reform process has been substantially evidence-based, arguably more so than other system reforms in mental health. However, while evidence is necessary, it is insufficient. It is also a by-product as well as a catalyst of reform. In early psychosis, we have also seen the evidence-based paradigm misused to frustrate overdue reform. Mental disorders are the chronic diseases of the young, with their onset and maximum impact in late adolescence and early adult life. A broader focus for early intervention would solve many of the second order issues raised by the early psychosis reform process, such as diagnostic uncertainty despite a clear-cut need for care, stigma and engagement, and should be more effective in mobilizing community support. Early intervention represents a vital and challenging project for early adopters in global psychiatry to consider.  相似文献   
18.
19.
The piecemeal development of capitalist socioeconomic systems in the colonial Chesapeake was deeply intertwined with projects of white ethnogenesis. Crafting a sense of "groupness" along lines perceived as racial required free "whites" to remain economically and socially interdependent. A variety of strategies and material forms—including reciprocal exchanges, hall-parlor house plans, and earthfast construction—facilitated this cohesion. Such integrative tactics coexisted in colonists' behavioral repertoires with more "capitalistic" strategies that prioritized private profit over social obligation. Colonists' deployment of diverse social strategies reflects a complex calculus assessing the benefits of economic autonomy against the benefits of ethnic ("white") solidarity. These dynamics can be illustrated through an 18th-century archaeological site at Flowerdew Hundred in the Chesapeake.  相似文献   
20.
SYNOPSIS. Notification is given of the deposit of paratype material of 36 species of Myxosporida. described by Prof. R. R. Kudo, with the U. S. National Museum.  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