全文获取类型
收费全文 | 552篇 |
免费 | 33篇 |
国内免费 | 9篇 |
出版年
2023年 | 14篇 |
2022年 | 8篇 |
2021年 | 14篇 |
2020年 | 18篇 |
2019年 | 49篇 |
2018年 | 34篇 |
2017年 | 22篇 |
2016年 | 14篇 |
2015年 | 15篇 |
2014年 | 25篇 |
2013年 | 70篇 |
2012年 | 24篇 |
2011年 | 23篇 |
2010年 | 15篇 |
2009年 | 18篇 |
2008年 | 21篇 |
2007年 | 15篇 |
2006年 | 22篇 |
2005年 | 19篇 |
2004年 | 13篇 |
2003年 | 13篇 |
2002年 | 8篇 |
2001年 | 10篇 |
2000年 | 9篇 |
1999年 | 11篇 |
1998年 | 10篇 |
1997年 | 4篇 |
1996年 | 5篇 |
1995年 | 5篇 |
1994年 | 3篇 |
1993年 | 8篇 |
1992年 | 1篇 |
1991年 | 7篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 6篇 |
1980年 | 4篇 |
1978年 | 1篇 |
1977年 | 4篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有594条查询结果,搜索用时 203 毫秒
1.
The results of a detailed analysis of 100 supernumerary limbs generated by 180° ipsilateral rotation (on the same limb stump) of regeneration blastemas is presented. The limbs were analyzed in terms of their position of origin, frequency, cartilage structure by Victoria blue staining, and muscle structure by serial sections. Single, double, or triple supernumeraries can be produced at no unique position of origin, although the posterodorsal quadrant was preferred. Four classes of supernumerary limbs were generated by such operations—normal; double dorsal or double ventral; part normal/part mirror imaged; part normal/part inverted in approximately equal frequencies. After amputation of these supernumeraries the same muscle patterns are faithfully regenerated. A hypothesis to explain the production of these abnormal limbs is proposed based on the observed phenomenon of fusion of supernumerary blastemata, but their regenerative behaviour presents problems for current models of pattern formation. Similar results have been obtained with developing limb buds and the relation between development and regeneration is discussed. 相似文献
2.
Felipe Silva de Andrade Thiago Ribeiro de Carvalho Lucas Borges Martins Ariovaldo Antonio Giaretta 《Studies on Neotropical Fauna and Environment》2017,52(2):157-167
In this study, we re-describe the advertisement and territorial calls of Boana pardalis, carry out an acoustic comparison between the studied species and the other congeners of the B. faber group, and report for the first time the tympanic amplexus for the studied species. The advertisement call of B. pardalis can be used to supplement its diagnosis in the B. faber group based on temporal call traits, e.g. emission rate and emission pattern, as well as the call envelope. 相似文献
3.
ANDREA S. WILEY 《American anthropologist》2007,109(4):666-677
Large-scale milk production and consumption historically have been localized to Europe and countries with large European-derived populations. However, global patterns have now shifted, with dramatic increases in milk consumption in Asian countries and flat or declining consumption in European and European-derived countries. Efforts to market it around the world emphasize milk's positive effects on child growth, and, by extension, the individual and national benefits that derive from that growth. At the same time, milk has newly emerged in milk promotions in the United States as food that facilitates weight loss. Milk has been able to achieve a global presence and continuing relevance in populations in which its consumption has been declining by continually transforming and repositioning itself as a "special" food with properties able to alleviate the health concerns seen as most salient at the time. 相似文献
4.
The floral biology ofTetrastylis ovalis and its pollination by bats was studied in southeastern Brazil. The flowers ofT. ovalis seem more fitted to bat-pollination than those ofPassiflora mucronata, the other known chiropterophilous passionflower. Two species of bats were observed visiting flowers ofT. ovalis: the phyllostomidsGlossophaga soricina andRhinophylla pumilio. Due to the attributes ofTetrastylis, its presumed primitiveness and a better defined bat-pollination syndrome we suggest that chiropterophily evolved independently in the former genus and inPassiflora. 相似文献
5.
This study is the first report of an algal disease, developed in close association with an endophytic organism, documented for the southeastern Pacific. We describe a disease affecting wild populations of the red alga Iridaea laminarioides Bory in central Chile, characterized by gall development on the surface of sporophytic, cystocarpic, and immature thalli. These abnormal growths result in severe morphological alterations of the affected thalli. Diseased fronds display an aggregated spatial distribution and occur throughout the year, with a maximum in summer followed by a decline in winter. The presence of galls was not associated with broken or torn fronds. Although causality has not been unequivocally demonstrated, our field and laboratory observations indicate a strong association of the galls with infections by an endophytic cyanobacterium, probably belonging to the genus Pleurocapsa. 相似文献
6.
Marjorie M. Holland 《Hydrobiologia》1993,251(1-3):331-340
Evidence suggests that the boundaries between rivers, floodplain wetlands, and adjacent upland communities are among the most important components of landscapes. In a landscape context, floodplain wetlands and their ecotones are important transition zones between uplands and aquatic ecosystems. Management of wetland patches and ecotones to achieve sustainable ecosystems requires action on a broad scale, giving consideration to all factors affecting wetlands and the drainage basins of which they are a part. In northeastern United States, a struggle to identify new sources of water has focussed attention on the sustainability of aquatic ecosystems. 相似文献
7.
Paula Meli Daniella Schweizer Leigh A. Winowiecki Susan Chomba Ermias Aynekulu Manuel R. Guariguata 《Restoration Ecology》2023,31(1):e13810
The strategy of the United Nations Decade on Ecosystem Restoration identifies three pathways for action for overcoming six global barriers thought to hamper upscaling. We evaluated 6,023 peer-reviewed and gray literature papers published over the last two decades to map the information landscape underlying the barriers and associated pathways for action across world regions, terrestrial ecosystem types, restorative interventions and their outcomes. Overall, the literature addressed more the financial and legislative barriers than the technical and research-related ones, supporting the view that social, economic and political factors hamper scaling up ecosystem restoration. Latin America, Africa, and North America were the most prominent regions in the literature, yet differed in the number of publications addressing each barrier. An overwhelming number of publications focused on forests (78%), while grasslands (6%), drylands (3%), and mangroves (2%) received less attention. Across the three pathways for action, the action lines on (1) promoting long-term ecosystem restoration actions and monitoring and (2) education on restoration were the most underrepresented in the literature. In general, restorative interventions assessed rendered positive outcomes except those of a political, legislative or financial nature which reported negative or inconclusive outcomes. Our indicative assessment reveals critical information gaps on barriers, pathways, and types of restorative interventions across world regions, particularly related to specific social issues such as education for ecosystem restoration. Finally, we call for refining “strength of evidence” assessment frameworks that can systematically appraise, synthesize and integrate information on traditional and practitioner knowledge as two essential components for improving decision-making in ecosystem restoration. 相似文献
8.
Dana A. Procsal Helen L. Henry Ernest J. Friedlander Anthony W. Norman 《Archives of biochemistry and biophysics》1977,179(1):229-234
The biological activity of 1α,24R,25-trihydroxyvitamin D3 [1α,24R,25(OH)3D3] was elevated in comparison to the hormonally active form of vitamin D3, 1α,25-dihydroxyvitamin D3 [1α,25(OH)2D3], in the rachitic chick in terms of its ability to (a) stimulate intestinal calcium absorption, (b) mobilize bone calcium, (c) induce intestinal calcium binding protein, (d) modulate the level of enzyme activity of the renal 25-OH-D3-1-hydroxylase system, and (e) interact with the intestinal cystosol-chromatin receptor system for the 1α,25(OH)2D3 receptor system. In each of these assays, the relative ratio of activity of 1α,24R,25(OH)3D3 to 1α,25(OH)2D3was (a) 25–50, (b) ca. 20, (c) 10, (d) 50, and (e) 36%, respectively. 相似文献
9.
Angel López Piñeiro Enrique Saiz 《International journal of biological macromolecules》1983,5(1):37-41
The Rotational Isomeric States model is applied to calculate dipole moments of polypeptides of the twenty natural α-amino acids in the random coil state. Dipole moments of each repeat unit (μi), are evaluated using a quantum mechanics procedure. Dipole moment ratios () of homopolypeptides are calculated and extrapolated to x →?. With a few exceptions, D? = 0.36 ± 0.1. Ten actual proteins and three enzymes are also studied; their dipole ratios (Dx′ =〈μ〉/x) range from 7.34 to 10.57 in 10?59 C2 m2 (6.6–9.5 D2). Diffferences in the values of Dx′ are due mainly to the different contributions, μi, of the amino acid residues contained in each polymer, whereas the sequence of amino acids has a very minor effect. 相似文献
10.
桂东、桂东南晚泥盆世一个弗拉斯期放射虫动物群 总被引:4,自引:2,他引:2
本文系统描述了广西玉林、贺县等地晚泥盆世榴江组硅质岩中的放射虫化石4属14种,其中有1新属2新种。这个放射虫动物群的多数分子曾见于苏联南乌拉尔地区和澳大利亚的弗拉斯阶。 相似文献