全文获取类型
收费全文 | 1130篇 |
免费 | 104篇 |
国内免费 | 2篇 |
出版年
2023年 | 6篇 |
2022年 | 18篇 |
2021年 | 30篇 |
2020年 | 17篇 |
2019年 | 21篇 |
2018年 | 29篇 |
2017年 | 18篇 |
2016年 | 34篇 |
2015年 | 49篇 |
2014年 | 42篇 |
2013年 | 62篇 |
2012年 | 87篇 |
2011年 | 66篇 |
2010年 | 43篇 |
2009年 | 36篇 |
2008年 | 47篇 |
2007年 | 50篇 |
2006年 | 35篇 |
2005年 | 51篇 |
2004年 | 44篇 |
2003年 | 39篇 |
2002年 | 34篇 |
2001年 | 23篇 |
2000年 | 30篇 |
1999年 | 20篇 |
1998年 | 11篇 |
1997年 | 11篇 |
1996年 | 15篇 |
1995年 | 12篇 |
1994年 | 11篇 |
1993年 | 10篇 |
1992年 | 15篇 |
1991年 | 13篇 |
1990年 | 18篇 |
1989年 | 14篇 |
1988年 | 6篇 |
1987年 | 7篇 |
1986年 | 9篇 |
1985年 | 21篇 |
1984年 | 11篇 |
1983年 | 9篇 |
1982年 | 13篇 |
1981年 | 10篇 |
1979年 | 12篇 |
1978年 | 9篇 |
1977年 | 5篇 |
1974年 | 9篇 |
1971年 | 4篇 |
1970年 | 6篇 |
1949年 | 3篇 |
排序方式: 共有1236条查询结果,搜索用时 15 毫秒
171.
Scanning electron microscopy of Acholeplasma colonies on agar 总被引:1,自引:0,他引:1
The colony formation and morphology of Acholeplasma laidlawii and of an Acholeplasma species was studied by scanning electron microscopy. In the colonies of both Acholeplasma spp. large irregularly shaped cells, spherical cells, chains of beads, and long filaments with small bulbous distensions were seen. The membrane of some of the large cells seemed to be perforated, giving the cell a pitted appearance. 相似文献
172.
Aerobiologia - Bioaerosols have gained importance in atmospheric sciences in the past few decades. Their exposure upsurges the health problems in sensitive population and is also known to... 相似文献
173.
The daily construction of a sleeping platform or “nest” is a universal behavior among large-bodied hominoids. Among chimpanzees, most populations consistently select particular tree species for nesting, yet the principles that guide species preferences are poorly understood. At Semliki, Cynometra alexandri constitutes only 9.6% of all trees in the gallery forest in which the study populations ranges, but it was selected for 73.6% of the 1,844 chimpanzee night beds we sampled. To determine whether physical properties influence nesting site selection, we measured the physical characteristics of seven common tree species at the Toro-Semliki Wildlife Reserve, Uganda. We determined stiffness and bending strength for a sample of 326 branches from the seven most commonly used tree species. We selected test-branches with diameters typically used for nest construction. We measured internode distance, calculated mean leaf surface area (cm2) and assigned a tree architecture category to each of the seven species. C. alexandri fell at the extreme of the sample for all four variables and shared a tree architecture with only one other of the most commonly selected species. C. alexandri was the stiffest and had the greatest bending strength; it had the smallest internode distance and the smallest leaf surface area. C. alexandri and the second most commonly selected species, Cola gigantea, share a ‘Model of Koriba’ tree architecture. We conclude that chimpanzees are aware of the structural properties of C. alexandri branches and choose it because its properties afford chimpanzees sleeping platforms that are firm, stable and resilient. 相似文献
174.
175.
176.
Mariano C. Chabi Anicet G. Dassou Innocent Dossou-Aminon David Ogouchoro Bonaventure Omondi Aman Alexandre Dansi 《Journal of ethnobiology and ethnomedicine》2018,14(1):78
Background
The cultivated banana and plantain (Musa spp.) are valuable for nutritional and socio-economic security for millions of people worldwide. In Benin, banana and plantain are among the most produced, consumed, and traded commodities. Its production is mainly for local consumption and remains insufficient to the demand. However, the varietal diversity of banana and plantain cultivated in Benin is not documented. This study aims at characterizing the banana and plantain cropping systems, genetic diversity, and production constraints as a baseline to the full utilization of this resource in crop improvement and to identify the potential production and agronomic qualities.Methods
A baseline investigation of ethnobotanical attributes of banana cultivars was done in 51 randomly chosen villages in southern Benin. Interviews with randomly selected representative farmers were carried out. Key informant interviews and focus group discussions were used for global confirmatory investigation of survey data. Socio-demographic data and indigenous knowledge on the farmer uses of banana and plantain diversity, such as cultural practices, origin, and availability of banana and plantain planting materials, and the constraints and criteria of varietal preference cited by farmers were ranked.Results
Eighty-seven locally recognized cultivars were found: 73 of banana and 14 of plantain groups. The most popular cultivars were Sotoumon (banana) (52.94%), Aloga (plantain) (41.17%), Planta (banana) (33.33%), and Adjangan (plantain) (27.45%). Of the eleven production constraints identified, the main biotic challenges were banana weevil Cosmopolites sordidus Germar and banana bunchy top virus (BBTV), while abiotic problems were drought and the wind. Some local varieties like Amandan, Assonwonnou, Coleti, and Ninkouin are extremely rare owing to agronomic and economic preference perceptions.Conclusion and implications
This study provides a baseline for banana diversity in Benin and the West African region and entry points for biological characterization and production improvement. This would enable the exploitation of this resource for plant breeding towards biotic and abiotic challenges facing banana production.177.
Coupling Input‐Output Tables with Macro‐Life Cycle Assessment to Assess Worldwide Impacts of Biofuels Transport Policies 下载免费PDF全文
Audrey Somé Thomas Dandres Caroline Gaudreault Guillaume Majeau‐Bettez Richard Wood Réjean Samson 《Journal of Industrial Ecology》2018,22(4):643-655
Many countries see biofuels as a replacement to fossil fuels to mitigate climate change. Nevertheless, some concerns remain about the overall benefits of biofuels policies. More comprehensive tools seem required to evaluate indirect effects of biofuel policies. This article proposes a method to evaluate large‐scale biofuel policies that is based on life cycle assessment (LCA), environmental extensions of input‐output (I‐O) tables, and a general equilibrium model. The method enables the assessment of indirect environmental effects of biofuels policies, including land‐use changes (LUCs), in the context of economic and demographic growth. The method is illustrated with a case study involving two scenarios. The first one describes the evolution of the world economy from 2006 to 2020 under business as usual (BAU) conditions (including demographic and dietary preferences changes), and the second integrates biofuel policies in the United States and the European Union (EU). Results show that the biofuel scenario, originally designed to mitigate climate change, results in more greenhouse gas emissions when compared to the BAU scenario. This is mainly due to emissions associated with global LUCs. The case study shows that the method enables a broader consideration for environmental effects of biofuel policies than usual LCA: Global economic variations calculated by a general equilibrium economic model and LUC emissions can be evaluated. More work is needed, however, to include new biofuel production technologies and reduce the uncertainty of the method. 相似文献
178.
Jens C. Frisvad Robert a. Samson Birgitte R. Rassing Marjolein I. van der Horst F.T.J. van Rijn J. Stark 《Antonie van Leeuwenhoek》1997,72(2):119-126
The new species Penicillium discolor, frequently isolated from nuts, vegetables and cheese is described. It is characterised by rough, dark green conidia, synnemateous growth on malt agar and the production of the secondary metabolites chaetoglobosins A, B and C, palitantin, cyclopenin, cyclopenol, cyclopeptin, dehydrocyclopeptin, viridicatin and viridicatol. It also produces the mouldy smelling compounds geosmin and 2-methyl-isoborneol, and a series of specific orange to red pigments on yeast extract sucrose agar, hence the epithet discolor. P. discolor resembles P. echinulatum morphologically but on basis of the secondary metabolites is also related to P. expansum, P. solitum and P. crustosum. 相似文献
179.
Samson A. Chow 《Methods (San Diego, Calif.)》1997,12(4):306-317
Integration of retroviral DNA, an essential step during the retroviral life cycle, is mediated by the viral protein integrase. Simplein vitroassays for measuring integrase activities are described, including catalysis (3′-end processing, 3′-end joining, disintegration), juxtaposition of viral DNA ends, DNA binding, and target site selection. The described assays will be useful in elucidating the molecular mechanism of retroviral integration and screening for integrase inhibitors as potential anti-retroviral drugs. 相似文献
180.
Nathalie Samson Marie St-Hilaire Elise Nsegbe Philippe Reix Fran?ois Moreau-Bussière Jean-Paul Praud 《Journal of applied physiology》2005,99(5):1636-1642
The present study was aimed at investigating the effects of nasal continuous positive airway pressure (nCPAP; 6 cmH2O) or intermittent positive pressure ventilation (nIPPV; 10/4 cmH2O) on nonnutritive swallowing (NNS) and on the coordination between NNS and phases of the respiratory cycle, while taking into account the potential effects of states of alertness. Twelve full-term lambs were chronically instrumented at 48 h after birth for polysomnographic recordings, including NNS, diaphragm electromyographic activity, respiratory movements, pulse oximetry, and states of alertness. Studies in control conditions, with nCPAP and nIPPV, were performed in random order in nonsedated lambs at 4, 5, and 6 days of life. Results demonstrate that nCPAP significantly decreased overall NNS frequency, more specifically isolated NNS during quiet sleep and bursts of NNS in active sleep. In comparison, the effects of nIPPV on NNS frequency were more variable, with an inhibition of NNS only in wakefulness and an increase in isolated NNS frequency in active sleep. In addition, neither nCPAP nor nIPPV disrupted the coordination between NNS and phases of the respiratory cycle. In conclusion, nCPAP inhibits NNS occurrence in newborn lambs. Clinical relevance of this novel finding is related to the importance of NNS for clearing the upper airways from secretions and gastric content frequently regurgitated in the neonatal period. 相似文献