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   245篇
  免费   7篇
  国内免费   2篇
  2017年   1篇
  2016年   2篇
  2015年   3篇
  2013年   4篇
  2012年   11篇
  2011年   14篇
  2010年   20篇
  2009年   18篇
  2008年   14篇
  2007年   24篇
  2006年   17篇
  2005年   8篇
  2004年   11篇
  2003年   4篇
  2002年   5篇
  2001年   3篇
  1999年   6篇
  1998年   9篇
  1997年   10篇
  1996年   5篇
  1995年   3篇
  1994年   3篇
  1993年   4篇
  1992年   5篇
  1991年   4篇
  1990年   3篇
  1989年   7篇
  1988年   3篇
  1987年   2篇
  1986年   1篇
  1985年   2篇
  1984年   1篇
  1983年   1篇
  1982年   2篇
  1980年   3篇
  1979年   1篇
  1978年   1篇
  1975年   2篇
  1974年   1篇
  1971年   2篇
  1959年   2篇
  1958年   4篇
  1956年   1篇
  1955年   4篇
  1953年   1篇
  1949年   2篇
排序方式: 共有254条查询结果,搜索用时 15 毫秒
81.
82.
Scoliosis is a three-dimensional deformity characterized by coronal, sagittal and axial rotation of the spine. Surgical fusion of the spine is required in severe cases. Assessment of the surgical procedure requires enough accuracy and flexibility to allow planning of individual interventions or implant designs. Conventional 2-D radiography and even 3-D CT scanning have limitations for in-depth analysis of scoliosis that limit the ability to see the three-dimensional deformity and expose the patient to considerable doses of radiation, respectively. Our stereophotogrammetric analysis is able to provide accurate, intraoperative measurement of vertebral movement during surgical manuevres. Stereophoto pairs taken at each stage of the operation and robust statistical techniques can be used to determine rotation, translation, goodness of fit, and overall spinal contour before, during, and after the surgical instrumentation. A demonstration of data available from this system is included.  相似文献   
83.
The public promotion of renewable energies is expected to increase the number of biogas plants and stimulate energy crops cultivation (e.g. maize) in Germany. In order to assess the indirect effects of the resulting land‐use changes on biodiversity, we developed six land‐use scenarios and simulated the responses of six farmland wildlife species with the spatially explicit agent‐based model system ALMaSS. The scenarios differed in composition and spatial configuration of arable crops. We implemented scenarios where maize for energy production replaced 15% and 30% of the area covered by other cash crops. Biogas maize farms were either randomly distributed or located within small or large aggregation clusters. The animal species investigated were skylark (Alauda arvensis), grey partridge (Perdix perdix), European brown hare (Lepus europaeus), field vole (Microtus agrestis), a linyphiid spider (Erigone atra) and a carabid beetle (Bembidion lampros). The changes in crop composition had a negative effect on the population sizes of skylark, partridge and hare and a positive effect on the population sizes of spider and beetle and no effect on the population size of vole. An aggregated cultivation of maize amplified these effects for skylark. Species responses to changes in the crop composition were consistent across three differently structured landscapes. Our work suggests that with the compliance to some recommendations, negative effects of biogas‐related land‐use change on the populations of the six representative farmland species can largely be avoided.  相似文献   
84.
Nine different classifications have been produced in the last 70 years for the horticulturally valuable genus Cyclamen , a small genus with fewer than 30 species. These classifications, generated by intuitive methods and cladistic analyses, incorporated a total of four infrageneric ranks above that of species and were based on data from morphology, cytology and DNA sequencing. Our results, based on cladistic analyses of three independent data sources − nrDNA ITS, cpDNA trn L intron and morphological data − reveal good resolution only in nrDNA sequence data. However, when these three data sources are combined they provide stronger resolution and support for three major clades, only one of which, subgenus Psilanthum , has been consistently supported in previous classifications. The differing infrageneric classifications produced in Cyclamen result from varying taxon sampling, differing interpretation of morphological data, changes in the sources and analysis of data, and inconsistent application of names. Extensive subdivision of small genera in the absence of adequate data that could provide evidence for consistent patterns of relationship is premature and leads to a proliferation of names.  © 2004 The Linnean Society of London, Botanical Journal of the Linnean Society , 2004, 146 , 339–349.  相似文献   
85.
  • 1 The European red fox Vulpes vulpes represents a continuing threat to both livestock and native vertebrates in Australia, and is commonly managed by setting ground‐level baits impregnated with 1080 (sodium fluoroacetate) poison. However, the long‐term effectiveness of such control campaigns is likely to be limited due to the ability of foxes to disperse over considerable distances and to swiftly recolonize areas from where they had been removed.
  • 2 To investigate the effectiveness of fox baiting in a production landscape, we assessed the potential for foxes to reinvade baited farm property areas within the jurisdiction of the Molong Rural Lands Protection Board (RLPB), an area of 815 000 ha on the central tablelands of New South Wales, Australia. The spatial distribution and timing of fox baiting campaigns between 1998 and 2002 was estimated from RLPB records and mapped using Geographical Information System software. The effectiveness of the control campaign was assessed on the basis of the likely immigration of foxes from non‐baited farms using immigration distances calculated from published relationships between dispersal distance and home range size.
  • 3 Few landholders undertook baiting campaigns in any given year, and the area baited was always so small that no baited property would have been sufficiently far from an unbaited property to have been immune from immigrating individuals. It is likely, therefore, that immigration onto farms negated any long‐term effects of baiting operations. This study highlights some of the key deficiencies in current baiting practices in south‐eastern Australia and suggests that pest management programmes should be monitored using such methods to ensure they achieve their goals.
  相似文献   
86.
Passerines are especially vulnerable to predation at the pre-independence stage. Although the role of nest success in British farmland passerine declines is contentious, improvement in nest success through sympathetic management could play a role in their reversal. Because habitat is known to interact with predation, management options for mitigation will need to consider effects of nest predation. We present results from an observational study of a population of Common Blackbird Turdus merula on a farm which has experienced a range of agri-environment and game-management options, including a period with nest predator control, as a case study to address some of these issues. We used an information theoretic model comparison procedure to look for evidence of interactions between habitat and nest predation, and then asked whether habitat management and nest predator abundances could explain population trends at the site through their effects on nest success. Interactions were detected between measures of predator abundance and habitat variables, and these varied with nest stage – habitat within the vicinity of the nest appeared to be important at the egg stage, and nest-placement characteristics were important at the nestling stage. Although predator control appeared to have a positive influence on Blackbird breeding population size, the non-experimental set-up meant we could not eliminate other potential explanations. Variation in breeding population size did not appear to be influenced by variation in nest success alone. Our study demonstrates that observational data can only go so far in detection of such effects, and we discuss how it might be taken further. Agri-environment and game-management techniques are likely to influence nest predation pressure on farmland passerines, but the patterns, mechanisms and importance to population processes remain not wholly understood.  相似文献   
87.
魔芋丛生芽诱导成苗及生根技术研究   总被引:1,自引:0,他引:1  
采用不同激素配方的1-9号培养基,探索了由魔芋丛生芽诱导试管苗的最佳培养基配方。试验结果表明:MS+6BA1.5 mg.L-1+NAA0.5 mg.L-1+IBA1.0 mg.L-1和MS+NAA1.0 mg.L-1+PP3331.0 mg.L-1两种培养基对于诱导出苗及生根效果最好,添加PP333不仅可加快出苗生根,还可促使试管苗矮壮。  相似文献   
88.
Expression of the adhesion molecules, ICAM-1, VCAM-1, NCAM, CD44, CD49d (VLA-4, a chain), and CDlla (LFA-1, a chain) on mouse oocytes, and pre- and peri-implantation stage embryos was examined by quantitative indirect immunofluorescence microscopy. ICAM-1 was most strongly expressed at the oocyte stage, gradually declining almost to undetectable levels by the expanded blastocyst stage. NCAM, also expressed maximally on the oocyte, declined to undetectable levels beyond the morula stage. On the other hand, CD44 declined from highest expression at the oocyte stage to show a second maximum at the compacted 8-cell/morula. This molecule exhibited high expression around contact areas between trophecto-derm and zona pellucida during blastocyst hatching. CD49d was highly expressed in the oocyte, remained significantly expressed throughout and after blastocyst hatching was expressed on the polar trophecto-derm. Like CD44, CD49d declined to undetectable levels at the blastocyst outgrowth stage. Expression of both  相似文献   
89.
90.
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