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   6803篇
  免费   1105篇
  国内免费   1247篇
  2024年   7篇
  2023年   242篇
  2022年   187篇
  2021年   330篇
  2020年   493篇
  2019年   490篇
  2018年   411篇
  2017年   429篇
  2016年   412篇
  2015年   398篇
  2014年   399篇
  2013年   458篇
  2012年   330篇
  2011年   313篇
  2010年   270篇
  2009年   364篇
  2008年   330篇
  2007年   390篇
  2006年   344篇
  2005年   304篇
  2004年   226篇
  2003年   222篇
  2002年   218篇
  2001年   180篇
  2000年   189篇
  1999年   161篇
  1998年   136篇
  1997年   110篇
  1996年   87篇
  1995年   97篇
  1994年   76篇
  1993年   71篇
  1992年   60篇
  1991年   58篇
  1990年   53篇
  1989年   39篇
  1988年   35篇
  1987年   22篇
  1986年   23篇
  1985年   32篇
  1984年   30篇
  1983年   20篇
  1982年   35篇
  1981年   19篇
  1980年   18篇
  1979年   10篇
  1978年   8篇
  1977年   5篇
  1976年   5篇
  1975年   5篇
排序方式: 共有9155条查询结果,搜索用时 0 毫秒
1.
Introduction: B cell chronic lymphocytic leukemia (B-CLL) is a hematological malignancy considered as the most common leukemia in the Western world. The understanding of B cell differentiation is crucial for the diagnosis, prognosis, and treatment of the disease.

Areas covered: In this review, B-cell ontogeny and its relation with the CLL development, in combination with the proteomic approaches which could provide a deep characterization of the disease through the characterization of the cellular signaling pathways involved in the pathological cells is described.

Expert commentary: Although conventional strategies (genome sequencing, morphology assays, and immunophenotyping by flow cytometry and/or immunochemistry) have allowed the establishment of the disease stage based on different parameters, it is still necessary to utilize novel approaches (e.g., proteomics) that have the potential to simultaneously analyze thousands of molecules to improve understanding of CLL.  相似文献   

2.
3.
孔雀河下游断流河道的环境特征及物种间关系   总被引:1,自引:0,他引:1  
基于孔雀河下游断流河道的环境因子和植被样地数据,采用聚类与CCA排序法,分析了生境的退化特征以及物种间的相互关系,结果表明:1)断流河道退化生境分为绿洲-荒漠过渡类型、轻度荒漠化类型和盐土荒漠化类型。绿洲-荒漠过渡类型地下水位低、盐分含量相对较低,植被盖度相对较高,土壤维持着原砂质壤土,为潜在退化型;轻度荒漠化类型地下水位、土壤质地与含盐量与前者基本相同,土壤未明显退化,但植被盖度低于10%,植物种类与个体数目都较低,属于轻度退化型;盐土荒漠化类型地下水位高、盐分含量高,土壤机械组成中砂粒比重较大、无建群种幸存,属于重度退化型。2)绿洲-荒漠过渡类型总体联结性为显著正联结,正负联结比小于1,生态系统表现为建群种维系物种关系的不稳定状态;轻度荒漠化类型总体联结性为不显著负联结,正负联结比小于1,表现出生态系统进入退化演替的阶段;盐土荒漠化类型总体联结性为显著正联结,正负联结比大于1,表现出重度退化群落的种间平衡状态,物种间以达到稳定共存,其中,真盐生植物对这种平衡的维持起着重要的作用。3)CCA排序表明,绿洲-荒漠过渡类型形成以胡杨为中心的种间正联结,幸存于盐分适中、水分养分相对较高的生境;轻度荒漠化类型,形成以多枝柽柳与刚毛柽柳相互依存的不显著负联结,幸存于土壤养分、水分相对较低的生境;盐土荒漠化类型形成以盐爪爪、盐节木、盐穗木等真盐生植物维系的显著正联结,幸存于土壤贫瘠、地下水位浅、盐分含量高、沙化严重的生境。  相似文献   
4.
Conservation management of the Tasmanian flora is now focusing on non-vascular plants. Major problems include the low level of information on the composition of the flora and the low number of competent specialists available to deal with the plants. Collation of information from literature and from collections in herbaria is required to establish exactly which data are available and their reliability. An environmental domain analysis covering all ecosystems would indicate which environments were under-represented or absent from current reserves and where needs for conservation lie. Within practical time-frames, this process is probably the best method of capturing unknown components of the flora whilst also catering for widespread species and those closely associated with particular environments. It also incorporates regional variability. Minor habitats, which are often floristically rich, and very rare species are best dealt with on an individual basis. Basic research into taxonomy and ecology is paramount. Reservation and conservation management must be based on well-established and maintained databases which are in turn based on a coherent taxonomy and sound biogoographical information. It is only by pursuing an active research programme that the necessary accurate information can be obtained and the success of the management procedures can be gauged.  相似文献   
5.
6.
7.
Environmental planning must determine management practices for a given territory based on the landscape processes that have occurred over time and their consequences. Therefore, environmental planning decisions must be based on strong empirical evidence that can be easily understood by all involved parties. Several studies have highlighted the methodological deficiencies that occur when obtaining and interpreting such issues, particularly in heterogeneous landscapes with complex interactions. In this paper, we evaluated two methodological approaches that are used in management planning, land use/cover change (LUCC) and mosaic change (MC) to compare their effectiveness and suitability for supporting decision-making. We applied these methods to the coastal landscape of São Sebastião Island, Brazil, which has undergone many changes in the last 50 years. For two years, land use/cover maps were produced using GIS and assessed according to changes in landscape elements (LUCC) and boundaries (MC). Overall, the LUCC failed to identify sets with similar structural heterogeneities in the landscape. However, the LUCC is easier for stakeholders to understand and apply than the MC. The MC method better presented the evolution of the relationship between the landscape elements and heterogeneity.  相似文献   
8.
目录     
《生态学杂志》2019,38(10):0
  相似文献   
9.
The demersal fish and cephalopod communities of the continental shelf and upper slope from 17 to 395m deep were studied during five annual cruises between Cape Agulhas and Port Alfred, South Africa. The cruises showed a consistent pattern of an inshore community (<100m), a shelf community ( c . 90–190m) and a shelf-edge/upper slope fauna (>200m). These groups were identified by dendrograms and multidimensional scaling cluster analysis, which supported on-board observations of catch variation with depth. Although the boundaries are not clearly defined, examination of physical features at the clustered stations suggests that depth, temperature and, to a lesser extent, oxygen concentration are important in the grouping. Occasional, apparently anomalous associations of inshore stations suggested that water temperature and oxygen may over-ride the normal depth distributions of the species groups. This intimates that patterns offish and cephalopod distribution may be dynamic and in part related to the physical parameters of the water body.  相似文献   
10.
The degree of aggregation of lettuce plants infected by aster yellows phytoplasma (AYP) was investigated in 12 fields from three experiments. Position of diseased and healthy plants was mapped in a 6–9×12-m section of each field; for most analyses, fields were divided into 10-plant quadrats. Mean disease incidence (p) ranged from 0.01 to 0.30. The frequency of diseased plants was described by the beta-binomial distribution, with an index of aggregation (θ) ranging from 0 to 0.17, positively correlated withp, and generally increasing over time within a field. Distance-class analysis revealed a core-cluster size of only a few plants. However, spatial autocorrelations ofp between quadrats were not significant, indicating that the scale of spatial pattern was small, generally less than 10 plants. An overall measure of aggregation was given by the slope parameter of the binary form of the power law, in which the log of the calculated variance is regressed on the log of the theoretical variance for a binomial distribution. The slope was 1.18 and significantly different from 1. Results for this “simple-interest” disease are interpreted in relation to the persistent transmission of AYP by its aster leafhopper vector.  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