全文获取类型
收费全文 | 11843篇 |
免费 | 24篇 |
国内免费 | 43篇 |
专业分类
系统科学 | 35篇 |
丛书文集 | 80篇 |
教育与普及 | 30篇 |
理论与方法论 | 42篇 |
现状及发展 | 5124篇 |
研究方法 | 565篇 |
综合类 | 5878篇 |
自然研究 | 156篇 |
出版年
2013年 | 96篇 |
2012年 | 191篇 |
2011年 | 330篇 |
2010年 | 85篇 |
2009年 | 59篇 |
2008年 | 190篇 |
2007年 | 222篇 |
2006年 | 203篇 |
2005年 | 224篇 |
2004年 | 208篇 |
2003年 | 207篇 |
2002年 | 198篇 |
2001年 | 379篇 |
2000年 | 364篇 |
1999年 | 263篇 |
1992年 | 232篇 |
1991年 | 180篇 |
1990年 | 204篇 |
1989年 | 190篇 |
1988年 | 201篇 |
1987年 | 205篇 |
1986年 | 160篇 |
1985年 | 248篇 |
1984年 | 170篇 |
1983年 | 149篇 |
1982年 | 166篇 |
1981年 | 135篇 |
1980年 | 170篇 |
1979年 | 385篇 |
1978年 | 291篇 |
1977年 | 287篇 |
1976年 | 249篇 |
1975年 | 287篇 |
1974年 | 309篇 |
1973年 | 308篇 |
1972年 | 344篇 |
1971年 | 344篇 |
1970年 | 428篇 |
1969年 | 361篇 |
1968年 | 381篇 |
1967年 | 351篇 |
1966年 | 319篇 |
1965年 | 203篇 |
1959年 | 107篇 |
1958年 | 204篇 |
1957年 | 137篇 |
1956年 | 122篇 |
1955年 | 105篇 |
1954年 | 81篇 |
1948年 | 83篇 |
排序方式: 共有10000条查询结果,搜索用时 397 毫秒
121.
目前,高分辨率遥感图象正愈来愈多地应用于土地与水资源的监测和管理,其中,IKONOS图象数据因其具有较高的空间分辨率和人机交互的解译效果.在省市土地利用信息获取及在城市研究中更具有广阔的应用前景.以南京幅的IKONOS图象数据为基础.以城乡接合部的不同试验区为对象,着重讨论了高分辨率遥感图象城市土地利用信息的提取与分类方法其主要思想是先应用常用的多光谱分类算法对试验区的土地利用现状进行初始的类别划分,然后采用Majority Filtering技术获取图象的上下信息,并给你根据地物的形状特征和空间关系对初始分类结果进行校正与调整.研究结果表明,当初始的类别划分能满足一定精度时,采用本所提出的方法能进一步改善高分辨率图象城市土地利用信息的分类效果. 相似文献
122.
Sara B. Seidelmann Janet K. Lighthouse Daniel M. Greif 《Cellular and molecular life sciences : CMLS》2014,71(11):1977-1999
Arteries consist of an inner single layer of endothelial cells surrounded by layers of smooth muscle and an outer adventitia. The majority of vascular developmental studies focus on the construction of endothelial networks through the process of angiogenesis. Although many devastating vascular diseases involve abnormalities in components of the smooth muscle and adventitia (i.e., the vascular wall), the morphogenesis of these layers has received relatively less attention. Here, we briefly review key elements underlying endothelial layer formation and then focus on vascular wall development, specifically on smooth muscle cell origins and differentiation, patterning of the vascular wall, and the role of extracellular matrix and adventitial progenitor cells. Finally, we discuss select human diseases characterized by marked vascular wall abnormalities. We propose that continuing to apply approaches from developmental biology to the study of vascular disease will stimulate important advancements in elucidating disease mechanism and devising novel therapeutic strategies. 相似文献
123.
Water mites Limnochares aquatica (L., 1758) during maintenance in the laboratory for a long period of time in constant conditions periodically produced certain whitish flocculent material consisting of long rigid unbranched tube-like threads 1.3 ± 0.3 µm in diameter crossing freely. These threads were studied using light-optical as well as transmission electron microscopical and scanning electron microscopical methods. Microbiological staining was also applied to the threads to exclude their bacterial or fungal origin. The thread wall is built of fine fibrils arranged at different angles to the long axis of threads that is reflected in a certain stratification of the wall. Threads are mostly hollow or may contain electron-dense homogeneous material. No cell components are present in the thread composition. Numerous dermal glands with their small slit-like orifice scattered throughout the mite body surface are thought to produce these threads. Most probably the thread formation is a reaction of mites to stress under laboratory conditions, and this is expected to be a type of defensive reaction. 相似文献
124.
125.
126.
127.
The purpose of the paper is to present a framework that enables action researchers to create quality action research projects within the organization development (OD) domain using the broad criteria of being rigorous, reflective and relevant and so contribute to the realm of practical knowing. What constitutes good quality action research within OD is a difficult question, given the broad range of approaches that operates in a wide variety of settings and with great diversity. It advances specific dimensions by which action researchers can create, review and assess quality in action research work. This integrative framework and criteria are practical tools to enable action researchers to create quality action research in OD. 相似文献
128.
129.
Pau B. Esparza-Moltó Cristina Nuevo-Tapioles José M. Cuezva 《Cellular and molecular life sciences : CMLS》2017,74(12):2151-2166
The mitochondrial H+-ATP synthase is a primary hub of cellular homeostasis by providing the energy required to sustain cellular activity and regulating the production of signaling molecules that reprogram nuclear activity needed for adaption to changing cues. Herein, we summarize findings regarding the regulation of the activity of the H+-ATP synthase by its physiological inhibitor, the ATPase inhibitory factor 1 (IF1) and their functional role in cellular homeostasis. First, we outline the structure and the main molecular mechanisms that regulate the activity of the enzyme. Next, we describe the molecular biology of IF1 and summarize the regulation of IF1 expression and activity as an inhibitor of the H+-ATP synthase emphasizing the role of IF1 as a main driver of energy rewiring and cellular signaling in cancer. Findings in transgenic mice in vivo indicate that the overexpression of IF1 is sufficient to reprogram energy metabolism to an enhanced glycolysis and activate reactive oxygen species (ROS)-dependent signaling pathways that promote cell survival. These findings are placed in the context of mitohormesis, a program in which a mild mitochondrial stress triggers adaptive cytoprotective mechanisms that improve lifespan. In this regard, we emphasize the role played by the H+-ATP synthase in modulating signaling pathways that activate the mitohormetic response, namely ATP, ROS and target of rapamycin (TOR). Overall, we aim to highlight the relevant role of the H+-ATP synthase and of IF1 in cellular physiology and the need of additional studies to decipher their contributions to aging and age-related diseases. 相似文献
130.
Carol D. Curtis Reema B. Davis Kyle G. Ingram Courtney T. Griffin 《Cellular and molecular life sciences : CMLS》2012,69(23):3921-3931
Vascular development is a dynamic process that relies on the coordinated expression of numerous genes, but the factors that regulate gene expression during blood vessel development are not well defined. ATP-dependent chromatin-remodeling complexes are gaining attention for their specific temporal and spatial effects on gene expression during vascular development. Genetic mutations in chromatin-remodeling complex subunits are revealing roles for the complexes in vascular signaling pathways at discrete developmental time points. Phenotypic analysis of these models at various stages of vascular development will continue to expand our understanding of how chromatin remodeling impacts new blood vessel growth. Such research could also provide novel therapeutic targets for the treatment of vascular pathologies. 相似文献