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   2959篇
  免费   135篇
  国内免费   100篇
系统科学   51篇
丛书文集   72篇
教育与普及   13篇
理论与方法论   5篇
现状及发展   58篇
综合类   2437篇
自然研究   558篇
  2025年   12篇
  2024年   44篇
  2023年   28篇
  2022年   36篇
  2021年   49篇
  2020年   72篇
  2019年   68篇
  2018年   68篇
  2017年   75篇
  2016年   59篇
  2015年   97篇
  2014年   97篇
  2013年   79篇
  2012年   559篇
  2011年   116篇
  2010年   112篇
  2009年   112篇
  2008年   96篇
  2007年   142篇
  2006年   136篇
  2005年   104篇
  2004年   100篇
  2003年   105篇
  2002年   84篇
  2001年   65篇
  2000年   82篇
  1999年   77篇
  1998年   68篇
  1997年   66篇
  1996年   46篇
  1995年   51篇
  1994年   75篇
  1993年   63篇
  1992年   38篇
  1991年   33篇
  1990年   24篇
  1989年   22篇
  1988年   12篇
  1987年   11篇
  1986年   7篇
  1985年   3篇
  1982年   1篇
排序方式: 共有3194条查询结果,搜索用时 0 毫秒
91.
《Journal of Natural History》2012,46(6):1353-1365
Two new genera of Pilumnidae each with a single new species are described from southeastern Queensland waters. Priapipilumnus is defined by the greatly swollen base and simple blunt apex of the male first pleopod and the concomitant posteriorly protruding, bilobed, third abdominal segment. Takedana is separable by the very short spinous anterolateral margins, the swollen branchial regions, the moderately short walking legs, and the strongly spined chelae. Actumnus pugilator is reassigned to Bathypilumnus Ng and Tan, 1984.  相似文献   
92.
报道贵州省织金洞弱蛛科蜘蛛一新属、新种:黔弱蛛属Qianleptoneta gen.nov.,五刺黔弱蛛Qianleptoneta quinquespinata sp.nov.。模式标本保存于河北大学博物馆。文中测量单位为mm。  相似文献   
93.
汪家社 《武夷科学》2006,22(1):34-36
通过对福建武夷山保护区松毛虫属及其近缘属昆虫调查和已有资料的分析研究,本文记述武夷山保护区松毛虫属及其近缘属共16种(杂毛虫属8种、松毛虫属6种、云毛虫属1种、丫毛虫属1种),占福建省已知种类89%。文中列出分属检索表。  相似文献   
94.
本文记录了内蒙古长蝽科Lygaeidae莎长蝽属Cymus Hahn 2种,其中棕莎长蝽C.claviculus (Fall.)为国内新纪录种。本文对内蒙古莎长蝽检索的同时,对国内新纪录种进行了描述。  相似文献   
95.
本文报道了在广西龙州发现的吉丁天牛属—新亚属胸刺吉丁天牛亚属和一新种龙州吉丁天牛。  相似文献   
96.
小卫星分布式雷达   总被引:6,自引:0,他引:6  
小卫星分布式雷达是一种新概念雷达系统 ,具有重要的应用前景。本文讨论了小卫星分布式雷达的基本概念 ,探讨了其在未来航天装备中的作用和地位 ,阐明了小卫星分布式雷达技术的科技内涵 ,最后提出了对我国发展天基雷达的几点思考  相似文献   
97.
Betel quid (BQ) chewing shows a strong correlation to the incidence of oral submucous fibrosis (OSF), leukoplakia and oral cancer. BQ contains mainly areca nut, lime, Piper betle leaf (PBL) and the inflorescence of P. betle (IPB). Hydroxychavicol (4-allyl-catechol, HC), as a major phenolic compound in PBL and IPB, is shown to induce oxidative stress, glutathione (GSH) depletion and cell cycle deregulation. Using bivariate BrdU/PI flow cytometry, KB cells in DNA synthesis (S phase) are shown to be sensitive to the toxic effect of HC and show cell cycle arrest and apoptosis following exposure to 0.1 and 0.3 mM HC. HC-induced apoptosis and cell cycle arrest are associated with mitochondrial membrane potential (m) depolarization as revealed by a decrease in rhodamine fluorescence. N-acetyl-L-cysteine (1 mM), superoxide dismutase (100 U/ml) and catalase (1000 U/ml) were effective in prevention of HC-induced GSH depletion (as indicated by chloromethylfluorescein fluorescence), reactive oxygen species (ROS) production (by dichlorofluorescein fluorescence), cell cycle arrest and apoptosis. However, dimethylthiourea (2 mM), neocuproine (1 mM), 1,10-phenanthroline (200 M) and desferrioxamine (0.5 mM) showed little effect on HC-induced cell changes. HC elevated the cellular and mitochondrial GSH levels at moderate concentrations (0.05–0.1 mM), whereas at a concentration of 0.3 mM, inhibitory effects were noted. These results indicate that HC consumption may be associated with BQ-chewing-related oral mucosal diseases via GSH depletion, ROS production, mitochondrial dysfunction, cell cycle disturbance and the induction of apoptosis. These events are related to the production of superoxide radicals and hydrogen peroxide.Received 9 July 2003; received after revision 28 September 2003; accepted 24 October 2003  相似文献   
98.
在提出施工企业安全绩效评价指标体系的基础上,运用灰色关联投影法对企业安全绩效进行评价。为避免指标的相对重要程度因人为赋权的主观性影响和指标权重分配的均衡化以及指标之间因量纲不同而对权重的影响,采取了变异系数法来对各指标确定权重。之后针对实地考察的施工企业进行实证分析,运用比较安全学的思想对安全绩效进行纵向对比,依据评价体系灰色关联度大小来判断施工企业安全绩效的水平高低。进而诊断出各企业安全绩效的优劣,为同类型的企业实际开展安全绩效评价提供指导性的方向和建议。  相似文献   
99.
100.
This study compares functional and morphological alterations caused by application of alloxan, streptozotocin, xanthine oxidase/hypoxanthine (generation of reactive oxygen species), or S-nitroso-N-acetyl-D,L-penicillamine (SNAP, liberation of nitric oxide) to isolated rat pancreatic islets in vitro. In perifusion experiments, membrane leakage—detected by non-stimulated insulin release—was found after application of all drugs, but showed a substance-specific time pattern. Twenty-four hours after application of the classical diabetogens (alloxan or streptozotocin), potassium chloride- and glucose-stimulated insulin secretion were markedly reduced, while a persistent reduction was observed neither after exposure to xanthine oxidase/hypoxanthine, nor to SNAP. Morphological analysis of the islets revealed that nearly all β-cells were destroyed following alloxan or streptozotocin treatment, while the majority of β-cells were configured regularly after application of xanthine oxidase/hypoxanthine or SNAP. Necrotic cells found after xanthine oxidase/hypoxanthine usually differed in morphology from those observed after application of the classical diabetogens. While the former cells were characterised by swollen nuclei, the latter had shrunken nuclei with irregular condensed chromatin. Apoptosis was found only following nitric oxide exposure. Due to these differences, it seems unlikely that alloxan, streptozotocin, xanthine oxidase/hypoxanthine, and nitric oxide have a common major feature in their toxic action. Received 16 September 1999; received after revision 15 November 1999; accepted 26 November 1999  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