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   18522篇
  免费   1945篇
  国内免费   1487篇
  21954篇
  2024年   47篇
  2023年   242篇
  2022年   578篇
  2021年   900篇
  2020年   653篇
  2019年   837篇
  2018年   830篇
  2017年   614篇
  2016年   859篇
  2015年   1076篇
  2014年   1306篇
  2013年   1319篇
  2012年   1561篇
  2011年   1457篇
  2010年   862篇
  2009年   827篇
  2008年   919篇
  2007年   829篇
  2006年   693篇
  2005年   643篇
  2004年   622篇
  2003年   640篇
  2002年   581篇
  2001年   451篇
  2000年   370篇
  1999年   306篇
  1998年   196篇
  1997年   155篇
  1996年   156篇
  1995年   115篇
  1994年   108篇
  1993年   90篇
  1992年   106篇
  1991年   109篇
  1990年   98篇
  1989年   82篇
  1988年   65篇
  1987年   66篇
  1986年   45篇
  1985年   67篇
  1984年   43篇
  1983年   41篇
  1982年   38篇
  1981年   29篇
  1979年   29篇
  1978年   25篇
  1975年   28篇
  1974年   21篇
  1973年   26篇
  1971年   41篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Aim Our aims were to verify the existence of phylogenetic disequilibrium between butterfly lineages at the subcontinental scale for islands and the nearest mainland and to test the capacity of islands for hosting ancestral populations of butterflies and the significance of such relict populations. Location The western Mediterranean continental area of Europe and North Africa together with several large and small islands (Balearics, Tuscan Archipelago, Aeolian Archipelago, Capri, Sardinia, Sicily, Corsica). Methods Using geometric morphometrics, the shape of male genitalia was analysed in two common butterflies (Pyronia cecilia and Pyronia tithonus), whose spatial heterogeneity in the Mediterranean region has recently been described. Observed patterns in genital shapes were compared with shapes predicted for islands and fossil islands to assess the contribution of historical and current events in accounting for the transition from a refugial model to an equilibrium model. Measurements were taken for 473 specimens in 90 insular and mainland sites. Results The shape of the genitalia of populations of most islands differed substantially from that predicted by the equilibrium hypothesis while closely fitting the refugial hypothesis. The comparison between different models strongly suggests that islands maintain ancestral lineages similar to those living in Spain (P. cecilia) and France (P. tithonus). A high correlation between observed and predicted patterns on islands and fossil islands occurs during the first steps of modelled introgressive hybridization while the following steps exposed a successively lower fit, suggesting that the process from a refugial to an equilibrium situation is highly skewed towards an earlier non‐equilibrium. Main conclusions The observed non‐equilibrium pattern supports the refugial hypothesis, suggesting that an ancestral lineage was originally distributed from Spain to Italy, and also occupied offshore islands. This lineage, replaced in Italy, has persisted on the islands owing to their isolation. A comparison of the distribution patterns for genetic and morphometric markers in several species indicates that the situation highlighted for Pyronia may represent a common biogeographic feature for many Mediterranean butterflies.  相似文献   
52.
M Schmitt  C Kempf  J Kempf 《Mutation research》1984,132(5-6):197-201
Cultures of adult mouse-lung fibroblasts have been treated by series of strong and weak muta-carcinogens. Unscheduled DNA synthesis has been measured by quantitative autoradiography using automatic image analysis. Some of the muta-carcinogens (AFB1, DMN, B(a)P, DMBA, MNNG, 4-NQO) yielded a measurable UDS response, whereas others (2-AA, AFB2, B(e)P, ICR-191) usually known as weak carcinogens, gave no response. The response was not improved by addition of liver S9. This shows that mouse-lung fibroblasts possess their own but limited metabolic activation systems.  相似文献   
53.
The neurocognitive basis of memory retrieval is often examined by investigating brain potential old/new effects, which are differences in brain activity between successfully remembered repeated stimuli and correctly rejected new stimuli in a recognition test. In this study, we combined analyses of old/new effects for words with an item-method directed-forgetting manipulation in order to isolate differences between the retrieval processes elicited by words that participants were initially instructed to commit to memory and those that participants were initially instructed to forget. We compared old/new effects elicited by to-be-forgotten (TBF) words with those elicited by to-be-remembered (TBR) words in both an explicit-memory test (a recognition test) and an implicit-memory test (a lexical-decision test). Behavioral results showed clear directed forgetting effects in the recognition test, but not in the lexical decision test. Mirroring the behavioral findings, analyses of brain potentials showed evidence of directed forgetting only in the recognition test. In this test, potentials from 450–650 ms (P600 old/new effects) were more positive for TBR relative to TBF words. By contrast, P600 effects evident during the lexical-decision test did not differ in magnitude between TBR and TBF items. When taken in the context of prior studies that have linked similar parietal old/new effects to the recollection of episodic information, these data suggest that directed-forgetting effects manifest primarily in greater episodic retrieval by TBR than TBF items, and that retrieval intention may be important for these directed-forgetting effects to occur.  相似文献   
54.
纳米TiO2在光催化下能产生活性羟基,超氧离子(O2-),过氧羟基(·OOH)和过氧化氢,这些产物具有很强的氧化性,因而具有广谱杀菌功能[1].目前国内外已广泛用于陶瓷洁具、玻璃表面、瓷砖釉面、水处理、空气净化等[2~8],但纳米TiO2在光催化下是否具有抗病毒的作用,目前尚未见报道.本文以载有纳米TiO2的陶瓷碟(以下简称纳米瓷碟)和发泡镍网(以下简称纳米镍网)为材料,对纳米TiO2复合材料在不同光源照射下杀灭乙型肝炎病毒(HBV)的效果进行了实验研究,取得了令人满意的结果.  相似文献   
55.
56.
Biomechanics and Modeling in Mechanobiology - The superficial femoral artery (SFA) is a typical atherosclerosis-prone site. We aimed to explore whether the tortuosity of the SFA associates with the...  相似文献   
57.
58.
植物中转录后基因沉默的启动、传导与抑制   总被引:4,自引:0,他引:4  
  相似文献   
59.
HCC has remained one of the challenging cancers to treat, owing to the paucity of drugs targeting the critical survival pathways. Considering the cancer cells are deficient in DNase activity, the increase of an autonomous apoptisis endonuclease should be a reasonable choice for cancer treatment. In this study, we investigated whether DNASE1L3, an endonuclease implicated in apoptosis, could inhibit the progress of HCC. We found DNASE1L3 was down-regulated in HCC tissues, whereas its high expression was positively associated with the favorable prognosis of patients with HCC. Besides, serum DNASE1L3 levels were lower in HCC patients than in healthy individuals. Functionally, we found that DNASE1L3 inhibited the proliferation of tumor cells by inducing G0/G1 cell cycle arrest and cell apoptosis in vitro. Additionally, DNASE1L3 overexpression suppressed tumor growth in vivo. Furthermore, we found that DNASE1L3 overexpression weakened glycolysis in HCC cells and tissues via inactivating the rate-limiting enzymes involved in PTPN2-HK2 and CEBPβ-p53-PFK1 pathways. Finally, we identified the HBx to inhibit DNASE1L3 expression by up-regulating the expression of ZNF384. Collectively, our findings demonstrated that DNASE1L3 could inhibit the HCC progression through inducing cell apoptosis and weakening glycolysis. We believe DNASE1L3 could be considered as a promising prognostic biomarker and therapeutic target for HCC.  相似文献   
60.
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