全文获取类型
收费全文 | 6750篇 |
免费 | 683篇 |
国内免费 | 829篇 |
专业分类
8262篇 |
出版年
2024年 | 33篇 |
2023年 | 114篇 |
2022年 | 225篇 |
2021年 | 385篇 |
2020年 | 311篇 |
2019年 | 345篇 |
2018年 | 305篇 |
2017年 | 224篇 |
2016年 | 330篇 |
2015年 | 471篇 |
2014年 | 526篇 |
2013年 | 523篇 |
2012年 | 611篇 |
2011年 | 560篇 |
2010年 | 363篇 |
2009年 | 294篇 |
2008年 | 373篇 |
2007年 | 385篇 |
2006年 | 320篇 |
2005年 | 262篇 |
2004年 | 218篇 |
2003年 | 213篇 |
2002年 | 189篇 |
2001年 | 99篇 |
2000年 | 73篇 |
1999年 | 73篇 |
1998年 | 71篇 |
1997年 | 41篇 |
1996年 | 36篇 |
1995年 | 22篇 |
1994年 | 23篇 |
1993年 | 30篇 |
1992年 | 26篇 |
1991年 | 22篇 |
1990年 | 13篇 |
1989年 | 26篇 |
1988年 | 14篇 |
1987年 | 13篇 |
1986年 | 13篇 |
1985年 | 10篇 |
1984年 | 8篇 |
1983年 | 8篇 |
1982年 | 7篇 |
1981年 | 7篇 |
1980年 | 4篇 |
1979年 | 8篇 |
1978年 | 5篇 |
1974年 | 5篇 |
1970年 | 4篇 |
1968年 | 4篇 |
排序方式: 共有8262条查询结果,搜索用时 0 毫秒
101.
Kai‐Qing Lu Min Li Guo‐Hong Wang Lian‐Sheng Xu David K. Ferguson Anjali Trivedi Jing Xuan Ying Feng Jin‐Feng Li Gan Xie Yi‐Feng Yao Yu‐Fei Wang 《植物分类学报:英文版》2019,57(2):190-199
Members of the Chenopodiaceae are the most dominant elements in the central Asian desert. The different genera and species within this family are common in desert vegetation types. Should it prove possible to link pollen types in this family to specific desert vegetation, it would be feasible to trace vegetation successions in the geological past. Nevertheless, the morphological similarity of pollen grains in the Chenopodiaceae rarely permits identification at the generic level. Although some pollen classifications of Chenopodiaceae have been proposed, none of them tried to link pollen types to specific desert vegetation types in order to explore their ecological significance. Based on the pollen morphological characters of 13 genera and 24 species within the Chenopodiaceae of eastern central Asia, we provide a new pollen classification of this family with six pollen types and link them to those plant communities dominated by Chenopodiaceae, for example, temperate dwarf semi‐arboreal desert (Haloxylon type), temperate succulent halophytic dwarf semi‐shrubby desert (Suaeda, Kalidium, and Atriplex types), temperate annual graminoid desert (Kalidium type), temperate semi‐shrubby and dwarf semi‐shrubby desert (Kalidium, Iljini, and Haloxylon types), and alpine cushion dwarf semi‐shrubby desert (Krascheninnikovia type). These findings represent a new approach for detecting specific desert vegetation types and deciphering ecosystem evolution in eastern central Asia. 相似文献
102.
Argon mediates protection by interleukin‐8 suppression via a TLR2/TLR4/STAT3/NF‐κB pathway in a model of apoptosis in neuroblastoma cells in vitro and following ischemia‐reperfusion injury in rat retina in vivo 下载免费PDF全文
103.
Luyao Zhang Kai Niu Kang Zhu Cui Xia Jing Yan Wei Zhao Junrong Wei Maoli Duan Guoxi Zheng 《PloS one》2016,11(11)
ObjectivesTo summarize the characteristics and long–term outcomes of olfactory neuroblastoma through the analysis of 13 cases in single institution, with the assessment of treatment modality, prognostic factors.MethodA retrospective study of thirteen cases diagnosed as olfactory neuroblastoma and underwent combined treatments during the period 2000–2010. Statistical analysis was performed to search for prognostic factors and compared different treatment modalities.Results13 patients were enrolled in this study, including 8 male and 5 female, ranging from 15 to 69 (median 43) years old. One patient at stage A was only treated with endoscopic endonasal surgery (EES). Seven patients were treated with preoperative radiotherapy and EES, two with EES and postoperative radiotherapy, and the other three with combined radiotherapy and chemotherapy. The range of follow-up time varied from 23 to 116 months (median 65 months). The 5-year overall survival rate was 46.2% (6/13). To date, these thirteen patients have not suffered local recurrences while two patients had lymph node recurrences and one had distant metastasis in the bone marrow. In 13 patients, 61.5% were diagnosed as late T stage (T3/4), 69.2% late Kadish stage (C/D) and 53.8% were high Hyams grade (I/ II), which indicated poor prognosis. Related prognostic factors were the TNM stage (T stage P = 0.028, N stage P = 0.000, M stage P = 0.007), Kadish stage (P = 0.025) and treatment modality (P = 0.015).ConclusionLate stage of TNM and Kadish staging system indicated a poor prognosis. Combined treatment modality, including endoscopic endonasal surgery, achieved a better outcome than non-surgical approach. 相似文献
104.
105.
Dynamic effects of predators on cyclic voles: field experimentation and model extrapolation 总被引:9,自引:0,他引:9
Korpimäki E Norrdahl K Klemola T Pettersen T Stenseth NC 《Proceedings. Biological sciences / The Royal Society》2002,269(1495):991-997
Mechanisms generating the well-known 3-5 year cyclic fluctuations in densities of northern small rodents (voles and lemmings) have remained an ecological puzzle for decades. The hypothesis that these fluctuations are caused by delayed density-dependent impacts of predators was tested by replicated field experimentation in western Finland. We reduced densities of all main mammalian and avian predators through a 3 year vole cycle and compared vole abundances between four reduction and four control areas (each 2.5-3 km(2)). The reduction of predator densities increased the autumn density of voles fourfold in the low phase, accelerated the increase twofold, increased the autumn density of voles twofold in the peak phase, and retarded the initiation of decline of the vole cycle. Extrapolating these experimental results to their expected long-term dynamic effects through a demographic model produces changes from regular multiannual cycles to annual fluctuations with declining densities of specialist predators. This supports the findings of the field experiment and is in agreement with the predation hypothesis. We conclude that predators may indeed generate the cyclic population fluctuations of voles observed in northern Europe. 相似文献
106.
Sutterwala SS Hsu FF Sevova ES Schwartz KJ Zhang K Key P Turk J Beverley SM Bangs JD 《Molecular microbiology》2008,70(2):281-296
Sphingolipids are essential components of eukaryotic membranes, and many unicellular eukaryotes, including kinetoplastid protozoa, are thought to synthesize exclusively inositol phosphorylceramide (IPC). Here we characterize sphingolipids from Trypanosoma brucei, and a trypanosome sphingolipid synthase gene family (TbSLS1-4) that is orthologous to Leishmania IPC synthase. Procyclic trypanosomes contain IPC, but also sphingomyelin, while surprisingly bloodstream-stage parasites contain sphingomyelin and ethanolamine phosphorylceramide (EPC), but no detectable IPC. In vivo fluorescent ceramide labelling confirmed stage-specific biosynthesis of both sphingomyelin and IPC. Expression of TbSLS4 in Leishmania resulted in production of sphingomyelin and EPC suggesting that the TbSLS gene family has bi-functional synthase activity. RNAi silencing of TbSLS1-4 in bloodstream trypanosomes led to rapid growth arrest and eventual cell death. Ceramide levels were increased more than threefold by silencing suggesting a toxic downstream effect mediated by this potent intracellular messenger. Topology predictions support a revised six-transmembrane domain model for the kinetoplastid sphingolipid synthases consistent with the proposed mammalian sphingomyelin synthase structure. This work reveals novel diversity and regulation in sphingolipid metabolism in this important group of human parasites. 相似文献
107.
The contrasting role of male relatedness in different mechanisms of sexual selection in red junglefowl 下载免费PDF全文
Cedric Kai Wei Tan Philippa Doyle Emma Bagshaw David S. Richardson Stuart Wigby Tommaso Pizzari 《Evolution; international journal of organic evolution》2017,71(2):403-420
In structured populations, competition for reproductive opportunities should be relaxed among related males. The few tests of this prediction often neglect the fact that sexual selection acts through multiple mechanisms, both before and after mating. We performed experiments to study the role of within‐group male relatedness across pre‐ and postcopulatory mechanisms of sexual selection in social groups of red junglefowl, Gallus gallus, in which two related males and one unrelated male competed over females unrelated to all the males. We confirm theoretical expectations that, after controlling for male social status, competition over mating was reduced among related males. However, this effect was contrasted by other sexual selection mechanisms. First, females biased male mating in favor of the unrelated male, and might also favor his inseminations after mating. Second, males invested more—rather than fewer—sperm in postcopulatory competition with relatives. A number of factors may contribute to explain this counterintuitive pattern of sperm allocation, including trade‐offs between male investment in pre‐ versus postcopulatory competition, differences in the relative relatedness of pre‐ versus postcopulatory competitors, and female bias in sperm utilization in response to male relatedness. Collectively, these results reveal that within‐group male relatedness may have contrasting effects in different mechanisms of sexual selection. 相似文献
108.
Gisela Drews Kai Graszynski 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1987,157(3):345-353
Summary Euryhaline Crustacea living in dilute media, counterbalance the salt loss by active absorption of NaCl across the gill epithelium. To investigate the mechanisms involved in salt absorption, transeptithelial potential difference (PDte) was measured in isolated, perfused gills of the fiddler crab,Uca tangeri. The influence of some specific inhibitors of epithelial ion transport on the PDte was tested.With symmetrical conditions on both sides of the epithelium, the posterior gills ofUca tangeri showed a spontaneous PDte of +5 to +10 mV, that is an active transport potential which was positive on the bath side as referred to the hemolymph side. This potential decreased considerably after application of KCN or 2,4-dinitrophenol (DNP) to the perfusion saline.Omission of K+ from the perfusion saline or addition of ouabain led to a reversible drop of the PDte, suggesting that the absorption of Na+ and also of Cl– is driven by the (Na++K+)ATPase located in the basolateral membrane of the epithelial cells.Perfusion of the hemolymph space with saline containing diphenylamine-2-carboxylate (DPC) or the loop diuretic furosemide resulted in a decrease of the PDte.After application of amiloride to the bath saline the PDte increased. Half-maximum response to amiloride was reached at a concentration of about 10–5 mol·l–1. This suggests that one of the Na+ pathways across the apical membrane may consist of Na+ channels.Abbreviations
PD
te
transepithelial potential difference
-
DPC
diphenylamine-2-carboxylate
-
R
ps
resistance of perfusate shunt
-
R
te
transepithelial resistance
-
R
in
input resistance
-
DNP
2,4-dinitrophenol
Parts of this study have been reported at the 1st Congress of Comparative Physiology and Biochemistry, Liège 1984, and at the Vth European Colloquium on Renal Physiology, Frankfurt, 1985 相似文献
109.
在菹草衰亡阶段对5个静水水体中菹草叶片表面附着物进行野外调查,并将其与水体营养盐浓度、沉水植物衰亡程度进行相关性分析。附着物共调查Chl.a含量、干重、有机质含量和藻类数量4个指标,沉水植物衰亡程度用单位面积叶片Chl.a含量表示,水体营养盐含量测量了TP、TN和N/P 3个指标。结果显示:附着物生物量与水体营养盐状况存在一定的正相关;各个点的附着物生物量与菹草衰亡状况存在一定相关性但相关性趋势与水体污染程度有关。在污染程度较高的水体中附着物生物量与菹草衰亡程度呈正相关,在污染程度较低的水体中附着物生物量与菹草衰亡程度呈负相关。结论为富营养化湖泊中营养盐含量的增加会导致附着物生物量的增加,但附着物只在污染程度较高的水体中促进植物衰亡。 相似文献
110.
通过田间小区试验,测定了3个春小麦品种(高原602、陇春8139和定西24)在不同生育期和不同种植密度的光谱反射率及对应叶面积指数(LAI)和地上生物量,分析了其光谱反射的一般特征和红边参数特征以及光谱变量与LAI和地上生物量的相关性.结果表明:在整个波段内,春小麦冠层光谱表现为高原602>陇春8139>定西24,其叶片光谱表现为定西24>陇春8139>高原602;春小麦冠层光谱在可见光波段和中红外波段成熟期明显大于孕穗期,而叶片光谱在近红外波段孕穗期明显大于成熟期;随着种植密度的提高,在近红外波段冠层和叶片的光谱反射率逐渐增加;冠层光谱的红边均具有"双峰"现象,从孕穗期到成熟期,冠层红边位置呈现"蓝移"现象;LAI和地上生物量与冠层光谱变量之间存在较好的相关性. 相似文献