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   4885篇
  免费   405篇
  国内免费   3篇
  2023年   11篇
  2022年   11篇
  2021年   74篇
  2020年   48篇
  2019年   61篇
  2018年   63篇
  2017年   75篇
  2016年   136篇
  2015年   221篇
  2014年   225篇
  2013年   307篇
  2012年   394篇
  2011年   391篇
  2010年   265篇
  2009年   231篇
  2008年   301篇
  2007年   328篇
  2006年   305篇
  2005年   262篇
  2004年   274篇
  2003年   246篇
  2002年   221篇
  2001年   50篇
  2000年   44篇
  1999年   57篇
  1998年   64篇
  1997年   42篇
  1996年   38篇
  1995年   49篇
  1994年   43篇
  1993年   40篇
  1992年   36篇
  1991年   35篇
  1990年   37篇
  1989年   28篇
  1988年   21篇
  1987年   18篇
  1986年   15篇
  1985年   26篇
  1984年   11篇
  1982年   11篇
  1981年   14篇
  1979年   13篇
  1978年   18篇
  1975年   12篇
  1974年   11篇
  1972年   15篇
  1971年   9篇
  1970年   11篇
  1969年   10篇
排序方式: 共有5293条查询结果,搜索用时 218 毫秒
1.
2.
Glucoraphasatin: Chemistry, occurrence, and biological properties   总被引:1,自引:0,他引:1  
Glucoraphasatin is an atypical glucosinolate mainly found in Raphanus sativus roots and sprouts. This review focuses on the chemistry, the occurrence, and the biological properties of glucoraphasatin.  相似文献   
3.
While magnetoreception in birds has been studied intensively, the literature on magnetoreception in bony fish, and particularly in non-migratory fish, is quite scarce. We examined alignment of common carps (Cyprinus carpio) at traditional Christmas sale in the Czech Republic. The sample comprised measurements of the directional bearings in 14,537 individual fish, distributed among 80 large circular plastic tubs, at 25 localities in the Czech Republic, during 817 sampling sessions, on seven subsequent days in December 2011. We found that carps displayed a statistically highly significant spontaneous preference to align their bodies along the North-South axis. In the absence of any other common orientation cues which could explain this directional preference, we attribute the alignment of the fish to the geomagnetic field lines. It is apparent that the display of magnetic alignment is a simple experimental paradigm of great heuristic potential.  相似文献   
4.
Summary Conditioned culture medium from Daudi cells was used as a source of soluble H-Y antigen. Concentrated culture medium was labeled with 125I and then fractionated by gel filtration. Column fractions were assayed for the presence of H-Y antigen by urease-ELISA. H-Y antigen-containing fractions were then pooled and subjected to an improved immunoprecipitation protocol. Three predominant H-Y antigenic proteins were identified with estimated molecular weights of above 200,000, 50,000, and 20,000.  相似文献   
5.
6.
Restoration of wetlands is a high priority world-wide. Peat extraction areas can be restored by rewetting, however affecting the environment. It could be expected to turn the drained peat-cutover area from a source to a sink of most elements. This study examined effects of such rewetting on peat, hydrology and water chemistry over 15 years at two sites in Sweden; the nutrient-poor Porla peatland and the nutrient-rich Västkärr peatland. Rewetting caused minor changes to peat chemistry, but at the Västkärr site ammonium concentrations increased in superficial peat layers while nitrate decreased. In terms of hydrology, rewetting of the Porla site decreased annual runoff and both high and low discharges. Water pH at the Porla site stayed fairly stable, but at the Västkärr site pH, after an initial 4 years dip, gradually increased to higher values than before rewetting. Water colour and organic matter content were fairly stable, but slightly lower values were found after 15 years than in initial 4–5 years. The concentrations of base cations and of inorganic N were lower after rewetting, while total P was higher. However, these impacts could change from an initial phase as the wetlands in the long-term perspective develop into mires.  相似文献   
7.
8.

Ge/Si ratios of plant phytoliths have been widely used to trace biogeochemical cycling of Si. However, until recently, information on how much of the Ge and Si transferred from soil to plants is actually stored in phytoliths was lacking. The aim of the present study is to (i) compare the uptake of Si and Ge in three grass species, (ii) localize Ge and Si stored in above-ground plant parts and (iii) evaluate the amounts of Ge and Si sequestrated in phytoliths and plant tissues. Mays (Zea mays), oat (Avena sativa) and reed canary grass (Phalaris arundinacea) were cultivated in the greenhouse on soil and sand to control element supply. Leaf phytoliths were extracted by dry ashing. Total elemental composition of leaves, phytoliths, stems and roots were measured by ICP-MS. For the localization of phytoliths and the determination of Ge and Si within leaf tissues and phytoliths scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX) and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) was used. The amounts of Si and Ge taken up by the species corresponded with biomass formation and decreased in the order Z. mays > P. arundinacea, A. sativa. Results from LA-ICP-MS revealed that Si was mostly localized in phytoliths, while Ge was disorderly distributed within the leaf tissue. In fact, from the total amounts of Ge accumulated in leaves only 10% was present in phytoliths highlighting the role of organic matter on biogeochemical cycling of Ge and the necessity for using bulk Ge/Si instead of Ge/Si in phytoliths to trace biogeochemical cycling of Si.

  相似文献   
9.
Abstract We have analyzed the sequence downstream of rpoN from Zcinetobacter calcoaceticus and identified an open reading frame encoding a protein with high similarity to UDP- N -acetylgucosamine 1-carboxyvinyl-transferase (MurZ). Multicopy plasmids encoding this enzyme conferred phosphomycin resistance to A. calcoaceticus . The polar effect of a rpoN mutation on the phosphomycin resistance level suggests that murZ is, in part, cotranscribed with rpoN . These observations confirm that A. calcoaceticus represents the first exceptin from a conserved genetic context of rpoN observed in several other Gram-negative bacteria.  相似文献   
10.
The airway mucosal epithelium is permanently exposed to airborne particles. A network of immune cells patrols at this interface to the environment. The interplay of immune cells is orchestrated by different mediators. In the current study we investigated the impact of neuronal signals on key functions of dendritic cells (DC). Using two-photon microscopic time-lapse analysis of living lung sections from CD11c-EYFP transgenic mice we studied the influence of neuropeptides on airway DC motility. Additionally, using a confocal microscopic approach, the phagocytotic capacity of CD11c+ cells after neuropeptide stimulation was determined. Electrical field stimulation (EFS) leads to an unspecific release of neuropeptides from nerves. After EFS and treatment with the neuropeptides vasoactive intestinal peptide (VIP) or calcitonin gene-related peptide (CGRP), airway DC in living lung slices showed an altered motility. Furthermore, the EFS-mediated effect could partially be blocked by pre-treatment with the receptor antagonist CGRP8–37. Additionally, the phagocytotic capacity of bone marrow-derived and whole lung CD11c+ cells could be inhibited by neuropeptides CGRP, VIP, and Substance P. We then cross-linked these data with the in vivo situation by analyzing DC motility in two different OVA asthma models. Both in the acute and prolonged OVA asthma model altered neuropeptide amounts and DC motility in the airways could be measured. In summary, our data suggest that neuropeptides modulate key features motility and phagocytosis of mouse airway DC. Therefore altered neuropeptide levels in airways during allergic inflammation have impact on regulation of airway immune mechanisms and therefore might contribute to the pathophysiology of asthma.  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