全文获取类型
收费全文 | 710336篇 |
免费 | 75572篇 |
国内免费 | 418篇 |
专业分类
786326篇 |
出版年
2018年 | 7264篇 |
2017年 | 6926篇 |
2016年 | 9687篇 |
2015年 | 13493篇 |
2014年 | 15026篇 |
2013年 | 22093篇 |
2012年 | 24904篇 |
2011年 | 24798篇 |
2010年 | 16401篇 |
2009年 | 14529篇 |
2008年 | 22056篇 |
2007年 | 22305篇 |
2006年 | 21218篇 |
2005年 | 20121篇 |
2004年 | 19643篇 |
2003年 | 18874篇 |
2002年 | 18133篇 |
2001年 | 28715篇 |
2000年 | 28802篇 |
1999年 | 23304篇 |
1998年 | 8581篇 |
1997年 | 8948篇 |
1996年 | 8584篇 |
1995年 | 7818篇 |
1994年 | 7928篇 |
1993年 | 7756篇 |
1992年 | 19617篇 |
1991年 | 19138篇 |
1990年 | 18698篇 |
1989年 | 18493篇 |
1988年 | 17060篇 |
1987年 | 16292篇 |
1986年 | 14970篇 |
1985年 | 15250篇 |
1984年 | 12505篇 |
1983年 | 10784篇 |
1982年 | 8368篇 |
1981年 | 7547篇 |
1980年 | 7211篇 |
1979年 | 12114篇 |
1978年 | 9379篇 |
1977年 | 8698篇 |
1976年 | 8261篇 |
1975年 | 8999篇 |
1974年 | 9639篇 |
1973年 | 9493篇 |
1972年 | 8493篇 |
1971年 | 7697篇 |
1970年 | 6866篇 |
1969年 | 6526篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
971.
Human umbilical vein endothelial cells were grown to confluence, as first passage cells, on coverslips. Treatment with ionomycin or histamine caused a sustained rise in intracellular Ca2+ (measured by Fura-2 fluorescence), but after treatment with thrombin, only a transient rise in Ca2+ was observed. Furthermore, the addition of thrombin after ionomycin or histamine lowered the raised Ca2+ back to near control levels. This effect of thrombin was concentration dependent, with increasing concentrations producing increases in both the rate and extent of the lowering of Ca2+. A similar effect of thrombin was seen on video imaging of Fura-2-loaded cell monolayers. The Ca2(+)-lowering effect of thrombin was not mimicked by phorbol 12-myristate 13-acetate nor blocked by staurosporine, indicating a lack of involvement of protein kinase C; intracellular pH also does not appear to be involved. The mechanism by which thrombin lowers cytoplasmic Ca2+ is due mainly to inhibition of Ca2+ entry since thrombin prevented the stimulated influx of Mn2+ caused by histamine or ionomycin. It may therefore be that in vivo under certain physiological or pathological conditions, thrombin's effects on intracellular Ca2+ are more transient than those of histamine, and thrombin also may induce transience in histamine's actions. 相似文献
972.
973.
The involvement of gibberellins in the control of flowering of sunflower was studied by direct application of GA3 to the apex of the plants, analysis of the endogenous levels of gibberellin-like substances at different plant ages, and indirectly by the application of paclobutrazol, an inhibitor of gibberellin synthesis. GA3 speeded-up flower initiation and floral apex development. The time of GA3 application was more critical than the amount of GA3 applied. The endogenous levels of gibberellin-like compounds increased significantly by day 15 after sowing. The application of paclobutrazol markedly delayed floral initiation and this effect was also depedent on plant age. Both GA3 and paclobutrazol had their greatest effects between 10 and 20 days after sowing suggesting that an increase in gibberellins in that time period plays a role in floral initiation. 相似文献
974.
J E Vancauwenberge P J Slininger R J Bothast 《Applied and environmental microbiology》1990,56(2):329-332
beta-hydroxypropionaldehyde (3-HPA) can be oxidized to acrylic acid, an industrially important chemical used in the manufacture of synthetic plastics and other polymers. Of 19 genera and 55 strains tested, 3 Klebsiella and 2 Enterobacter strains produced 3-HPA. The most efficient strain was Klebsiella pneumoniae NRRL B-4011. Under optimum conditions (28 degrees C; 40 g of semicarbazide hydrochloride per liter, 70 g of glycerol per liter; and pH 6.0), 3.1 g of B-4011 cells per liter accumulated 22 g of 3-HPA per liter at a specific rate of 0.83 g/g per h; however, 14.5 g of cells per liter accumulated 46 g of 3-HPA per liter at a specific rate of 0.41 g/g per h. 相似文献
975.
Preventing the establishment of ectoparasitic poultry red mite (Dermanyssus gallinae) populations is key in ensuring welfare and egg production of laying hens and absence of allergic reactions of workers in poultry facilities. Using the Hazard Analysis and Critical Control Point method, a panel of experts identified hazards and associated risks concerning the introduction and spread of this mite in poultry facilities. Together we provide an overview of possible corrective actions that can be taken to prevent population establishment. Additionally, a checklist of the most critical control points has been devised as management tool for poultry farmers. This list was evaluated by Dutch and British poultry farmers. They found the checklist feasible and useful. 相似文献
976.
Se-Chan Kim Alexander Ghanem Heidi Stapel Klaus Tiemann Pascal Knuefermann Andreas Hoeft Rainer Meyer Christian Grohé Anne A Knowlton Georg Baumgarten 《BMC physiology》2007,7(1):5
Backgound
It has been reported that Toll-like receptor 4 (TLR4) deficiency reduces infarct size after myocardial ischemia/reperfusion (MI/R). However, measurement of MI/R injury was limited and did not include cardiac function. In a chronic closed-chest model we assessed whether cardiac function is preserved in TLR4-deficient mice (C3H/HeJ) following MI/R, and whether myocardial and systemic cytokine expression differed compared to wild type (WT). 相似文献977.
978.
Sandhya Ravishankar Meera Pandey R. P. Tewari V. Krishna 《World journal of microbiology & biotechnology》2006,22(10):1021-1025
Summary The sporophores of Pleurotus are gymnocarpous and continuously release spores in the atmosphere causing respiratory allergies like hay fever and farmer’s lung disease among workers. The allergy is caused by the antigens present on the walls of the spores. Apart from this, during commercial production, these spores settle on the fruit bodies, germinate and form a velvety film which gives an unpleasant appearance to the mushrooms. The spores emitted may include new genotypes likely to attack wood or trees. Spore allergy is one of the most important limiting factors for the large scale cultivation of this species. Different approaches are being adopted at IIHR for the production of commercial sporeless/low-sporing strains of Pleurotus to alleviate the spore allergy problem. Attempts were made during the present investigation to produce sporeless or low-sporing mutants through u.v. mutation. Mutation of the mycelium did not yield the desired results. Mutation of the spores of Pleurotus sajor-caju yielded an extremely low-sporing mutant after 75 min exposure. The character has been found to be stable for more than 10 generations of subculturing. 相似文献
979.
Jan Suchá?ek Petr Sed’a Václav Friedrich Renata Wachowiak-Smolíková Mark P. Wachowiak 《PloS one》2016,11(11)
Media, and particularly TV media, have a great impact on the general public. In recent years, spatial patterns of information and the relevance of intangible geographies have become increasingly important. Gatekeeping plays a critical role in the selection of information that is transformed into media. Therefore, gatekeeping, through national media, also co-forms the generation of mental maps. In this paper, correspondence analysis (a statistical method) combined with cloud lines (a new visual analytics technique) is used to analyze how individual major regional events in one of the post-communist countries, the Czech Republic, penetrate into the media on a national scale. Although national news should minimize distortions about regions, this assumption has not been verified by our research. Impressions presented by the media of selected regions that were markedly influenced by one or several events in those regions demonstrate that gatekeepers, especially news reporters, functioned as a filter by selecting only a few specific, and in many cases, unusual events for dissemination. 相似文献
980.