全文获取类型
收费全文 | 46096篇 |
免费 | 4566篇 |
国内免费 | 11篇 |
专业分类
50673篇 |
出版年
2022年 | 355篇 |
2021年 | 692篇 |
2020年 | 515篇 |
2019年 | 589篇 |
2018年 | 770篇 |
2017年 | 705篇 |
2016年 | 1175篇 |
2015年 | 1931篇 |
2014年 | 2125篇 |
2013年 | 2592篇 |
2012年 | 3211篇 |
2011年 | 3145篇 |
2010年 | 2082篇 |
2009年 | 1769篇 |
2008年 | 2536篇 |
2007年 | 2504篇 |
2006年 | 2369篇 |
2005年 | 2153篇 |
2004年 | 2130篇 |
2003年 | 1889篇 |
2002年 | 1867篇 |
2001年 | 852篇 |
2000年 | 863篇 |
1999年 | 791篇 |
1998年 | 499篇 |
1997年 | 398篇 |
1996年 | 399篇 |
1995年 | 396篇 |
1994年 | 355篇 |
1993年 | 347篇 |
1992年 | 525篇 |
1991年 | 475篇 |
1990年 | 460篇 |
1989年 | 432篇 |
1988年 | 466篇 |
1987年 | 416篇 |
1986年 | 344篇 |
1985年 | 379篇 |
1984年 | 378篇 |
1983年 | 324篇 |
1982年 | 309篇 |
1981年 | 277篇 |
1980年 | 244篇 |
1979年 | 283篇 |
1978年 | 220篇 |
1977年 | 237篇 |
1976年 | 214篇 |
1975年 | 246篇 |
1974年 | 198篇 |
1973年 | 209篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
Abstract. Small-scale species frequency and cumulative species frequency were studied in four plots in limestone grassland of the Veronica spicata-Avenula pratensis association on Stora Alvaret on the Baltic island of Öland, Sweden. Species mobility was expressed as increase in cumulative species frequency in 20 subplots of 100 cm2. Observed cumulative frequencies from 1985–1989 in all four plots, and from 1985–1995 in one plot were compared with values following from two null models, a ‘minimal mobility’ model and a random mobility model. In ca. 50 % of the cases the observed cumulative frequency was not significantly different from the random expectation. However, in many such cases the mean annual frequency was either very high or very low. Three ways of calculating the mobility rate are presented though only one is used: (observed cumulative frequency -lowest annual frequency) / expected cumulative frequency. Values × 100 range from 0 to 100. There were slight differences between the four plots which were interpreted in terms of differences in grazing intensity and soil depth. It is stressed that the idea of the Carousel model has never been meant to suggest that all species would show random mobility, which we now quantify, but that species differ in their mobility rate and that the mean rate is much higher than generally realized. 相似文献
43.
44.
The low-molecular-mass form of two distinct catechol O-methyltransferase activities (S-adenosyl-L-methionine: catechol O-methyltransferase, COMT, EC 2.1.1.6) has been purified to homogeneity from rat liver using 40-70% ammonium sulfate precipitation, gel filtration on Sephadex G-100, adsorption on hydroxyapatite C and ion-exchange chromatography on DEAE-Sepharose CL-6B. The relative molecular mass Mr, determined by sodium dodecyl sulfate/polyacrylamide gel electrophoresis is 22 400 +/- 500. Irradiation of the enzyme in the presence of 8-azido-[methyl-3H]AdoMet results in the specific labeling of the catalytic site of the enzyme. Photolabeling was successful with crude COMT preparations and with the isolated enzyme. Immunocytochemical studies present new information about the localization of the low-molecular-mass form in the liver parenchyma. Subcellularly COMT immunoreactivity could be attributed exclusively to the compartment with glycogen granules. Nucleus, mitochondria and endoplasmic reticulum showed no immunostaining. 相似文献
45.
Ulrike Rosenfellner Franz Zehetner Martin H. Gerzabek 《Soil & Sediment Contamination》2009,18(6):685-687
Data from eight published studies were combined to show that the influence of traffic density on Pb contents in roadside soils increases with proximity to the road. 相似文献
46.
47.
Wounding of quiescent white potato tuber tissue enhances chromatin-boundprotein phosphokinase activity, which exhibits two distinctphases during wound-healing. A moderate activation of the enzymesup to 20 hr after injury is followed by a dramatic increasein activity with a peak at 50 hr. This time-course resemblesthat of chromatinbound DNA-dependent RNA polymerase with a peakin activity at about 48 hr after wounding. The kinases phosphorylateendogenous proteins as well as added histones, phosvitin andcasein. The incorporated phosphate is stable under standardassay conditions, indicating the absence of protein phosphatases.Sensitivity of the incorporated phosphate toward trypsin andalkali, but not DNase, RNase, hydroxylamine or succinic acidpoints to seryl- and threonyl-bonds and proteins as acceptormolecules. Kinases from resting tissues are only weakly stimulatedeven by 100 mM MgCl2, those from wounded tissues exhibit pronouncedMg$$-optima at 510 mM with endogenous proteins, phosvitinand casein and 50 mM MgCl2 with histones. Wounding also increasesthe sensitivity of the kinases toward p-hydroxymercuribenzoate. Chromatin preparations from both resting and wounded tissuescontain about 40 protein bands after polyacrylamide disc gelelectrophoresis. In vitro phosphorylation of these proteinsin chromatin from quiescent tissues is comparably low and uniform.Wounding induces changes in the protein and phosphorylationpattern with a general enhancement of phosphorylative capacityand preferential phosphorylation of low molecular weight proteins. (Received August 10, 1981; Accepted November 18, 1981) 相似文献
48.
49.
Michael Gewecke Martin Woike 《Ethology : formerly Zeitschrift fur Tierpsychologie》1978,47(3):293-298
Analysis of the flight behaviour of siskins (Carduelis spinus) was made possible by tethering them with a harness to a flight balance in a wind tunnel. It could be shown that the breast feathers work as an air-current sense organ influencing the siskins' flight pattern and the aerodynamic parameters. 相似文献
50.