全文获取类型
收费全文 | 23603篇 |
免费 | 1839篇 |
国内免费 | 15篇 |
专业分类
25457篇 |
出版年
2023年 | 108篇 |
2022年 | 241篇 |
2021年 | 463篇 |
2020年 | 275篇 |
2019年 | 389篇 |
2018年 | 468篇 |
2017年 | 413篇 |
2016年 | 657篇 |
2015年 | 1115篇 |
2014年 | 1272篇 |
2013年 | 1540篇 |
2012年 | 1803篇 |
2011年 | 1661篇 |
2010年 | 1084篇 |
2009年 | 1009篇 |
2008年 | 1291篇 |
2007年 | 1207篇 |
2006年 | 1202篇 |
2005年 | 1162篇 |
2004年 | 1105篇 |
2003年 | 970篇 |
2002年 | 1024篇 |
2001年 | 295篇 |
2000年 | 230篇 |
1999年 | 278篇 |
1998年 | 289篇 |
1997年 | 216篇 |
1996年 | 240篇 |
1995年 | 216篇 |
1994年 | 227篇 |
1993年 | 228篇 |
1992年 | 202篇 |
1991年 | 162篇 |
1990年 | 135篇 |
1989年 | 145篇 |
1988年 | 157篇 |
1987年 | 108篇 |
1986年 | 103篇 |
1985年 | 122篇 |
1984年 | 148篇 |
1983年 | 117篇 |
1982年 | 143篇 |
1981年 | 118篇 |
1980年 | 112篇 |
1979年 | 95篇 |
1978年 | 93篇 |
1977年 | 86篇 |
1976年 | 62篇 |
1974年 | 59篇 |
1973年 | 66篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Jan Vymazal 《Hydrobiologia》1988,166(3):225-237
The results of experiments on the efficiency of periphyton communities for nutrient removal from polluted streams in a continuous flow-through are given. The artifical stream (5 m × 0.7 m × 0.5 m) was made of wood, with silon (a kind of nylon) screens, as a substratum for periphyton growth. The elimination of nutrients was monitored by ammonium, nitrite, nitrate and orthophosphate analyses. In addition, the elimination of organics and the decrease in trophic state were determined. During two field experiments a marked elimination of nitrogen and phosphorus was demonstrated. The maximum efficiency of ammonium and orthophosphate removal was 80% and 70%, respectively. Organic removal reached 35% (C.O.D.Mn-Kubel) and 54% (B.O.D.5). Inorganic and organic nutrient elimination caused significant changes in periphyton community structure in the outflow portion of the through, evaluated by the saprobic index and the similarity coefficient. The experiments confirmed that periphyton communities are a useful means of nutrient removal from polluted streams. 相似文献
102.
Endoparasitic copepods are very numerous in Indo-West Pacific corals. In West Indian corals they were thought to be absent, but recent studies have shown that a varied endoparasitic copepod fauna exists as well. Striking is the taxonomic composition of the coral-inhabiting copepods:In the Indo-West Pacific Lichomolgidae and Xarifiidae are the dominant families, both are absent in the West Indies. On the other hand, Corallovexiidae and Asterocheridae dominate in the West Indies; the former family is absent and the latter is apparently rare and not very diversified in the Indo-West Pacific. 相似文献
103.
Niels Peter Revsbech Lars Peter Nielsen Peter Bondo Christensen Jan Srensen 《Applied microbiology》1988,54(9):2245-2249
The construction of a microsensor which can be used to measure O2 and N2O simultaneously is described. The microsensor exhibited a linear response to both O2 and N2O, and the response to N2O was independent of the O2 concentration and vice versa. The N2O detection limit of a microsensor with a tip diameter of 20 μm was around 1 μmol liter−1. The signals for O2 and N2O were affected by hydrogen sulfide, but other interfering agents were not observed in the biofilms and sediments analyzed. Microprofiles of O2 and N2O were measured in a biofilm which was exposed to acetylene to block the N2O reductase activity of denitrifying bacteria. O2 penetrated about 0.5 mm into the biofilm and was not affected by acetylene, but the N2O concentration at 1.4 mm depth increased from 32 to 411 μmol liter−1 after the addition of the inhibitor. The shape of the N2O profile after the addition of acetylene showed that denitrification (denitrifying activity) was detectable in all anoxic layers of the biofilm. 相似文献
104.
Christophe N. N'soukpo-Kossi Jan Sielewiesiuk Roger M. Leblanc Richard A. Bone John T. Landrum 《生物化学与生物物理学报:生物膜》1988,940(2)
The linear dichroism of single monolayers of lutein, zeaxanthin and a mixture of lutein and synthetic phosphatidylcholine has been measured. The angle of orientation of the carotenoid molecules was found to lie between 45° and 51° relative to the plane of the solid support. Although the adsorbed monolayers were mostly in a monomeric state, microscopic observations, as well as the II-A isotherms, indicated the existence of crystalline islets. The results have been interpreted in connection with Haidinger's polarization brushes. 相似文献
105.
W Korytowski C C Felix B Kalyanaraman 《Biochemical and biophysical research communications》1988,154(2):781-788
Oxidase electrode measurements as well as optical and electron spin resonance spectroscopic data have shown that synthetic neuromelanin oxidizes the neurotoxin metabolite 1-methyl-4-phenyl-2,3-dihydropyridinium in a dose-dependent manner forming 1-methyl-4-phenylpyridinium and hydrogen peroxide. Hydroxyl radicals are formed in this reaction which is promoted by iron chelates. In contrast, neither 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine nor 1-methyl-4-phenylpyridinium reacts with synthetic neuromelanin in a similar fashion. The mechanism of selective toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in pigmented neuronal cells is discussed in the light of these findings. 相似文献
106.
A burst of CO2 from photosynthesizing leaves after a temperature decrease under constant light conditions 总被引:1,自引:0,他引:1
A transient CO2 burst from seedlings of some plant species was observed after a rapid temperature decrease. The magnitude of the CO2 release depended on initial temperature, oxygen concentration and light intensity. To obtain a maximal value of CO2 release, the temperature had to decrease by more than 8°C. The phenomenon was detected only in the light, and was confined to C3 species. It was inhibited by low oxygen concentration, indicating its possible connection with photorespiration. 相似文献
107.
108.
109.
Cytotoxicity, chromosome aberrations, and mutations to 6-thioguanine resistance were synergistically increased by incubating the ultraviolet light (UV)-irradiated Chinese hamster ovary (CHO) cells in medium containing sodium arsenite. However, the frequencies of sister-chromatid exchanges and mutations to ouabain resistance induced by UV were not synergistically increased by sodium arsenite. The synergistic effect of sodium arsenite on UV-induced chromosome aberrations varied with cell-harvesting time and decreased with increasing time intervals between UV and sodium arsenite treatments. 相似文献
110.
Requirement for the action of endogenous ethylene during germination of non-dormant seeds of Amaranthus caudatus 总被引:2,自引:0,他引:2
The role of endogenous ethylene during germination of non-dormant seeds of Amaranthus caudatus L. was investigated. The seeds readily germinated in water and darkness at 24°C. Application of ethylene or of its precursor I-aminocyclopropane-I-carboxylic acid (ACC) slightly increased the rate of germination. Both compounds effectively antagonized osmotic inhibition by polyethyleneglycol. Application of aminoethoxyvinylglycine (AVG) reduced ethylene production by 90% but did not inhibit germination. However, germination was inhibited by 2,5-norbornadiene, a competitive inhibitor of ethylene action. This inhibition was counteracted by ethylene, ethephon or ACC and enforced by AVG. It is concluded that the action of endogenous ethylene is an indispensable factor during germination of non-dormant seeds of A. caudatus. Ethylene action is required from the start of imbibition on. In water, low levels of endogenous ethylene are sufficient for this action. PEG increased the ethylene requirement considerably. 相似文献