全文获取类型
收费全文 | 4506篇 |
免费 | 442篇 |
国内免费 | 170篇 |
出版年
2024年 | 8篇 |
2023年 | 51篇 |
2022年 | 86篇 |
2021年 | 109篇 |
2020年 | 151篇 |
2019年 | 135篇 |
2018年 | 154篇 |
2017年 | 112篇 |
2016年 | 116篇 |
2015年 | 149篇 |
2014年 | 194篇 |
2013年 | 252篇 |
2012年 | 97篇 |
2011年 | 205篇 |
2010年 | 171篇 |
2009年 | 250篇 |
2008年 | 274篇 |
2007年 | 252篇 |
2006年 | 249篇 |
2005年 | 197篇 |
2004年 | 213篇 |
2003年 | 171篇 |
2002年 | 158篇 |
2001年 | 99篇 |
2000年 | 93篇 |
1999年 | 86篇 |
1998年 | 89篇 |
1997年 | 84篇 |
1996年 | 85篇 |
1995年 | 71篇 |
1994年 | 78篇 |
1993年 | 63篇 |
1992年 | 63篇 |
1991年 | 37篇 |
1990年 | 37篇 |
1989年 | 52篇 |
1988年 | 38篇 |
1987年 | 46篇 |
1986年 | 33篇 |
1985年 | 69篇 |
1984年 | 61篇 |
1983年 | 39篇 |
1982年 | 38篇 |
1981年 | 31篇 |
1980年 | 26篇 |
1979年 | 16篇 |
1978年 | 6篇 |
1977年 | 6篇 |
1976年 | 6篇 |
1975年 | 3篇 |
排序方式: 共有5118条查询结果,搜索用时 15 毫秒
31.
Changes in Nine Enzyme Markers for Neurons, Glia, and Endothelial Cells in Agonal State and Huntington''s Disease Caudate Nucleus 总被引:2,自引:2,他引:0
John Butterworth 《Journal of neurochemistry》1986,47(2):583-587
Enzymes considered to be markers for neurons (angiotensin converting enzyme, thermolysin-like metalloendopeptidase, alanine aminopeptidase, and glutamate-oxaloacetate transaminase), glia (glutamine synthetase, pyruvate carboxylase, and beta-glucuronidase), and endothelial cells (alkaline phosphatase and plasminogen activator) were measured in caudate nucleus from 10 sudden death controls, eight agonal state controls, and 16 Huntington's disease patients. Glutamate-oxaloacetate transaminase was slightly reduced by agonal state. The four enzymes with a neuronal distribution were all correlatively reduced in Huntington's disease caudate nucleus. Glutamine synthetase activity was reduced and beta-glucuronidase mean activity increased over twofold in Huntington's disease caudate nucleus, with the two enzyme activities being inversely related. Pyruvate carboxylase was markedly affected by agonal state and was very variable in Huntington's disease caudate nucleus. The two endothelial enzymes were unaltered in Huntington's disease caudate nucleus. The findings are indicative of neuronal loss, an increased proportion of altered glia, and also of maintained vasculature in Huntington's disease caudate nucleus. Measurement of enzyme activities can help to delineate the types of cell altered in Huntington's disease. 相似文献
32.
Chemical limnology of two artificial lakes used for both stormwater management and recreation 总被引:1,自引:1,他引:0
The aims of this study were to document the mainly chemical behaviour of two linked artificial lakes used for both stormwater management and recreation in the new town of Craigavon. Further, the understanding of their behaviour should help in their management and the design of other similar lakes.The lake mean total phosphorus (73 µg P l–1), nitrate (0.50 mg N l–1) and chlorophyll a (25 µg l–1) concentrations, Secchi depth (1.2 m) and the estimated total phosphorus loading (1.98 g m–2 a–1) all classify the main lake as eutrophic. An important source of the phosphorus load on the lakes is the urban area of Craigavon (52% of the total load). The interrelationships between total phosphorus, chlorophyll a and Secchi depth in the main lake are similar to those in natural ones. In addition, the lake follows the total phosphorus load — trophic state relationships (lake total phosphorus and chlorophyll a concentrations and Secchi depth) found to apply elsewhere. These two points indicate that the artificial lakes in Craigavon behave similarly to natural ones. 相似文献
33.
Edward S. Deevey Michael W. Binford Mark Brenner Thomas J. Whitmore 《Hydrobiologia》1986,143(1):49-53
Transfer functions relating trophic state (Carlson's TSIchlorophyll-a) to present day accumulation rate of (1) nutrients, (2) cations, and (3) organic sediment, are computed using Binford's 210Pb-dilution method. As computed from surficial sediments of 27 lakes, former trophic states are reconstructed for recent (210Pb-dated) sedimentary histories of 14 lakes. Of the three kinds of models potentially available, model (3) (TSI vs. organic accumulation) is the weakest statistically, and may be unduly influenced by exceptional deposition and/or preservation of allochthonous organic matter. At present, however, it is the only model applicable to all 14 of the histories tested. Results are encouraging in that model accurately predicts observed TSI's in several mesotrophic and eutrophic lakes. Clearly significant increases (accelerations) are inferred only for a of the most eutrophic lakes of the set, while the model consistently overpredicts TSI's of the 7 most oligotrophic lakes. As Whitmore's diatom-assemblage index is a better predictor of TSI than is model (3) in the one eutrophic lake in which it has been tested, we expect more persuasive results when models (1) and (2) can be tested within a more complete set of analytical data. We were surprised to find 3 severely disturbed lakes among the 12 that show little or no acceleration in rate of eutrophication in recent decades, but we defer attempts at explanation until former nutrient loading can be tested by model (1). 相似文献
34.
The assumption that landscapes dominated by mature vegetation are presently in carbon steady state with the atmosphere is challenged. Evidence suggests that the vegetation and soils of these landscapes are frequently disturbed and over short time periods (<300 yr) slowly sequester atmospheric carbon. The critical consideration in this argument is the time interval used to evaluate a steady state. Current models of carbon flux through the terrestrial biota limit their time considerations to 120 yr, a short and inadequate time interval for realistic assumptions about steady state in the carbon cycle of vegetation.Research performed under subcontract 19B-07762C with S. Brown and 19X-43326C with the Center for Energy and Environment Research of the University of Puerto Rico (A. E. Lugo) under Martin Marietta Energy Systems, Inc., contract DE-AC05-840R21400 with the U.S. Department of Energy. 相似文献
35.
A combined single-turnover flash and 35Cl NMR technique has been used to monitor S-state dependence of Cl− binding to PS-II particles derived from mangrove (Avicennia marina). No detectable high-affinity binding was found to particles in the S0 and S1 states, but binding with an affinity comparable to that which activates O2 evolution was found in the S2 and S3 states. 相似文献
36.
The araBAD operon of Salmonella typhimurium LT2. I. Nucleotide sequence of araB and primary structure of its product, ribulokinase 总被引:6,自引:0,他引:6
Hybrid plasmids containing the araBAD operon of Salmonella typhimurium LT2 were characterized by Southern blot and genetic analyses. The nucleotide sequence of araB was determined. The araB gene product, ribulokinase (EC 2.7.1.16), was purified and the results of amino acid composition analysis and partial amino acid sequence are in agreement with predictions from the DNA sequence. Ribulokinase is 569 amino acid residues long and has a calculated Mr of 61 793. Ribulokinase shares significant homology with xylulose kinase from Escherichia coli. Codon usage in the araB gene does not favor those codons which have intermediate codon-anticodon binding energy. 相似文献
37.
38.
Nonrandom insertion of Tn5 into cloned human adenovirus DNA 总被引:4,自引:0,他引:4
39.
Haruki Ochi 《Environmental Biology of Fishes》1985,12(2):155-160
Synopsis Reproductive behavior of the temperate damselfish, Chromis notata, was investigated on the island of Mukaishima, Japan, almost daily during the breeding season in 1982. Both males and females
repeated reproductive cycles many times during the breeding season. Females had a strong tendency to spawn a whole clutch
on one nest during a few hours. The average number of eggs which a male gained per reproductive cycle was estimated at 38560
(480–131100 eggs). Males ordinarily cared for eggs until just prior to hatching, but abandoned more than half of the nests
with the eggs numbering less than 11568.
Contribution 207 from the Mukaishima Marine Biological Station. 相似文献
40.
Tetsuo Kuwamura 《Environmental Biology of Fishes》1985,13(1):17-24
Synopsis Social and reproductive behavior of three paternal mouthbrooding cardinalfishes (Apogonidae) were investigated in the shallow marine waters of Shirahama, Japan. The solitary species Apogon doederleini and A. niger bred in transient pairs, in which a male and female associated for only a few hours of each afternoon on less than 5 successive days. The prespawning behavior was the same as the courtship display on days prior to spawning. After spawning, egg-incubating males were usually left alone. The gregarious species Apogon notatus formed territorial lasting pairs, which resided at given sites from dawn to dusk on each day during a period of a month or more. After spawning, the egg-incubating male either continued to stay with his mate in the territory, or left it to enter into an aggregation. In the latter case, the female continued to reside in the territory, pairing with a new male whom she brought from an aggregation. It is suggested that in paternal apogonids the prolonged pair bond and territoriality should have developed only in gregarious species as secondary adaptation for reproductive success: to avoid conspecific interference during spawning. 相似文献