全文获取类型
收费全文 | 6467篇 |
免费 | 776篇 |
国内免费 | 3篇 |
专业分类
7246篇 |
出版年
2023年 | 18篇 |
2022年 | 40篇 |
2021年 | 107篇 |
2020年 | 53篇 |
2019年 | 76篇 |
2018年 | 60篇 |
2017年 | 71篇 |
2016年 | 140篇 |
2015年 | 247篇 |
2014年 | 272篇 |
2013年 | 303篇 |
2012年 | 456篇 |
2011年 | 477篇 |
2010年 | 251篇 |
2009年 | 255篇 |
2008年 | 392篇 |
2007年 | 426篇 |
2006年 | 384篇 |
2005年 | 417篇 |
2004年 | 391篇 |
2003年 | 409篇 |
2002年 | 367篇 |
2001年 | 66篇 |
2000年 | 41篇 |
1999年 | 90篇 |
1998年 | 115篇 |
1997年 | 83篇 |
1996年 | 81篇 |
1995年 | 69篇 |
1994年 | 76篇 |
1993年 | 68篇 |
1992年 | 50篇 |
1991年 | 51篇 |
1990年 | 41篇 |
1989年 | 41篇 |
1988年 | 39篇 |
1987年 | 57篇 |
1986年 | 35篇 |
1985年 | 44篇 |
1984年 | 57篇 |
1983年 | 49篇 |
1982年 | 80篇 |
1981年 | 50篇 |
1980年 | 52篇 |
1979年 | 38篇 |
1978年 | 26篇 |
1977年 | 38篇 |
1976年 | 19篇 |
1975年 | 20篇 |
1974年 | 18篇 |
排序方式: 共有7246条查询结果,搜索用时 0 毫秒
21.
22.
23.
Species-specific algal responses to zooplankton: experimental and field observations in three nutrient-limited lakes 总被引:3,自引:0,他引:3
Elser James J.; Goff Nancy C.; MacKay Neil A.; Amand Ann L.St.; Elser Monica M.; Carpenter Stephen R. 《Journal of plankton research》1987,9(4):699-717
A series of 4-day manipulations of zooplankton biomass and nutrientavailability was performed in enclosures in three lakes to determinespecies-specific algal responses to herbivory and nutrient enrichment.Algal performance in enclosures was compared to the relationshipsbetween weekly algal growth rates and the zooplankton in situ.When in situ growth rates were significant functions of zooplanktonbiomass, the responses were generally consistent with responsesin the enclosure experiments. The importance of both nutrientsand zooplankton in mediating algal growth was demonstrated bynumerous observations: strong algal community response to enrichment,unimodal or positive responses of certain algal taxa to zooplanktonbiomass, differences in degree of nutrient limitation amongthe algal response types, lack of nutrient limitation of non-grazedalgal taxa and a preponderance of taxa with no net responseto increasing zooplankton biomass. Variation in the zooplanktoncommunity may be the largest source of variability in nutrientsupply rate during summer in stratified lakes, and causes substationalvariability in the algae. Algae responded more strongly to changesin zooplankton composition than to changes in zooplankton biomass.We conclude that, due to the close coupling of phytoplanktonand zooplankton communities in these nutrient-limited lakes,major compositional changes in the zooplankton have greatereffects on the algae than do changes in biomass of grazers alreadypresent.
1Present address: Division of Environmental Studies, Universityof California, Davis, CA 95616, USA
2Present address: Division of Biological Sciences, Universityof California, Davis, CA 95616, USA 相似文献
24.
In rats fed a normal (22% protein) diet, injection of clenbuterol (1 mg/kg/d for 21 d) did not affect energy intake, energy expenditure or weight gain, but reduced energetic efficiency, and fat and energy gains and increased body protein content. Presenting a low-protein (8%) diet reduced energy intake, gain and efficiency, body protein content and the mass of the gastrocnemius muscle when compared to rats fed the control diet. Injection of the protein-deficient rats with clenbuterol (1 mg/kg/d for 21 d) caused hypophagia and reduced body weight and energy gains, energy expenditure and total body fat. However, the total body content of protein was not significantly reduced and the percentage of body protein in this protein deficient, clenbuterol-treated group was greater than that of untreated rats on both the high- and low-protein diets. The ratio of body protein to fat following clenbuterol treatment was increased by over 50% in both normal and protein-deficient rats. The results show that in protein deficient animals, clenbuterol treatment may help conserve body protein at the expense of fat, resulting in a smaller, but leaner body mass. 相似文献
25.
Effects of water stress on production of ethylene and its precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), by loblolly pine ( Pinus taeda L.) seedlings from a Texas drought-hardy and a Virginia Coastal Plain source were investigated. Ethylene production rates in needles from the Virgnia source increased slightly with initial stress (-1.3 MPa), declined until water potential reached -1.6 MPa and then increased sharply at -2.5 MPa. The ethylene production rates in needles from the Texas also increased slightly with initial stress, then decreased with decreasing water potential. Ethylene production by root tissue was two to three times higher than needle tissue and decreased with decreasing water potential. ACC concentrations in needles of both seed sources decreased as water potential began decreasing. Below -1.4 MPa, ACC levels started increasing (Texas source) or remained constant until -2.8 MPa (Virginia source) at which time its level increased three-fold. Mean ACC levels in root tissue [122 nmol (g dry weight)−1 ] were slightly higher than the mean levels in the needle tissue [92 nmol (g dry weight) −1 ]; roots apparently were more efficient in converting it to ethylene since ethylene production was two to three times higher than needle tissue. The modulation of ethylene synthesis by ACC synthase and ethyleneforming enzyme appeared to be influenced by stress level, organ and seed source. 相似文献
26.
Nancy Scheper-Hughes 《Culture, medicine and psychiatry》1987,11(1):53-78
The deinstitutionalization of psychiatric patients is a deeply cultural as well as political task. It entails the sharing of responsibility for human distress with family and community. Consequently, the locus of social control has also shifted from psychiatric and medical expertise to community and legal institutions. Diagnosis and treatment models must be more compatible with lay explanatory models. This paper explores the various meanings of going mental and being mental in the white, working class, ethnic neighborhood of South Boston. The data are extracted from a study of the impact of deinstitutionalization on a cohort of middle-aged, psychiatric patients discharged from Boston State Hospital in the attempt to return them to community living. Individual, family, and community responses to, and interpretations of, the symptoms of mental distress are discussed. The study indicates that even seriously disturbed individuals are sensitive to cultural meanings and social cues regarding the perception, expression, and content of psychiatric episodes. While madness invariably disenfranchises, it does not necessarily deculturate the individual. 相似文献
27.
d-Aspartate in Human Brain 总被引:3,自引:3,他引:0
Eugene H. Man George H. Fisher Iris L. Payan Rodolfo Cadilla-Perezrios Nancy M. Garcia Radhika Chemburkar Georgine Arends William H. Frey II 《Journal of neurochemistry》1987,48(2):510-515
The presence of the biologically uncommon D-aspartic acid (D-aspartate) in human brain white matter has been previously reported. The earlier study has now been expanded to include D/L-aspartate ratios from 67 normal brains. The data show that the D-aspartate content increases rapidly from 1 year to approximately 35 years of age, levels off in middle age, and then appears to decrease somewhat. The D-aspartate content in gray matter remains at a consistently low level (half of that found in white matter) throughout the human life span. Within the limitations of current analytical methods, there was no detectable difference in D/L-aspartate ratios in white and gray matter of brains with Alzheimer's disease and several other pathologies when compared with brains of normal subjects. However, the presence of a significant D-aspartate level in white matter during the adult life span may lead to changes in protein configuration related to dysfunctions associated with the aging brain. 相似文献
28.
Freeze-Thawing of Aquaspirillum magnetotacticum Cells Selectively Releases Periplasmic Proteins 总被引:1,自引:1,他引:0 下载免费PDF全文
Lawrence C. Paoletti Kevin A. Short Nancy Blakemore Richard P. Blakemore 《Applied microbiology》1987,53(10):2590-2592
Cells of the gram-negative bacterium Aquaspirillum magnetotacticum, when suspended in buffer and freeze-thawed, produced pinkish orange supernatant fluid. The fluid contained ≤2.0% of total extractable outer membrane component 2-keto-3-deoxyoctonate or of the cytoplasmic membrane marker succinic dehydrogenase. Electrophoretic banding patterns and difference spectra of proteins and hemoproteins released by freeze-thawing cells were distinct from those of membrane-associated substances and similar to those of periplasmic substances obtained by applying conventional fractionation methods to this organism. 相似文献
29.
Characterization and in Vivo Cloning of prlC, a Suppressor of Signal Sequence Mutations in Escherichia coli K12 总被引:19,自引:1,他引:18 下载免费PDF全文
The prlC gene of E. coli was originally identified as an allele, prlC1, which suppresses certain signal sequence mutations in the genes for several exported proteins. We have isolated six new alleles of prlC that also confer this phenotype. These mutations can be placed into three classes based on the degree to which they suppress the lamB signal sequence deletion, lamBs78. Genetic mapping reveals that the physical location of the mutations in prlC correlates with the strength of the suppression, suggesting that different regions of the gene can be altered to yield a suppressor phenotype. We also describe an in vivo cloning procedure using lambda placMu9H. The procedure relies on transposition and illegitimate recombination to generate a specialized transducing phage that carries prlC1. This method should be applicable to any gene for which there is a mutant phenotype. 相似文献
30.