全文获取类型
收费全文 | 18945篇 |
免费 | 1772篇 |
国内免费 | 8篇 |
专业分类
20725篇 |
出版年
2024年 | 27篇 |
2023年 | 117篇 |
2022年 | 268篇 |
2021年 | 621篇 |
2020年 | 340篇 |
2019年 | 443篇 |
2018年 | 501篇 |
2017年 | 444篇 |
2016年 | 705篇 |
2015年 | 1172篇 |
2014年 | 1239篇 |
2013年 | 1290篇 |
2012年 | 1767篇 |
2011年 | 1778篇 |
2010年 | 1098篇 |
2009年 | 951篇 |
2008年 | 1229篇 |
2007年 | 1228篇 |
2006年 | 1070篇 |
2005年 | 971篇 |
2004年 | 911篇 |
2003年 | 723篇 |
2002年 | 647篇 |
2001年 | 126篇 |
2000年 | 78篇 |
1999年 | 108篇 |
1998年 | 115篇 |
1997年 | 79篇 |
1996年 | 70篇 |
1995年 | 52篇 |
1994年 | 45篇 |
1993年 | 48篇 |
1992年 | 34篇 |
1991年 | 25篇 |
1990年 | 20篇 |
1989年 | 28篇 |
1988年 | 18篇 |
1987年 | 16篇 |
1986年 | 14篇 |
1985年 | 24篇 |
1984年 | 21篇 |
1983年 | 25篇 |
1982年 | 22篇 |
1981年 | 18篇 |
1980年 | 13篇 |
1979年 | 12篇 |
1978年 | 13篇 |
1976年 | 11篇 |
1963年 | 10篇 |
1960年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
The use of plantations to manage extensive tracks of deforested lands in the tropics is a conservation strategy that has recently received considerable attention. Plantation trees can promote seed dispersal by attracting dispersers and creating favorable site conditions, leading to increased germination and establishment of indigenous trees. Subsequently, plantation trees can be harvested for profit or left to senesce, leaving a native tree community. We evaluated the effect of vine, grass, and shrub cutting (weeding) over a 3‐year period on regeneration of indigenous trees subsequent to the removal of plantation softwoods in Kibale National Park, Uganda. Counter to what would be expected if weeding released trees from competition, we found no difference in the total number of stems or in the stems greater than 10 cm diameter at breast height between control and weeded plots; there were more stems greater than 1 cm diameter at breast height in the control plots. For species found in both control and weeded plots, the maximum size of individuals did not differ. At the end of the study, 61 species were found in the control plots and 43 species were found in the weeded plots, and in both types of plots the three most abundant species were the same. The number of species and stems classified as early or middle successional species did not differ between weeded and control plots. The fact that weeding did not promote regeneration of indigenous trees after the removal of plantation trees illustrates the importance of evaluating and field‐testing potential management options. 相似文献
42.
43.
44.
Mitochondrial DNA of the extinct quagga: Relatedness and extent of postmortem change 总被引:4,自引:0,他引:4
Russell G. Higuchi Lisa A. Wrischnik Elizabeth Oakes Matthew George Benton Tong Allan C. Wilson 《Journal of molecular evolution》1987,25(4):283-287
Sequences are reported for portions of two mitochondrial genes from a domestic horse and a plains zebra and compared to those published for a quagga and a mountain zebra. The extinct quagga and plains zebra sequences are identical at all silent sites, whereas the horse sequence differs from both of them by 11 silent substitutions. Postmortem changes in quagga DNA may account for the two coding substitutions between the quagga and plains zebra sequences. The hypothesis that the closest relative of the quagga is the domestic horse receives no support from these data. From the extent of sequence divergence between horse and zebra mitochondrial DNAs (mtDNAs), as well as from information about the fossil record, we estimate that the mean rate of mtDNA divergence in Equus is similar to that in other mammals, i.e., roughly 2% per million years. 相似文献
45.
Increased frequency of 6-thioguanine-resistant peripheral blood lymphocytes in Werner syndrome patients 总被引:1,自引:1,他引:0
Ken-ichiro Fukuchi Kiyoji Tanaka Yuichi Kumahara Kazuo Marumo Matthew B. Pride George M. Martin Raymond J. Monnat Jr. 《Human genetics》1990,84(3):249-252
Summary The frequency of spontaneous 6-thioguanine (TG)-resistant peripheral blood lymphocytes in five unrelated Werner syndrome (WS) patients was determined using an autoradiographic labeling assay. The average frequency of TG-resistant lymphocytes was eightfold higher in WS patients than in sex- and age-matched normal control donors. This finding and previous identification of increased spontaneous chromosomal rearrangements and deletions in WS cells or cell lines suggest that WS is a human genomic instability or mutator syndrome. 相似文献
46.
47.
Chemotaxonomic studies were performed on some heterofermentative lactobacilli of uncertain taxonomic position. Two strains from beer and six strains from a variety of habitats were found to be distinct from each other and all other Lactobacillus species examined on the basis of DNA-DNA hybridizations and warrant new species for which the names L. malefermentans and L. parabuchneri , respectively, are proposed. 相似文献
48.
Thomas L. Pazdernik Matthew Layton Stanley R. Nelson Fred E. Samson 《Neurochemical research》1992,17(1):11-21
This overview presents data showing that glucose use increases and that excitatory amino acids (i.e., glutamate, aspartate), taurine and ascorbate increase in the extracellular fluid during seizures. During the cellular hyperactive state taurine appears to serve as an osmoregulator and ascorbate may serve as either an antioxidant or as a pro-oxidant. Finally, a unifying hypothesis is given for seizure-induced brain damage. This unifying hypothesis states that during seizures there is a release of excitatory amino acids which act on glutamatergic receptors, increasing neuronal activity and thereby increasing glucose use. This hyperactivity of cells causes an influx, of calcium (i.e. calcium stress) and water movements (i.e., osmotic stress) into the cells that culminate in brain damage mediated by reactive oxygen species.Special issue dedicated to Dr. Frederick E. Samson 相似文献
49.
Clostridium mayombei sp. nov., a previously undescribed H2-oxidizing CO2-reducing acetogenic bacterium, was isolated from gut contents of the African soilfeeding termite, Cubitermes speciosus. Cells were anaerobic, Gram positive, catalase and oxidase negative, endospore-forming motile rods which measured 1×2 – 6 m and which had a DNA base composition of 25.6 mol% G+C (strain SFC-5). Optimum conditions for growth on H2+CO2 were at 33°C and pH 7.3, and under these conditions cells produced acetate according to the equation: 4 H2+2 CO2CH3COOH+2 H2O. Other substrates supporting good growth included carbohydrates (e.g. glucose, xylose, starch), sugar alcohols, and organic and amino acids, and with these substrates acetate was almost always the principle fermentation product. Comparative analysis of 16S rRNA nucleotide sequences confirmed that C. mayombei was closely related to various members of the genus Clostridium. However, morphological and physiological differences between C. mayombei and other homoacetogenic clostridia were deemed significant enough to warrant creation of a new taxon. Results are discussed in light of the diversity of H2/CO2 acetogens recently isolated from various termites, and in terms of the relative importance of H2/CO2 acetogenesis to termite nutrition. 相似文献
50.
pH-dependent processes in proteins 总被引:8,自引:0,他引:8
J B Matthew F R Gurd B Garcia-Moreno M A Flanagan K L March S J Shire 《CRC critical reviews in biochemistry》1985,18(2):91-197
Recent improvements in the understanding of electrostatic interactions in proteins serve as a focus for the general topic of pH-dependent processes in proteins. The general importance of pH-dependent processes is first set out in terms of hydrogen ion equilibria, stability, ligand interactions, assembly, dynamics, and events in related molecular systems. The development of various theoretical treatments includes various formalisms in addition to the solvent interface model developed by Shire et al. as an extension of the Tanford-Kirkwood treatment. A number of detailed applications of the model are presented and future potentialities are sketched. 相似文献