全文获取类型
收费全文 | 635317篇 |
免费 | 70652篇 |
国内免费 | 405篇 |
专业分类
706374篇 |
出版年
2018年 | 5350篇 |
2016年 | 7287篇 |
2015年 | 9886篇 |
2014年 | 11374篇 |
2013年 | 17208篇 |
2012年 | 18856篇 |
2011年 | 19161篇 |
2010年 | 12817篇 |
2009年 | 11869篇 |
2008年 | 17304篇 |
2007年 | 17642篇 |
2006年 | 16963篇 |
2005年 | 16098篇 |
2004年 | 15909篇 |
2003年 | 15456篇 |
2002年 | 14991篇 |
2001年 | 27752篇 |
2000年 | 27894篇 |
1999年 | 22446篇 |
1998年 | 8138篇 |
1997年 | 8581篇 |
1996年 | 8282篇 |
1995年 | 7517篇 |
1994年 | 7616篇 |
1993年 | 7510篇 |
1992年 | 18994篇 |
1991年 | 18567篇 |
1990年 | 18189篇 |
1989年 | 18054篇 |
1988年 | 16605篇 |
1987年 | 15904篇 |
1986年 | 14632篇 |
1985年 | 14849篇 |
1984年 | 12136篇 |
1983年 | 10475篇 |
1982年 | 8117篇 |
1981年 | 7270篇 |
1980年 | 7011篇 |
1979年 | 11784篇 |
1978年 | 9140篇 |
1977年 | 8449篇 |
1976年 | 8032篇 |
1975年 | 8769篇 |
1974年 | 9374篇 |
1973年 | 9230篇 |
1972年 | 8325篇 |
1971年 | 7508篇 |
1970年 | 6706篇 |
1969年 | 6364篇 |
1968年 | 5736篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
982.
F H Stephenson 《Gene》1985,35(3):313-320
983.
Control of breathing in Sherpas at low and high altitude 总被引:2,自引:0,他引:2
Hackett P. H.; Reeves J. T.; Reeves C. D.; Grover R. F.; Rennie D. 《Journal of applied physiology》1980,49(3):374-379
984.
985.
J. Paupério J. S. Herman J. Melo‐Ferreira M. Jaarola P. C. Alves J.B. Searle 《Molecular ecology》2012,21(24):6015-6032
Species are generally described from morphological features, but there is growing recognition of sister forms that show substantial genetic differentiation without obvious morphological variation and may therefore be considered ‘cryptic species’. Here, we investigate the field vole (Microtus agrestis), a Eurasian mammal with little apparent morphological differentiation but which, on the basis of previous sex‐linked nuclear and mitochondrial DNA (mtDNA) analyses, is subdivided into a Northern and a Southern lineage, sufficiently divergent that they may represent two cryptic species. These earlier studies also provided limited evidence for two major mtDNA lineages within Iberia. In our present study, we extend these findings through a multilocus approach. We sampled 163 individuals from 46 localities, mainly in Iberia, and sequenced seven loci, maternally, paternally and biparentally inherited. Our results show that the mtDNA lineage identified in Portugal is indeed a distinct third lineage on the basis of other markers as well. In fact, multilocus coalescent‐based methods clearly support three separate evolutionary units that may represent cryptic species: Northern, Southern and Portuguese. Divergence among these units was inferred to have occurred during the last glacial period; the Portuguese lineage split occurred first (estimated at c. 70 000 bp ), and the Northern and Southern lineages separated at around the last glacial maximum (estimated at c. 18 500 bp ). Such recent formation of evolutionary units that might be considered species has repercussions in terms of understanding evolutionary processes and the diversity of small mammals in a European context. 相似文献
986.
H. Elenga O. Peyron R. Bonnefille D. Jolly R. Cheddadi J. Guiot V. Andrieu S. Bottema G. Buchet J.-L. De Beaulieu A. C. Hamilton J. Maley R. Marchant R. Perez-Obiol M. Reille G. Riollet L. Scott H. Straka D. Taylor E. Van Campo A. Vincens F. Laarif H. Jonson 《Journal of Biogeography》2000,27(3):621-634
Pollen data from 18,000 14C yr bp were compiled in order to reconstruct biome distributions at the last glacial maximum in southern Europe and Africa. Biome reconstructions were made using the objective biomization method applied to pollen counts using a complete list of dryland taxa wherever possible. Consistent and major differences from present‐day biomes are shown. Forest and xerophytic woods/scrub were replaced by steppe, both in the Mediterranean region and in southern Africa, except in south‐western Cape Province where fynbos (xerophytic scrub) persisted. Sites in the tropical highlands, characterized today by evergreen forest, were dominated by steppe and/or xerophytic vegetation (cf. today’s Ericaceous belt and Afroalpine grassland) at the last glacial maximum. Available data from the tropical lowlands are sparse but suggest that the modern tropical rain forest was largely replaced by tropical seasonal forest while the modern seasonal or dry forests were encroached on by savanna or steppe. Montane forest elements descended to lower elevations than today. 相似文献
987.
A Adam J Damas P Franchimont 《Comptes rendus des séances de la Société de biologie et de ses filiales》1980,174(5):856-862
A radioimmunoassay for low molecular weight (LMW) human Kininogen has been carried out. The first step was to prepare LMW Kininogen from human plasma. The proposed method allowed to get chemically pure and biologically active LMW Kininogen. This preparation was used to induce antibody. Optimal conditions for labelling and incubation were determined. This method may be applied to the assay of Kininogen in human plasma. 相似文献
988.
Fast product formation and slow product release are important features in a hysteretic reaction mechanism of glutathione transferase T2-2. 总被引:1,自引:0,他引:1
The reaction mechanism of rat glutathione transferase T2-2 has been studied using pre-steady-state and steady-state kinetics. Several parts of the catalytic cycle including binding of substrates, product formation, and product release were investigated. Under saturating conditions, a two-step product release was found to be rate limiting in the enzyme-catalyzed reactions between the nucleophilic substrate glutathione and either of the two electrophilic substrates 1-menaphthyl sulfate and 4-nitrobenzyl chloride. The rate constant for pre-steady-state product formation on rat glutathione transferase T2-2 has an observed pK(a) value of 5.7 apparently due to ionization of the sulfhydryl group of glutathione. This rate constant is approximately 2 orders of magnitude higher than k(cat) at pH values of >6. It can be predicted from the pH dependence that product formation would be the sole rate-limiting step at pH values of <3. A hysteretic mechanism of rGST T2-2 is proposed based on a slow conformational transition detected in pre-steady-state displacement experiments. 相似文献
989.
Cortinarius sarcoflammeus is proposed as a new species belonging to subgenus Dermocybe, on the basis of its morphological, chemical and ecological characters. The strong red-orange colour of the context and stipe
base, large spores, sphagnicolous habitat and high dermorubin content are characteristic for the new species. Holotypes of
C. huronensis and C. huronensis var. olivaceus have been examined for comparison, and their differences discussed. Photographs and line drawings of C. sarcoflammeus are added.
Received February 13, 2001 Accepted March 23, 2001 相似文献
990.
The development of gene therapy is hampered by the difficulty of producing large stocks of retroviral vectors at high titer. This study aimed to improve culture conditions and to intensify the production of retroviruses by FLYRD18, a packaging cell line derived from the HT1080 human fibrosarcoma line. Batch virus production proved to be feasible in unsupplemented basal medium and provided significantly higher titers and productivities than medium supplemented with 10% serum. For longer-term production, however, AIM-V complete serum-free medium and basal medium supplemented with 2% serum gave superior results. Serum supplementation should nevertheless be optimized to take into account the presence of inhibitors of viral production. In monolayer cultures with 0.2 mL/cm(2), the cell concentration was increased up to 2 x 10(6) cells/mL without loss of cell productivity. A semicontinuous production process, which enables the collection of larger amounts of viruses from the same culture, has also been successfully used. Suspension culture processes were prevented by the anchorage dependency of the FLYRD18 cell line. Microcarrier cultures were able to produce viruses but will require further investigation and optimization for their performance to become competitive with monolayer cultures. In the course of this study, more than a 10-fold increase of titer has been achieved. 相似文献