全文获取类型
收费全文 | 3539篇 |
免费 | 266篇 |
国内免费 | 435篇 |
出版年
2024年 | 23篇 |
2023年 | 83篇 |
2022年 | 108篇 |
2021年 | 85篇 |
2020年 | 112篇 |
2019年 | 190篇 |
2018年 | 128篇 |
2017年 | 117篇 |
2016年 | 128篇 |
2015年 | 88篇 |
2014年 | 227篇 |
2013年 | 244篇 |
2012年 | 170篇 |
2011年 | 148篇 |
2010年 | 124篇 |
2009年 | 186篇 |
2008年 | 223篇 |
2007年 | 214篇 |
2006年 | 188篇 |
2005年 | 153篇 |
2004年 | 125篇 |
2003年 | 113篇 |
2002年 | 71篇 |
2001年 | 64篇 |
2000年 | 61篇 |
1999年 | 66篇 |
1998年 | 64篇 |
1997年 | 44篇 |
1996年 | 31篇 |
1995年 | 43篇 |
1994年 | 35篇 |
1993年 | 39篇 |
1992年 | 31篇 |
1991年 | 34篇 |
1990年 | 27篇 |
1989年 | 24篇 |
1988年 | 20篇 |
1987年 | 23篇 |
1985年 | 24篇 |
1984年 | 47篇 |
1983年 | 43篇 |
1982年 | 39篇 |
1981年 | 33篇 |
1980年 | 34篇 |
1979年 | 34篇 |
1978年 | 21篇 |
1977年 | 25篇 |
1976年 | 28篇 |
1975年 | 16篇 |
1974年 | 17篇 |
排序方式: 共有4240条查询结果,搜索用时 31 毫秒
1.
The RAINFOR database: monitoring forest biomass and dynamics 总被引:1,自引:0,他引:1
Problem: Data from over 100 permanent sample plots which have been studied for 10–20 years need a suitable system for storage which allows simple data manipulation and retrieval for analysis. Methods: A relational database linking tree records, taxonomic nomenclature and corresponding environmental data has been built in MS Access as part of the RAINFOR project. Conclusion: The database allows flexible and long‐term use of a large amount of data: more than 100 tree plots across Amazonia, incorporating over 80 000 records of individual trees and over 300 000 total records of tree diameter measurements from successive censuses. The database is designed to enable linkages to existing soil, floristic or plant‐trait databases. This database will be a useful tool for exploring the impact of environmental factors on forest structure and dynamics at local to continental scales, and long term changes in forest ecology. As an early example of its potential, we explore the impact of different methodological assumptions on estimates of tropical forest biomass and carbon storage. 相似文献
2.
The l-thyroxine binding site in human serum thyroxine-binding globulin was investigated by affinity labeling with N-bromoacetyl-l-thyroxine (BrAcT4). Competitive binding studies showed that, in the presence of 100 molar excess of BrAcT4, binding of thyroxine to thyroxine-binding globulin was nearly totally abolished. The reaction of BrAcT4 to form covalent binding was inhibited in the presence of thyroxine and the affinity-labeled thyroxinebinding globulin lost its ability to bind thyroxine. These results indicate BrAcT4 and thyroxine competed for the same binding site. Affinity labeling with 2 mol of BrAcT4/mol of thyroxine-binding globulin resulted in the covalent attachment of 0.7 mol of ligand. By amino acid analysis and high voltage paper electrophoresis, methionine was identified as the major residue labeled (75%). Lysine, tyrosine, and histidine were also found to be labeled to the extent of 8, 8, and 5%, respectively. 相似文献
3.
4.
5.
6.
Flow cytometry: rapid biochemical analysis of single cells 总被引:7,自引:0,他引:7
H S Kruth 《Analytical biochemistry》1982,125(2):225-242
7.
The results of a detailed analysis of 100 supernumerary limbs generated by 180° ipsilateral rotation (on the same limb stump) of regeneration blastemas is presented. The limbs were analyzed in terms of their position of origin, frequency, cartilage structure by Victoria blue staining, and muscle structure by serial sections. Single, double, or triple supernumeraries can be produced at no unique position of origin, although the posterodorsal quadrant was preferred. Four classes of supernumerary limbs were generated by such operations—normal; double dorsal or double ventral; part normal/part mirror imaged; part normal/part inverted in approximately equal frequencies. After amputation of these supernumeraries the same muscle patterns are faithfully regenerated. A hypothesis to explain the production of these abnormal limbs is proposed based on the observed phenomenon of fusion of supernumerary blastemata, but their regenerative behaviour presents problems for current models of pattern formation. Similar results have been obtained with developing limb buds and the relation between development and regeneration is discussed. 相似文献
8.
In vitro growth of murine T cells. IV. Use of T-cell growth factor to clone lymphoid cells 总被引:3,自引:0,他引:3
Murine bone marrow cells can suppress the in vitro primary antibody response of normal spleen cells without apparent cytotoxicity. The bone marrow cells suppress the response to both T-dependent (SRBC) and T-independent (DNP-Ficoll) antigens. When bone marrow cells are fractionated on a sucrose density gradient, the suppressive activity is found in the residue rather than the lymphocyte fraction. The suppressive activity is either unaffected or enhanced by treatment with anti-T- and anti-B-cell serums. Pretreatment of mice with phenylhydrazine which reduces the number of pre-B cells did not reduce the suppressive activity of their bone marrow cells. Suppressive activity is abolished by irradiation of the marrow cells in vitro with 1000 R prior to assay. The activity is present in the marrow of thymus deficient (nude) mice, infant mice, and mice which have been made polycythemic by transfusion. Furthermore, the suppressor cell can phagocytize iron carbonyl particles, is slightly adherent to plastic and Sephadex G-10, and can bind to EA monolayers. We conclude that the suppressor cell is not a mature lymphocyte or granulocyte nor a member of the erythrocytic series, but is likely to be an immature cell possibly of the myeloid series. We speculate on the physiologic role of this cell. 相似文献
9.
10.
A heptageniid mayfly species Paegniodes dao sp. nov. (Ephemeroptera: Heptageniidae) is described from Vietnam. The larva is distinguished by round anterolateral margins of pronotum, triangular‐shaped labrum, rudimentary lamella and well developed fibrillae of gills 1, and light brown body that lacks distinct markings. Diagnosis, line drawings, material, and habitat data are provided. 相似文献