全文获取类型
收费全文 | 87439篇 |
免费 | 20220篇 |
国内免费 | 33篇 |
专业分类
107692篇 |
出版年
2022年 | 507篇 |
2021年 | 1251篇 |
2020年 | 2653篇 |
2019年 | 4257篇 |
2018年 | 4567篇 |
2017年 | 4778篇 |
2016年 | 5348篇 |
2015年 | 6323篇 |
2014年 | 6110篇 |
2013年 | 7379篇 |
2012年 | 6027篇 |
2011年 | 5753篇 |
2010年 | 5758篇 |
2009年 | 4225篇 |
2008年 | 4250篇 |
2007年 | 3932篇 |
2006年 | 3723篇 |
2005年 | 3805篇 |
2004年 | 3714篇 |
2003年 | 3510篇 |
2002年 | 3500篇 |
2001年 | 995篇 |
2000年 | 749篇 |
1999年 | 895篇 |
1998年 | 923篇 |
1997年 | 645篇 |
1996年 | 657篇 |
1995年 | 645篇 |
1994年 | 629篇 |
1993年 | 640篇 |
1992年 | 572篇 |
1991年 | 493篇 |
1990年 | 421篇 |
1989年 | 436篇 |
1988年 | 425篇 |
1987年 | 365篇 |
1986年 | 373篇 |
1985年 | 426篇 |
1984年 | 491篇 |
1983年 | 409篇 |
1982年 | 497篇 |
1981年 | 450篇 |
1980年 | 394篇 |
1979年 | 259篇 |
1978年 | 317篇 |
1977年 | 305篇 |
1976年 | 252篇 |
1975年 | 236篇 |
1974年 | 265篇 |
1973年 | 232篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
This paper is concerned with gene survival in a population which may increase without density dependence according to a generalization of the Moran model for haploid individuals. A selective advantage to one allele and the possibility of differential reproductive rates are allowed. Simple conditions are given for ultimate homozygosity to be certain and for the possibility of ultimate polymorphism. The results complement and extend those of Heyde (1981, 1982). 相似文献
2.
3.
4.
5.
6.
Peter Buchwald 《Theoretical biology & medical modelling》2009,6(1):5-13
Background
The function and viability of cultured, transplanted, or encapsulated pancreatic islets is often limited by hypoxia because these islets have lost their vasculature during the isolation process and have to rely on gradient-driven passive diffusion, which cannot provide adequate oxygen transport. Pancreatic islets (islets of Langerhans) are particularly susceptible due to their relatively large size, large metabolic demand, and increased sensitivity to hypoxia. Here, finite element method (FEM) based multiphysics models are explored to describe oxygen transport and cell viability in avascular islets both in static and in moving culture media. 相似文献7.
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
8.
9.
Peter J Stephens 《BMJ (Clinical research ed.)》2005,330(7506):1508-1509
10.