全文获取类型
收费全文 | 1936785篇 |
免费 | 191220篇 |
国内免费 | 1663篇 |
专业分类
2129668篇 |
出版年
2021年 | 18676篇 |
2018年 | 20632篇 |
2017年 | 19473篇 |
2016年 | 30244篇 |
2015年 | 43851篇 |
2014年 | 52085篇 |
2013年 | 78242篇 |
2012年 | 57528篇 |
2011年 | 51204篇 |
2010年 | 51133篇 |
2009年 | 50211篇 |
2008年 | 46965篇 |
2007年 | 46896篇 |
2006年 | 47512篇 |
2005年 | 48409篇 |
2004年 | 47267篇 |
2003年 | 44389篇 |
2002年 | 42142篇 |
2001年 | 64838篇 |
2000年 | 62855篇 |
1999年 | 54674篇 |
1998年 | 29471篇 |
1997年 | 29325篇 |
1996年 | 28217篇 |
1995年 | 26380篇 |
1994年 | 25943篇 |
1993年 | 25536篇 |
1992年 | 46556篇 |
1991年 | 45163篇 |
1990年 | 43767篇 |
1989年 | 43736篇 |
1988年 | 40275篇 |
1987年 | 39001篇 |
1986年 | 36436篇 |
1985年 | 37794篇 |
1984年 | 34001篇 |
1983年 | 29874篇 |
1982年 | 26483篇 |
1981年 | 24888篇 |
1980年 | 23395篇 |
1979年 | 30917篇 |
1978年 | 26574篇 |
1977年 | 24623篇 |
1976年 | 23556篇 |
1975年 | 24570篇 |
1974年 | 26137篇 |
1973年 | 26128篇 |
1972年 | 23026篇 |
1971年 | 21123篇 |
1970年 | 18404篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
891.
The dynamics of fish populations in two African rivers are being investigated with a view to developing fishery management models for tropical non-floodplain rivers. Preliminary work was carried out in the Rufiji River, East Africa, to establish growth and mortality rates along with L ∞ and K for six species in a river where fishing mortality is zero. These parameters, derived from scale-reading and length frequency analysis, also allowed the development of phased management plans for the fishery resulting from impoundment.
A subsequent investigation has begun on a West African river, the Taia in Sierra Leone, to pursue observations over four seasons, two of which are now complete. A systematic daily sampling programme is being carried out of the whole community, to accumulate similar but more extensive data to those obtained previously. In the Taia, however, fishing mortality must also be estimated from catch surveys, and analysis is carried out using the recently developed ELEFAN programs which can be compared with results obtained by other methods such as scale-reading. 相似文献
A subsequent investigation has begun on a West African river, the Taia in Sierra Leone, to pursue observations over four seasons, two of which are now complete. A systematic daily sampling programme is being carried out of the whole community, to accumulate similar but more extensive data to those obtained previously. In the Taia, however, fishing mortality must also be estimated from catch surveys, and analysis is carried out using the recently developed ELEFAN programs which can be compared with results obtained by other methods such as scale-reading. 相似文献
892.
Z I Lebedeva E A Kabanova T T Berezov 《Biulleten' eksperimental'no? biologii i meditsiny》1985,100(12):696-698
The effect of substrate analogues on glutamin-(asparagin-)ase from Pseudomonas aurantiaca-548 has been studied. The enzyme was demonstrated to be highly sensitive to the the action of 6-diazo-5-oxo-L-norleucine and azaserine. L-isomers of glutamine, aspartate, glutamate and several other substrate analogues with free alpha-amino groups protected the enzyme against the inhibitory DON effect. Thus, thorough preliminary selection of appropriate inhibitors, their dosage and treatment duration is needed for the recommendation of combined enzyme-inhibitor application in anti-tumour chemotherapy. 相似文献
893.
894.
895.
896.
897.
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
898.
E. G. Korvin-Pavlovskaya I. V. Neklyudova L. V. Beloussov 《Russian Journal of Developmental Biology》2006,37(2):77-84
Circumferential and radial components of the yolk cell surface movements were measured in the loach embryos at the late blastula stage within 40–50 min after puncture or indentation by an obliquely directed glass rod. The yolk cell surface was preliminarily marked by coal particles. It was shown that even closely located regions of the surface differed markedly in the rate and direction of their movements. In the vicinity of puncture, the yolk cell surface at first contracted in both circumferential and radial directions and then widened, but did not reach the initial values. In more remote areas, this surface continued to contract in the circumferential direction, but was extended in the radial direction. The degree of its contraction along different radii was unequal. The reaction to oblique indentation was anisotropic: the closest area of the yolk cell surface, located along the direction of indentation, contracted in both circumferential and radial directions and formed a fold “leaking” onto the rod, while the opposite area contracted in the circumferential direction, but extended in the radial direction. A conclusion was drawn that the yolk cell surface is a multivariant mechanosensitive system. Its active responses to mechanical influences obey the same patterns as multicellular embryonic tissues. 相似文献
899.
900.