全文获取类型
收费全文 | 61886篇 |
免费 | 20篇 |
国内免费 | 74篇 |
专业分类
61980篇 |
出版年
2020年 | 4篇 |
2019年 | 3篇 |
2018年 | 6794篇 |
2017年 | 6777篇 |
2016年 | 4469篇 |
2015年 | 272篇 |
2014年 | 37篇 |
2013年 | 49篇 |
2012年 | 2594篇 |
2011年 | 7836篇 |
2010年 | 6495篇 |
2009年 | 6327篇 |
2008年 | 6334篇 |
2007年 | 8027篇 |
2006年 | 107篇 |
2005年 | 1303篇 |
2004年 | 759篇 |
2003年 | 1012篇 |
2002年 | 927篇 |
2001年 | 143篇 |
2000年 | 209篇 |
1999年 | 7篇 |
1998年 | 11篇 |
1997年 | 20篇 |
1996年 | 25篇 |
1995年 | 6篇 |
1994年 | 16篇 |
1993年 | 30篇 |
1992年 | 19篇 |
1991年 | 28篇 |
1990年 | 9篇 |
1989年 | 19篇 |
1988年 | 4篇 |
1987年 | 13篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 14篇 |
1983年 | 18篇 |
1982年 | 16篇 |
1981年 | 3篇 |
1978年 | 3篇 |
1975年 | 11篇 |
1972年 | 613篇 |
1971年 | 536篇 |
1970年 | 6篇 |
1965年 | 13篇 |
1962年 | 10篇 |
1956年 | 9篇 |
1944年 | 9篇 |
1940年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
F. Hofhansl W. Wanek S. Drage W. Huber A. Weissenhofer A. Richter 《Biogeochemistry》2011,106(3):371-396
Bulk precipitation and throughfall were collected in a wet lowland rainforest in SW Costa Rica on an event basis to allow
modelling the contributions of dry deposition and canopy exchange to nutrient inputs and internal cycling of nutrients. Estimates
based on bulk precipitation underestimated total atmospheric deposition to tropical rainforests by up to 10-fold ignoring
the contributions of dry deposition. Canopy exchange contributed most of the aboveground inputs to the forest soil of Na+, about half for K+, 10% for P and Mg2+ and negligible for N, C and other elements. Tree species composition did not account for the differences found in net throughfall
between forest sites, and vegetation structure (plant area index) had only a small effect on net throughfall. Forest regrowth
affected net throughfall through reduced soil fertility and differences in leaf traits. Topography most significantly affected
net throughfall via increased dry deposition at sites of higher elevation and via soil fertility and increased canopy exchange
at down slope sites. 相似文献
72.
Emin Oguzhan Oguz Hayati Yuksel Yasar Enli A. Cevik Tufan Gunfer Turgut 《Biological trace element research》2010,133(3):335-341
Copper is an essential trace element that is extremely toxic to organisms and organs at high doses. We have investigated the
histological and biochemical effects of a toxic dose of copper sulfate on the liver of term Ross broiler chicks. Fertilized
eggs were divided into three groups: experimental, injected with 50 mcg/0.1 ml copper sulfate in the air chambers on day 1;
sham, injected with 0.1 ml saline; and control, no injection. Term chicks were killed and their livers investigated histologically,
with hematoxylin–eosin-stained sections examined under light microscopy, and biochemically, for malondialdehyde and glutathione
levels. Histological examinations showed copper-treated samples with granular degeneration and necrosis of hepatocytes and
impairment to the cell lining of the remark cords. The samples had a congestive appearance, with blood in the vena centralis
and sinusoids, slight connective tissue increase, and lymphocyte infiltration. Control and sham group sections had normal
appearances. As oxidative damage parameters, in the copper-treated group, malondialdehyde levels were increased and glutathione
levels decreased. In the sham and control groups, there were no significant differences. At this toxic dose, copper sulfate
shows oxidative damage according to the histology of term chick liver that are confirmed biochemically by the changes in malondialdehyde
and glutathione levels. 相似文献
73.
74.
THERE are two principal groups of theories of ageing—those which hold that random cell damage is chiefly responsible for the events characteristic of ageing, which culminate in death and those which hold that ageing and death are genetically controlled. It is too soon to decide between these points of view and in any case Bullough1 has shown that they are not mutually exclusive. So far experiments to test the random error theories of ageing, involving exposure of organisms to unnaturally large or even small amounts of agents such as X-rays and mutagenic agents (for reviews, see refs. 2 and 3), have been controversial and inconclusive. 相似文献
75.
The purpose of this review is to provide a global perspective on Oncorhynchus salmonine introductions and put-and-take fisheries based on modern stocking programs, with special emphasis on freshwater
ecosystems. We survey the global introductions of nine selected salmonines of the genus Oncorhynchus: golden trout, cutthroat trout, pink salmon, chum salmon, coho salmon, masu/cherry salmon, rainbow trout/steelhead, sockeye
salmon/kokanee, and chinook salmon. The information is organized on a geographical basis by continent, and then by species
and chronology. Two different objectives and associated definitions of ‘success’ for introductions are distinguished: (a)
seed introduction: release of individuals with the purpose of creating a wild-reproducing, self-sustaining population; and
(b) put-and-take introduction: release of individuals with the purpose of maintaining some level of wild population abundance,
regardless of wild reproduction. We identify four major phenomena regarding global salmonine introductions: (1) general inadequacy
of documentation regarding introductions; (2) a fundamental disconnect between management actions and ecological consequences
of introductions; (3) the importance of global climate change on success of previous and future introductions; and (4) the
significance of aquaculture as a key uncertainty in accidental introductions. We conclude this review with a recognition of
the need to terminate ongoing stocking programs for introduced salmonines worldwide. 相似文献
76.
77.
In this paper the investigation results of successional processes in the vegetative cover appearing due to succession starting
in native ecosystem inundation zone under Sayan-Shush reservoir influence are given. 相似文献
78.
Divergence time estimates of mammals from molecular clocks and fossils: Relevance of new fossil finds from India 总被引:1,自引:0,他引:1
G. V. R. Prasad 《Journal of biosciences》2009,34(5):649-659
This paper presents a brief review of recent advances in the classification of mammals at higher levels using fossils and
molecular clocks. It also discusses latest fossil discoveries from the Cretaceous — Eocene (66–55 m.y.) rocks of India and
their relevance to our current understanding of placental mammal origins and diversifications. 相似文献
79.
80.