全文获取类型
收费全文 | 1601204篇 |
免费 | 145897篇 |
国内免费 | 1424篇 |
出版年
2021年 | 18210篇 |
2019年 | 16193篇 |
2018年 | 19485篇 |
2017年 | 18325篇 |
2016年 | 29226篇 |
2015年 | 42750篇 |
2014年 | 50852篇 |
2013年 | 76956篇 |
2012年 | 46230篇 |
2011年 | 37583篇 |
2010年 | 47384篇 |
2009年 | 47562篇 |
2008年 | 34511篇 |
2007年 | 33667篇 |
2006年 | 36383篇 |
2005年 | 37726篇 |
2004年 | 36552篇 |
2003年 | 33932篇 |
2002年 | 31867篇 |
2001年 | 47773篇 |
2000年 | 45176篇 |
1999年 | 41868篇 |
1998年 | 27117篇 |
1997年 | 26829篇 |
1996年 | 26133篇 |
1995年 | 24285篇 |
1994年 | 23908篇 |
1993年 | 23189篇 |
1992年 | 35464篇 |
1991年 | 34070篇 |
1990年 | 32600篇 |
1989年 | 33040篇 |
1988年 | 30218篇 |
1987年 | 29001篇 |
1986年 | 27307篇 |
1985年 | 28804篇 |
1984年 | 27074篇 |
1983年 | 23927篇 |
1982年 | 22680篇 |
1981年 | 21680篇 |
1980年 | 20276篇 |
1979年 | 23221篇 |
1978年 | 20912篇 |
1977年 | 19645篇 |
1976年 | 18915篇 |
1975年 | 19012篇 |
1974年 | 19856篇 |
1973年 | 20108篇 |
1972年 | 17352篇 |
1971年 | 15764篇 |
排序方式: 共有10000条查询结果,搜索用时 647 毫秒
691.
692.
CHRISTIAN K. FELD 《Freshwater Biology》2013,58(4):715-729
1. Although many studies have focussed on the effects of catchment land use on lotic systems, the importance of broad (catchment) and fine (segment/reach) scale effects on stream assemblages remain poorly understood. 2. Nine biological metrics for macrophytes (498 sites), benthic macroinvertebrates (491) and fish (478) of lowland and mountain streams in four ecoregions of France and Germany were related to catchment and riparian buffer land use using partial Redundancy Analysis and Boosted Regression Trees (BRTs). 3. Lotic fauna was better correlated (mean max., r = 0.450) than flora (r = 0.277) to both scales of land use: the strongest correlations were noted for mountain streams. BRTs revealed strong non‐linear relationships between mountain assemblage metrics and land use. Correlations increased with increasing buffer lengths, suggesting the importance of near‐stream land use on biotic assemblages. 4. Several metrics changed markedly between 10–20% (mountain ecoregions) and 40–45% (lowland) of arable land use, irrespective of the buffer size. At mountain sites with >10% catchment arable land use, metric values differed between sites with <30% and sites with >30% forest in the near‐stream riparian area. 5. These findings support the role of riparian land use in catchment management; however, differences between mountain and lowland ecoregions support the need for ecoregion‐specific management. 相似文献
693.
X-IRRADIATION of mammalian cells in culture yields a survival curve of the threshold type (for review see ref. 1). It isjnter-esting to ask how one can enhance the radiation response by small changes of the physical environment of the cells, as can be done chemically, for example, by incorporation of 5-bromo-deoxyuridine into DNA1,2. Elevation of the temperature is a likely prospect for enhancement of radiosensitivity for the following reasons. It is known that proteins are heat labile and that temperature sensitive mutants of bacteria and phage can be obtained for many different enzymes3 which are operative at 37° C but not at 42° or 43°C. For example4, DNA polymerase is reversibly temperature sensitive; it is rendered inoperative above 42°C, but will be functional again when the temperature is lowered. It is not unreasonable to expect that temperature sensitive mutations for many enzymes occur frequently and that the use of temperatures somewhat higher than the normal range at which the cells grow might disclose sensitivities for specific enzymes in normal cells of higher organisms. 相似文献
694.
695.
JOHNSTONE O. YOUNG 《Freshwater Biology》1972,2(4):355-359
Five new British records of freshwater Microturbellaria are illustrated: Gieysztoria infundibuliformis, Castrada lanceola, Castrada neocomensis, Castrada viridis and Strongylostoma elongatum. These species together with a new species, Macrostomum johni described by Young (1972), are woven into the fabric of the existing key for the group (Young, 1970). All six species were recorded from the littoral zone of lakes in Caernarvonshire, N. Wales. The times of the recordings are indicated. 相似文献
696.
697.
698.
A Ba?sset J Cotonat P Montastruc 《Comptes rendus des séances de la Société de biologie et de ses filiales》1975,169(2):444-448
The authors have studied in the dog--normal or diabetic insipidus after hypothalamic lesion--submitted to oral ingestion of ethanol the effects of a few antidiuretic drug: chlorpropamid, atromid-S, carbamazepin and carisoprodol. The carisoprodol only can reduce the diuresis and the drinking provoked by ethanol. 相似文献
699.
700.