全文获取类型
收费全文 | 46267篇 |
免费 | 4819篇 |
国内免费 | 13篇 |
出版年
2022年 | 283篇 |
2021年 | 558篇 |
2020年 | 381篇 |
2019年 | 464篇 |
2018年 | 593篇 |
2017年 | 563篇 |
2016年 | 977篇 |
2015年 | 1755篇 |
2014年 | 1734篇 |
2013年 | 2296篇 |
2012年 | 2618篇 |
2011年 | 2332篇 |
2010年 | 1621篇 |
2009年 | 1483篇 |
2008年 | 2037篇 |
2007年 | 2044篇 |
2006年 | 1842篇 |
2005年 | 1788篇 |
2004年 | 1706篇 |
2003年 | 1474篇 |
2002年 | 1481篇 |
2001年 | 1384篇 |
2000年 | 1371篇 |
1999年 | 1241篇 |
1998年 | 692篇 |
1997年 | 656篇 |
1996年 | 629篇 |
1995年 | 616篇 |
1994年 | 582篇 |
1993年 | 610篇 |
1992年 | 1172篇 |
1991年 | 897篇 |
1990年 | 930篇 |
1989年 | 886篇 |
1988年 | 768篇 |
1987年 | 701篇 |
1986年 | 727篇 |
1985年 | 807篇 |
1984年 | 629篇 |
1983年 | 493篇 |
1982年 | 424篇 |
1981年 | 379篇 |
1980年 | 318篇 |
1979年 | 463篇 |
1978年 | 392篇 |
1977年 | 295篇 |
1976年 | 275篇 |
1975年 | 249篇 |
1974年 | 333篇 |
1973年 | 302篇 |
排序方式: 共有10000条查询结果,搜索用时 390 毫秒
81.
82.
83.
W. O. van der Knaap Jacqueline F. N. van Leeuwen Tomasz Goslar Robert Krisai Willy Tinner 《Vegetation History and Archaeobotany》2012,21(1):37-60
Three mires and a small lake in the Swiss and Austrian Alps were studied palynologically at high resolution, covering the
last 1,000, 400, 50 and 1,200 years, respectively. Methodological lessons include: (1) Sub-decadal resolution in upper, little-decomposed
peat layers reveals recurrent marked fluctuations in both percentages and influx of regional tree-pollen types, reflecting
variations in pollen production rather than in plant-population sizes. (2) Intermittent, single-spectrum pollen maxima in
samples of sub-decadal resolution indicate pollen transport in clumps. This type of pollen transport may remain unrecognized
in sections with lower sampling resolution, which may then lead to inappropriate interpretation in terms of plant-population
sizes. (3) The detection of short-lived phases of human impact in decomposed peat requires sampling intervals as close as
0.2 cm. (4) PAR (pollen influx) may reflect vegetation dynamics more faithfully than percentages. Reliable PAR, however, is
difficult to achieve in Alpine mires due to past human impact on peat growth, even when complex depth–age modelling techniques
are used. Critical comparison of PAR with percentages is therefore essential. (5) Careful consideration of spatial scales
in pollen signals (local–regional and subdivisions) is essential for a realistic palaeo-ecological interpretation. Results
in terms of past human impact on vegetation are summarized as follows: (1) Trends in pollen types reflecting regional human
action are in general agreement with earlier findings for the western Swiss Alps, allowing for regional differences. (2) All
mires in the Alps investigated here and in an earlier study experienced human impact during the last millennium. The studied
small lake, lying in sub-alpine pasture, records forest dynamics at a lower elevation since a.d. 800. 相似文献
84.
The inner-dune zone between the villages of Bakkum and Egmond in the North-Holland Dune Reserve is one of the few remaining examples of the semi-natural boundary between the dunes and the polders. For several centuries this area has been used for various purposes, in particular for grazing. Large-scale excavating, especially after about 1900, has encroached upon the dunes. Vegetation and land-use history were studied and a provisional scheme on the vegetation development is presented. Management policies discussed are non-intervention, mowing in order to start natural processes such as rabbit grazing, and the continuation of traditional livestock grazing. Proposals for external management policies were made in order to maintain or enhance the original transition zone, based on comprehensive physical-geographical studies of the whole inner-dune zone. 相似文献
85.
86.
Albinism and mouse behaviour 总被引:1,自引:0,他引:1
87.
Intestinal synthesis of 24-keto-1,25-dihydroxyvitamin D3. A metabolite formed in vivo with high affinity for the vitamin D cytosolic receptor 总被引:1,自引:0,他引:1
J L Napoli B C Pramanik P M Royal T A Reinhardt R L Horst 《The Journal of biological chemistry》1983,258(15):9100-9107
24-Keto-1,25-dihydroxyvitamin D3 has been identified as an intestinal metabolite of 1,25-dihydroxyvitamin D3 by ultraviolet absorbance, mass spectroscopy, and chemical reactivity. The metabolite was produced from 1,25-dihydroxyvitamin D3 and 1,24R,25-trihydroxyvitamin D3 in rat intestinal mucosa homogenates. 24-Keto-1,25-dihydroxyvitamin D3 is present in vivo in the plasma and small intestinal mucosa of rats fed a stock diet, receiving no exogenous 1,25-dihydroxyvitamin D3, and in the plasma and small intestinal mucosa of rats dosed chronically with 1,25-dihydroxyvitamin D3. 24-Keto-1,25-dihydroxyvitamin D3 has affinity equivalent to 1,24R,25-trihydroxyvitamin D3 for the 3.7 S cytosolic receptor specific for 1,25-dihydroxyvitamin D3 in the intestine and thymus. In cytosolic preparations contaminated with the 5 S vitamin D-binding protein, both metabolites are about 7-fold less potent than 1,25-dihydroxyvitamin D3. In contrast, in cytosolic preparations largely free of the 5 S binding protein, both metabolites are equipotent with the parent compound. No evidence was obtained supporting a substantial presence of 23-keto-1,25-dihydroxyvitamin D3 in vivo; nor was the latter compound generated in detectable amounts from 1,25-dihydroxyvitamin D3 by intestinal homogenates. Thus, C-24 oxidation is a significant pathway of intestinal 1,25-dihydroxyvitamin D3 metabolism that produces metabolites with high affinity for the cytosolic receptor which mediates vitamin D action. 相似文献
88.
89.
Dr. Horst Lyr 《Planta》1955,46(4):408-413
Ohne Zusammenfassung 相似文献
90.