全文获取类型
收费全文 | 48822篇 |
免费 | 5124篇 |
国内免费 | 24篇 |
出版年
2021年 | 691篇 |
2020年 | 442篇 |
2019年 | 555篇 |
2018年 | 726篇 |
2017年 | 692篇 |
2016年 | 1148篇 |
2015年 | 2054篇 |
2014年 | 2047篇 |
2013年 | 2704篇 |
2012年 | 3115篇 |
2011年 | 2818篇 |
2010年 | 1925篇 |
2009年 | 1726篇 |
2008年 | 2427篇 |
2007年 | 2353篇 |
2006年 | 2138篇 |
2005年 | 2078篇 |
2004年 | 1940篇 |
2003年 | 1692篇 |
2002年 | 1681篇 |
2001年 | 1317篇 |
2000年 | 1280篇 |
1999年 | 1179篇 |
1998年 | 714篇 |
1997年 | 598篇 |
1996年 | 582篇 |
1995年 | 569篇 |
1994年 | 539篇 |
1993年 | 551篇 |
1992年 | 1033篇 |
1991年 | 779篇 |
1990年 | 804篇 |
1989年 | 775篇 |
1988年 | 679篇 |
1987年 | 621篇 |
1986年 | 636篇 |
1985年 | 725篇 |
1984年 | 557篇 |
1983年 | 426篇 |
1982年 | 370篇 |
1981年 | 338篇 |
1980年 | 278篇 |
1979年 | 404篇 |
1978年 | 359篇 |
1977年 | 257篇 |
1976年 | 231篇 |
1975年 | 206篇 |
1974年 | 310篇 |
1973年 | 259篇 |
1972年 | 205篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
82.
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
83.
Sandra Danner Christiane Kirchhoff Richard Ivell 《Reproductive biology and endocrinology : RB&E》2009,7(1):67-14
Background
Post-meiotically expressed genes in the testis are essential for the proper progression of spermatogenesis, and yet, aside from the construction of individual transgenic mice using specific promoters to drive reporter plasmids, there are only very limited possibilities for relevant and quantitative analysis of gene promoters. This is due to the special nature of post-meiotic haploid cells, which to date are not represented in any appropriate cell-lines. This article reports the development of novel methodology using isolated and cultured rat seminiferous tubules in a multiwell format, into which promoter-reporter constructs can be introduced by a combination of microinjection and electroporation. 相似文献84.
85.
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
86.
Albinism and mouse behaviour 总被引:1,自引:0,他引:1
87.
88.
Ante M. Krstulovi Laura Bertani-Dziedzic Sandra Bautista-Cerqueira Stanley E. Gitlow 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1982,227(2):379-389
We describe a direct analysis for the simultaneous quantitative determination of 4-hydroxy-3-methoxyphenylacetic (homovanillic) acid and other monoamine metabolites in human lumbar cerebrospinal fluid, utilizing reversed-phase high-performance liquid chromatography with amperometric detection. In addition, a rapid isocratic separation was developed for homovanillic acid in the presence of other endogenous compounds.Twenty-five unselected diagnostic specimens of human lumbar cerebrospinal fluid were extracted with ethyl acetate and subsequently analyzed using the described method. Chromatographic peaks were identified on the basis of retention behavior and ratio of responses at several oxidation potentials.Although our quantitative results correlate well with the literature values, the data were not interpreted clinically since samples were obtained from routine, diagnostic testing of patients admitted to the medical or neurologic services at the Mount Sinai Hospital. 相似文献
89.
90.
Changes are reported of Crataegus-Betula dune woodlands between 1950–1980 from Meijendel, a calcareous coastal dune system near the city of The Hague, The Netherlands, which is used as a water catchment area. It concerns both woodlands in degradation stage and, more common, woodlands on the increase. Changes were recorded with help of successive vegetation maps, air photos and permanent plot observations. Woodland increase usually occurs on places with groundwater at or near the surface, either because of natural circumstances or as a result of artificial groundwater recharge through infiltration as part of the water catchment activities. The management of both types of woodland is discussed.Nomenclature follows Heukels-van der Meijden (1983), Flora van Nederland, 20th ed. Wolters-Noordhoff, Groningen. Nomenclature of mosses follows Margadant & During (1982), Beknopte Flora van Nederlandse Blad- en Levermossen, Thieme, Zutphen.The authors have pleasure in thanking their colleagues from the Dune Water Works who supplied data on hydrology. 相似文献