全文获取类型
收费全文 | 87822篇 |
免费 | 7662篇 |
国内免费 | 27篇 |
出版年
2023年 | 372篇 |
2022年 | 844篇 |
2021年 | 1674篇 |
2020年 | 1009篇 |
2019年 | 1239篇 |
2018年 | 1642篇 |
2017年 | 1458篇 |
2016年 | 2465篇 |
2015年 | 3988篇 |
2014年 | 4151篇 |
2013年 | 5541篇 |
2012年 | 6363篇 |
2011年 | 5968篇 |
2010年 | 3798篇 |
2009年 | 3373篇 |
2008年 | 4622篇 |
2007年 | 4567篇 |
2006年 | 4084篇 |
2005年 | 3792篇 |
2004年 | 3561篇 |
2003年 | 3287篇 |
2002年 | 3132篇 |
2001年 | 1704篇 |
2000年 | 1584篇 |
1999年 | 1547篇 |
1998年 | 1079篇 |
1997年 | 908篇 |
1996年 | 885篇 |
1995年 | 843篇 |
1994年 | 784篇 |
1993年 | 806篇 |
1992年 | 1294篇 |
1991年 | 1014篇 |
1990年 | 1044篇 |
1989年 | 963篇 |
1988年 | 846篇 |
1987年 | 767篇 |
1986年 | 767篇 |
1985年 | 871篇 |
1984年 | 705篇 |
1983年 | 548篇 |
1982年 | 476篇 |
1981年 | 432篇 |
1980年 | 382篇 |
1979年 | 501篇 |
1978年 | 414篇 |
1977年 | 335篇 |
1976年 | 303篇 |
1974年 | 356篇 |
1973年 | 327篇 |
排序方式: 共有10000条查询结果,搜索用时 533 毫秒
111.
112.
Adriana Calderaro Giovanna Piccolo Chiara Gorrini Sara Montecchini Sabina Rossi Maria Cristina Medici Carlo Chezzi Georges Snounou 《PloS one》2012,7(10)
It has been proposed that ovale malaria in humans is caused by two closely related but distinct species of malaria parasites: P. ovale curtisi and P. ovale wallikeri. We have extended and optimized a Real-time PCR assay targeting the parasite’s small subunit ribosomal RNA (ssrRNA) gene to detect both these species. When the assay was applied to 31 archival blood samples from patients diagnosed with P. ovale, it was found that the infection in 20 was due to P. ovale curtisi and in the remaining 11 to P. ovale wallikeri. Thus, this assay provides a useful tool that can be applied to epidemiological investigations of the two newly recognized distinct P. ovale species, that might reveal if these species also differ in their clinical manifestation, drugs susceptibility and relapse periodicity. The results presented confirm that P. ovale wallikeri is not confined to Southeast Asia, since the majority of the patients analyzed in this study had acquired their P. ovale infection in African countries, mostly situated in West Africa. 相似文献
113.
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
114.
115.
Jamilly de Souza Costa Maria de Fátima Arruda 《Marine and Freshwater Behaviour and Physiology》2016,49(6):375-389
This study characterized the behavioral activity of Macrobrachium rosenbergii in the early stages of development, under different stocking densities (25 and 40 animals/m2), and during the light and dark phases of a 24-h cycle. Observations of individuals were made in 8 aquariums. Behavioral recording lasted 15 min/aquarium, 4 times/day and 4 days/week, 4 weeks in total. Food was offered twice daily. Observational methods included a combination of behavioral sampling and scan sampling. During the light phase, inactivity, cleaning and remaining in a shelter were the most frequent behaviors. During the dark phase the subjects displayed a higher frequency of feeding, exploration, swimming, and digging. At low density, the animals gained more weight and exhibited greater growth overall. These results indicate a behavioral pattern that is more favorable to animals in the lower density cultivation environment that can also create better living conditions for these shrimp, favor survival rates and therefore improve management success. 相似文献
116.
117.
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
118.
119.
Albinism and mouse behaviour 总被引:1,自引:0,他引:1
120.