全文获取类型
收费全文 | 31382篇 |
免费 | 2768篇 |
国内免费 | 25篇 |
专业分类
34175篇 |
出版年
2023年 | 161篇 |
2022年 | 367篇 |
2021年 | 776篇 |
2020年 | 415篇 |
2019年 | 549篇 |
2018年 | 654篇 |
2017年 | 537篇 |
2016年 | 965篇 |
2015年 | 1631篇 |
2014年 | 1709篇 |
2013年 | 1999篇 |
2012年 | 2632篇 |
2011年 | 2680篇 |
2010年 | 1632篇 |
2009年 | 1353篇 |
2008年 | 2009篇 |
2007年 | 1986篇 |
2006年 | 1930篇 |
2005年 | 1656篇 |
2004年 | 1680篇 |
2003年 | 1476篇 |
2002年 | 1439篇 |
2001年 | 275篇 |
2000年 | 201篇 |
1999年 | 265篇 |
1998年 | 298篇 |
1997年 | 214篇 |
1996年 | 200篇 |
1995年 | 167篇 |
1994年 | 149篇 |
1993年 | 143篇 |
1992年 | 114篇 |
1991年 | 140篇 |
1990年 | 102篇 |
1989年 | 92篇 |
1988年 | 83篇 |
1987年 | 66篇 |
1986年 | 83篇 |
1985年 | 90篇 |
1984年 | 93篇 |
1983年 | 93篇 |
1982年 | 114篇 |
1981年 | 112篇 |
1980年 | 89篇 |
1979年 | 61篇 |
1978年 | 45篇 |
1977年 | 47篇 |
1976年 | 47篇 |
1975年 | 46篇 |
1974年 | 46篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
Andrew P. Vayda 《Human ecology: an interdisciplinary journal》1987,15(4):519-521
42.
43.
Andrew A. Lackner Morten Schidt Gary C. Armitage Peter F. Moore Robert J. Munn Preston A. Marx Murray B. Gardner Linda J. Lowenstine 《Journal of medical primatology》1989,18(3-4):195-207
Simian acquired immune deficiency syndrome (SAIDS) caused by the type D retrovirus SRV-1 results in opportunistic infections and a spectrum of oral lesions similar to those seen in humans with AIDS. To better understand the pathogenesis of these oral lesions we have retrospectively examined the oral mucosa from ten rhesus monkeys that died with SAIDS and prospectively examined the oral mucosa of ten additional animals inoculated with SRV-1 to determine at what time, and in what cells SRV-1 infection of the oral mucosa occurs. Using single and double label immunohistologic techniques, and electron microscopy we detected SRV-1 in clusters of oral epithelial cells and rare Langerhans cells as early as 1 month postinoculation. 相似文献
44.
Andrew Clarke J. Cynan Ellis-Evans Mark W. Sanders Lesley J. Holmes 《Hydrobiologia》1989,172(1):183-191
The copepod Pseudoboeckella poppei (Daday) (Calanoida, Centropagidae) was sampled from Sombre and Heywood Lakes on Signy Island, Antarctica (60° S, 45° W) between January 1984 and March 1985. Sombre Lake is clear and oligotrophic with little phytoplankton and a bottom sediment low in organic content. By contrast Heywood Lake is turbid and mesotrophic; a substantial phytoplankton develops in summer and the bottom sediments are comparatively rich in organics. Both lakes freeze over for much of the year, forcing the copepods to adopt a benthic feeding strategy over winter. Adult Pseudoboeckella feed on phytoplankton when this is available, but also on detritus, diatoms and short algal filaments stirred up from the sediment. In Heywood Lake, male copepods show a smooth seasonal trend in lipid content with lipid being synthesised in early summer and utilised in late summer and winter. The summer increase in lipid content is associated with an increase in dry weight. Female lipid contents show evidence of two peaks of egg production. In Sombre Lake both male and female copepods increase in size during summer and show a wider range of lipid contents than in Heywood Lake; it is likely that this is due to the poorer winter feeding conditions which necessitate the synthesis of a much larger store of reserves during the summer. In contrast to marine calanoid copepods, lipid stores are exclusively triacylglycerol with no trace of wax ester. 相似文献
45.
Andrew N. Webber Kathryn A. Platt-Aloia Robert L. Heath William W. Thomson 《Physiologia plantarum》1988,72(2):288-297
The detergent Tween-20 solubilized preferentially portions of the marginal regions of Spinacea oleracea L. thylakoid membranes and, thus, opened the inside of the grana to the external media. Differential centrifugation. following Tween-20 solubilization. enabled separate fractions of grana and stromal-exposed membranes to be isolated. Analysis of Tween-20 solubilized material, after pelleting all membrane material by centrifugation at 100 000 g, revealed polypeptides associated with the coupling factor (CF1 ) particles, cytochrome b6 /f and photosystem II complexes, suggesting that the marginal membranes contain these proteins. Concomitantly, the 100 000 g pellet was depleted in cytochrome b6 /f and P700, determined spectroscopically, Thus. our results reveal the margin to be a distinct membrane region, which does not contain the light-harvesting centers of photosystem II (LHC II). The implication of these results, in terms of the energetic interaction of components of granal and stromalexposed membrane regions, is discussed. 相似文献
46.
This article applied distributed artificial intelligence to the real-time planning and control of flexible manufacturing systems (FMS) consisting of asynchronous manufacturing cells. A knowledge-based approach is used to determine the course of action, resource sharing, and processor assignments. Within each cell there is an embedded automatic planning system that executes dynamic scheduling and supervises manufacturing operations. Because of the decentralized control, real-time task assignments are carried out by a negotiation process among cell hosts. The negotiation process is modeled by augmented Petri nets —the combination of production rules and Petri nets—and is excuted by a distributed, rule-based algorithm. 相似文献
47.
Detailed examination of a sample plot covered by the Lindero membranaceae-Fagetum crenatae association on Mt. Sanpoiwadake,
Hakusan National Park, revealed a number of correlations between the distribution of subassociations and environmental factors.
The subassociations on the south-facing slopes receive deep snow cover in winter with rapid melting in the spring. They occur
on porous, freely draining soils, typical of the general range of brown forest soils. Conversely, on the north-eastern slopes
there are widespread late-snow patches which delay leaf development and expansion and which provide an abundant water supply
well into early summer. Under these conditions, bleached soil horizons have developed with iron pan formation, resulting in
poor soil drainage, strongly correlated with quite different plant communities. 相似文献
48.
Summary The sensitivity to calcium of the human squamous carcinoma cell line, SCC-13, was demonstrated and characterized. Cultures
grown to confluence in the presence of 0.2 to 2 mM calcium had approximately 10-fold higher levels of particulate transglutaminase activity and envelope competence than those
grown in low calcium (0.025 to 0.05 mM) medium. Raising the calcium from 0.025 to 1.8 mM induced expression of this enzyme and of competence over the course of a week. Conversely, for cultures grown to confluence
in 1.8 mM calcium, subsequent reduction of calcium to 0.025 mM resulted in a substantial decline in transglutaminase over a similar time period. Immunoprecipitable transglutaminase was
clearly identifiable in cultures grown in 1.8 mM calcium-containing medium but not in those grown in low calcium medium or in the presence of retinoic acid, suggestive of
regulation at the level of mRNA accumulation or translation rather than posttranslational modification.
This research was supported by Public Health Service grant AR 27130 from the National Institute of Arthritis, Musculoskeletal
and Skin Diseases, Bethesda, MD, and National Research Service postdoctoral fellowship ES 05336 from the National Institute
of Environmental Health Sciences, Research Triangle Park, NC. 相似文献
49.
Stefan Jansson Eran Pichersky Roberto Bassi Beverley R. Green Masahiko Ikeuchi Anastasios Melis David J. Simpson Michael Spangfort L. Andrew Staehelin J. Philip Thornber 《Plant Molecular Biology Reporter》1992,10(3):242-253
We propose a nomenclature for the genes encoding the chlorophylla/b-binding proteins of the light-harvesting complexes of photosystem I and II. The genes encoding LHC I and LHC II polypeptides
are namedLhca1 throughLhca4 andLhcb1 throughLhcb6, respectively. The proposal follows the general format recommended by the Commision on Plant Gene Nomenclature. We also present
a table for the conversion of old gene names to the new nomenclature. 相似文献
50.