全文获取类型
收费全文 | 75135篇 |
免费 | 5302篇 |
国内免费 | 124篇 |
出版年
2022年 | 388篇 |
2021年 | 1028篇 |
2020年 | 771篇 |
2019年 | 959篇 |
2018年 | 1751篇 |
2017年 | 1522篇 |
2016年 | 2149篇 |
2015年 | 3157篇 |
2014年 | 3244篇 |
2013年 | 4406篇 |
2012年 | 5029篇 |
2011年 | 4635篇 |
2010年 | 3116篇 |
2009年 | 2401篇 |
2008年 | 3694篇 |
2007年 | 3636篇 |
2006年 | 3528篇 |
2005年 | 3065篇 |
2004年 | 3074篇 |
2003年 | 2797篇 |
2002年 | 2744篇 |
2001年 | 2018篇 |
2000年 | 1898篇 |
1999年 | 1604篇 |
1998年 | 726篇 |
1997年 | 630篇 |
1996年 | 546篇 |
1995年 | 563篇 |
1994年 | 421篇 |
1993年 | 443篇 |
1992年 | 1024篇 |
1991年 | 929篇 |
1990年 | 875篇 |
1989年 | 807篇 |
1988年 | 690篇 |
1987年 | 673篇 |
1986年 | 638篇 |
1985年 | 674篇 |
1984年 | 593篇 |
1983年 | 525篇 |
1982年 | 413篇 |
1981年 | 385篇 |
1979年 | 531篇 |
1978年 | 397篇 |
1977年 | 400篇 |
1976年 | 383篇 |
1975年 | 396篇 |
1974年 | 419篇 |
1973年 | 401篇 |
1972年 | 352篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
Y. Salas A. Márquez J. Canelón Y. Perazzo V. Colmenárez J. A. López 《Mycopathologia》2012,174(5-6):511-517
Pythium insidiosum is a pathogenic oomycete known since 1890 that causes pythiosis in mammals. In this report, seven P. insidiosum isolates were recovered from Venezuelan horses and were characterized. The strains were recovered from biopsied tissues and kunkers collected from granulomatous masses located on the hind limb and from a nodular lesion in the left upper eyelid, which decrease the ability of the horses to be used for working purposes. The methods used to identify P. insidiosum isolates were based on the production of sporangia and zoospores, histopathology and PCR assay. To further characterize these strains, portions of the 18S rRNA genes of the seven isolates were sequenced. The sequences showed high homology to previously described P. insidiosum DNA sequences available in GenBank. Similar studies based on the morphological, histological and molecular data identified the etiological agent in samples of granulomatous lesions in these equines as P. insidiosum. In America, the infection has been diagnosed more frequently in equines of Brazil, Colombia, Costa Rica and the United States of America. 相似文献
3.
Temporal variability of top-down forces and their role in host choice evolution of phytophagous arthropods 总被引:1,自引:0,他引:1
The role of top-down forces in host choice evolution of phytophagous arthropods is the subject of a vividly animated debate. Empirical evidence for the evolutionary role of top-down forces comes from studies showing that phytophagous arthropods prefer hosts that entail enemy-free space. The aim of this paper is to draw the attention of plant–arthropod researchers to the potentially, temporally variable nature of third trophic level effects. We show that this aspect is largely neglected in studies on enemy-free space, despite the fact that relative enemy impact varies seasonally among plants in at least some studies. We conclude that rigorous testing of the enemy-free space hypothesis can only be performed when within and between season variation in higher trophic level effects is taken into account. 相似文献
4.
5.
6.
T. E. Graedel Julian Allwood Jean‐Pierre Birat Matthias Buchert Christian Hagelüken Barbara K. Reck Scott F. Sibley Guido Sonnemann 《Journal of Industrial Ecology》2011,15(3):355-366
The recycling of metals is widely viewed as a fruitful sustainability strategy, but little information is available on the degree to which recycling is actually taking place. This article provides an overview on the current knowledge of recycling rates for 60 metals. We propose various recycling metrics, discuss relevant aspects of recycling processes, and present current estimates on global end‐of‐life recycling rates (EOL‐RR; i.e., the percentage of a metal in discards that is actually recycled), recycled content (RC), and old scrap ratios (OSRs; i.e., the share of old scrap in the total scrap flow). Because of increases in metal use over time and long metal in‐use lifetimes, many RC values are low and will remain so for the foreseeable future. Because of relatively low efficiencies in the collection and processing of most discarded products, inherent limitations in recycling processes, and the fact that primary material is often relatively abundant and low‐cost (which thereby keeps down the price of scrap), many EOL‐RRs are very low: Only for 18 metals (silver, aluminum, gold, cobalt, chromium, copper, iron, manganese, niobium, nickel, lead, palladium, platinum, rhenium, rhodium, tin, titanium, and zinc) is the EOL‐RR above 50% at present. Only for niobium, lead, and ruthenium is the RC above 50%, although 16 metals are in the 25% to 50% range. Thirteen metals have an OSR greater than 50%. These estimates may be used in considerations of whether recycling efficiencies can be improved; which metric could best encourage improved effectiveness in recycling; and an improved understanding of the dependence of recycling on economics, technology, and other factors. 相似文献
7.
R M Arahuetes J López M Ramos 《Comparative biochemistry and physiology. B, Comparative biochemistry》1986,83(1):231-233
Glucose and ketone bodies production has been studied in hepatocytes isolated from fetuses at term of fed and fasted adrenalectomized mothers. Maternal adrenalectomy diminishes the fetal liver weight. This effect is increased when the adrenalectomized pregnant rat is fasted for the last 2 days of gestation. Maternal adrenalectomy diminishes glucose production in hepatocytes isolated from fetuses at term. This diminution is markedly greater when the adrenalectomized pregnant rat is fasted for the last 48 hr of gestation. Maternal adrenalectomy diminishes ketone bodies production in hepatocytes isolated from fetuses at term. 相似文献
8.
9.
10.
The relations between the saccus lacrimalis and different portions of the musculus orbicularis oculi were studied in orbital regions of human fetuses sectioned into numbered series. No insertions of the pars lacrimalis or Horner's muscle on the saccus were found. These muscular fibres pass along the dorsal wall of the saccus and are separated from it by the reflex tendon of the ligamentum palpebrale mediale. The only muscular fibres that insert on the saccus are those that approach the anterior face of the saccus and the fornix. The fibres that insert on the anterior face proceed from the deep bundles of the pars preseptalis of the lower eyelids, and those that insert on the fornix derive from the deep bundles of the pars preseptalis of the upper eyelid. 相似文献