全文获取类型
收费全文 | 1311057篇 |
免费 | 112013篇 |
国内免费 | 1482篇 |
专业分类
1424552篇 |
出版年
2021年 | 17933篇 |
2020年 | 12688篇 |
2019年 | 16241篇 |
2018年 | 17525篇 |
2017年 | 16281篇 |
2016年 | 27996篇 |
2015年 | 42546篇 |
2014年 | 50627篇 |
2013年 | 76970篇 |
2012年 | 37065篇 |
2011年 | 25597篇 |
2010年 | 43524篇 |
2009年 | 45003篇 |
2008年 | 24695篇 |
2007年 | 22428篇 |
2006年 | 27988篇 |
2005年 | 28931篇 |
2004年 | 28387篇 |
2003年 | 25753篇 |
2002年 | 23926篇 |
2001年 | 29216篇 |
2000年 | 26249篇 |
1999年 | 27519篇 |
1998年 | 25011篇 |
1997年 | 24812篇 |
1996年 | 24426篇 |
1995年 | 22423篇 |
1994年 | 22392篇 |
1993年 | 21379篇 |
1992年 | 24319篇 |
1991年 | 22928篇 |
1990年 | 21508篇 |
1989年 | 22650篇 |
1988年 | 20678篇 |
1987年 | 19680篇 |
1986年 | 18646篇 |
1985年 | 20704篇 |
1984年 | 21009篇 |
1983年 | 18598篇 |
1982年 | 19747篇 |
1981年 | 19160篇 |
1980年 | 17834篇 |
1979年 | 17126篇 |
1978年 | 16500篇 |
1977年 | 16158篇 |
1976年 | 15371篇 |
1975年 | 14653篇 |
1974年 | 15099篇 |
1973年 | 15455篇 |
1972年 | 12934篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
221.
222.
223.
Bidens cordylocarpa is a high polyploid species restricted in distribution to stream sides in the mountains of Jalisco, Mexico. The morphologically
enigmatic species was originally described as a member of the genus Coreopsis, but later transferred to Bidens, largely because the involucral bracts appear most similar to Bidens. Characters of the cypselae, often useful in generic placement, are of no value for this species because the fruits have
features not detected in either Bidens or Coreopsis. Sequences from the internal transcribed spacer region of nuclear ribosomal DNA (ITS) were used to assess the relationships
of Bidens cordylocarpa. The molecular phylogeny places B. cordylocarpa in a strongly supported clade of Mexican and South American Bidens, and provides more definitive evidence of relationships than morphology, chromosome number, or secondary chemistry. Molecular,
morphological, and chromosomal data suggest that B. cordylocarpa is an ancient polyploid, perhaps the remnant of a polyploid complex.
Received August 28, 2000 Accepted February 11, 2001 相似文献
224.
225.
Ramakrishna A. Bangaru Yatish K. Bansal A. R. M. Rao T. P. Gandhi 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》2000,739(2)
A rapid, simple and sensitive reversed-phase high-performance liquid chromatographic (HPLC) method has been developed for the measurement of acyclovir concentrations in human plasma and its use in bioavailability studies is evaluated. Unchanged acyclovir has been quantified without the introduction of an internal standard using the present method. Human plasma proteins were selectively precipitated by the addition of 7% perchloric acid to spiked plasma samples or to the plasma samples obtained after acyclovir administration to human volunteers and the mixture was spun at 1000 g for 10 min. The supernatant was directly injected into a Novaflex C18 column and detected at 254 nm. The mobile phase consisted of octane sulfonic acid buffer (pH 2.5) and methanol (92:08). The limit of quantitation for acyclovir in plasma was 20 ng/ml, which enabled the determination of the area under the curve (AUC) more precisely, that is, it is much closer to its extrapolated value. The present method has been successfully applied to samples from bioavailability studies. 相似文献
226.
227.
M. A. Riddin G. J. Venter K. Labuschagne M. H. Villet 《Medical and veterinary entomology》2019,33(4):467-475
An upsurge in African horse sickness (AHS) in the Eastern Cape, South Africa, from 2006 led to an epidemiological reassessment of the disease there. Light trapping surveys carried out near horses, donkeys and zebras in 2014–2016 collected 39 species of Culicoides midge (Diptera: Ceratopogonidae) that are potential vectors of AHS. To establish if these midges fed on equids, DNA sequences were obtained from the gut contents of 52 female midges (35 freshly blood‐fed, 13 gravid and four parous), representing 11 species collected across 11 sites. Culicoides leucostictus fed on all three equids. Culicoides bolitinos, Culicoides imicola and Culicoides magnus fed on both horses and donkeys. Culicoides onderstepoortensis fed on donkeys, and Culicoides similis and Culicoides pycnostictus fed on zebras. Bloodmeals from cows, pigs, warthogs, impalas and a domestic dog were also identified in various species, but none of the midges tested had fed on birds. These results contribute to knowledge of the vectorial capacity of several species of Culicoides with regard to AHS in the Eastern Cape and point to potential reservoir hosts, of which donkeys, zebras and domestic dogs have previously been found to harbour AHS. Blood‐fed midges were also obtained throughout winter, indicating the potential for endemic AHS in the province. 相似文献
228.
229.
Patterns of Sleeping Site and Sleeping Tree Selection by Black-and-Gold Howler Monkeys (Alouatta caraya) in Northern Argentina 总被引:1,自引:0,他引:1
Brividoro Melina V. Kowalewski Martin M. Scarry Clara J. Oklander Luciana I. 《International journal of primatology》2019,40(3):374-392
International Journal of Primatology - The selection of sleeping sites and sleeping trees in nonhuman primates is related to social and ecological factors. We investigate the role of body... 相似文献
230.
Stephen M. Reilly 《Journal of morphology》1987,191(2):205-214
Comparison of metamorphosis of skull and hyobranchial system in two species of neotenic salamanders reveals two different types of neoteny. Ambystoma talpoideum is completely neotenic owing to delayed metamorphosis. Notophthalmus viridescens exhibits limited neoteny as a result of incomplete metamorphosis. Morphological details of neoteny are compared to life history in both species in order to discuss the ecological morphology of the two neotenic strategies. Comparisons to Taricha granulosa, Triturus vulgaris, and Ambystoma gracile indicate that these two strategies are widely employed and may represent familial patterns. 相似文献