全文获取类型
收费全文 | 2378篇 |
免费 | 223篇 |
国内免费 | 1篇 |
出版年
2021年 | 23篇 |
2019年 | 24篇 |
2018年 | 26篇 |
2017年 | 25篇 |
2016年 | 41篇 |
2015年 | 62篇 |
2014年 | 96篇 |
2013年 | 87篇 |
2012年 | 145篇 |
2011年 | 127篇 |
2010年 | 96篇 |
2009年 | 71篇 |
2008年 | 132篇 |
2007年 | 132篇 |
2006年 | 120篇 |
2005年 | 107篇 |
2004年 | 113篇 |
2003年 | 113篇 |
2002年 | 115篇 |
2001年 | 50篇 |
2000年 | 60篇 |
1999年 | 44篇 |
1998年 | 30篇 |
1997年 | 44篇 |
1996年 | 37篇 |
1995年 | 43篇 |
1994年 | 31篇 |
1993年 | 22篇 |
1992年 | 37篇 |
1991年 | 27篇 |
1990年 | 31篇 |
1989年 | 37篇 |
1988年 | 27篇 |
1987年 | 45篇 |
1986年 | 27篇 |
1985年 | 20篇 |
1984年 | 27篇 |
1983年 | 19篇 |
1982年 | 24篇 |
1980年 | 19篇 |
1979年 | 19篇 |
1978年 | 14篇 |
1977年 | 16篇 |
1976年 | 18篇 |
1975年 | 15篇 |
1974年 | 16篇 |
1973年 | 16篇 |
1972年 | 19篇 |
1971年 | 18篇 |
1969年 | 13篇 |
排序方式: 共有2602条查询结果,搜索用时 15 毫秒
1.
2.
3.
International Research Ethics 总被引:2,自引:0,他引:2
This article provides a critical overview of the most important issues pertaining to the ongoing debate on international research ethics. It critically describes three problems of continuing concern: 1) the question of whether the distinction between therapeutic and non-therapeutic research should be upheld; 2) the questions of whether the currently demanded best proven diagnostic and therapeutic method of treatment for all research subjects is feasible both in developed and in developing countries, and whether it should be upheld; 3) the questions of who owns international research ethics guidelines and regulatory frameworks and, how decisions about changes to such international guidelines can possibly be achieved, given that it seems to be the case that genuine disagreement about issues of content is possible and likely. 相似文献
4.
5.
W E Berdel P Schick H Sedlmeier U Fink J Rastetter O Messerschmidt 《Radiation research》1983,94(1):166-170
Synthetic lysophospholipids represent a variety of analogs of the naturally occurring 2-lysophosphatidylcholine. Some of these compounds showed significant therapeutic effects on the survival of mice following radiation injury when administered after various doses of whole-body X irradiation. Such therapeutic effects were discernible even when the treatment was given 6 hr after irradiation, and both intravenous and oral application were effective. Intravenous application of 2 X 25 mg/kg lysophospholipid after whole-body X irradiation around the LD50 resulted in significantly higher numbers of surviving animals. The mode of action remains speculative. 相似文献
6.
7.
8.
Summary Different mushrooms, mosses and corresponding soil samples have been collected mainly from two sites in the alpine region of southwestern Bavaria. At the end of the growthseason, September 1986, gamma spectroscopic analysis showed that the moss-, mould, and needle-layer contained considerably more 134Cs and 137Cs activity per unit fresh weight than eight different species of mushroom. These two isotopes were carried into the biotop mainly as a consequence of the Chernobyl accident. 131J could not be found any more in the samples ca. 5–6 months after the catastrophe. The activity of the cesium isotopes decreased with increasing soil depth. In the mushrooms the activity was relatively high in Xerocomus badius and surprisingly low in Boletus edulis; samples of the latter and of Cantharellus cibarius collected in September 1985 (before the accident) and kept deep frozen contained almost identical amounts of 137Cs as those collected from August to October 1986. Mushrooms contained considerably more of the natural isotope 40K than the needlelayers and the soil samples in the neighbourhood. In all mushrooms except Xerocomus badius the activity of 40K was generally higher than the 137Cs activity. The results indicate that except Xerocomus badius the analyzed mushrooms do not actively take up Cs from the soil, in contrast to K. 相似文献
9.
Summary Cucumber seedlings were grown in a Portsmouth soil-sand system to study how varying soil clay and organic matter content might
modify cucumber seedling response to ferulic acid, a reported allelopathic agent. Leaf area expansion of cucumber seedlings,
soil respiration, and soil solution concentrations of ferulic acid were monitored. Leaf area, mean absolute rates of leaf
expansion, and shoot dry weight of cucumber seedlings were significantly reduced by ferulic acid concentrations ranging from
10 to 70 μg/g dry soil. Ferulic acid was applied every other day, since it rapidly disappeared from soil solution as a result
of retention by soil particles, utilization by microbes and/or uptake by roots. The amount of ferulic acid retained (i.e., adsorbed, polymerized,etc.) by soil particles appeared to be secondary to microbial utilization and/or uptake by roots. Varying clay (5.3 to 9.8 g/cup)
and organic matter (2.0 to 0.04g/cup) contents of the soil appeared to have little impact on the disappearance of ferulic
acid from soil solution under “ideal” growth conditions for cucumber seedlings unless larger amounts of ferulic acid were
added to the soil; in this case 200 μg/g. The addition of ferulic acid to the soil materials substantially increased the activity
of the soil microbes. This latter conclusion is based on recovery of ferulic acid from soil solution and soil respiration
measurements.
Paper No. 10347 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, N C 27695-7601. The use
of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of the product
named, nor criticism of similar ones not mentioned. 相似文献
10.
Summary Periwinkle (Catharanthus roseus) cells cultured in 1-B 5 medium display the ultrastructure of parenchyma cells. The parenchyma character remained unchanged when cells were exposed to any one of three different conditions effecting alkaloid accumulation. Transfer of cells to alkaloid production medium for 2 weeks (condition 1) accorded two special features,i.e., unusually big lipid droplets in the cytoplasm and, upon fixation, one or several electron-dense droplets of spongy precipitate in vacuoles. Among hormone-autotrophic cultures (condition 2) some cells showed a fine electron-dense vacuolar precipitate. Addition ofPhythium homogenate (fungal elicitor) to cells cultured in 1-B 5-medium for 10 days (condition 3), cells showed a frequent appearance of singular big lipid droplets in the cytoplasm, whereas vacuoles remained devoid of precipitate. The appearance of big lipid droplets and of vacuolar precipitate is interpreted as progressing cytodifferentiation, but is coincidental with alkaloid accumulation.NRCC no. 24524. 相似文献