全文获取类型
收费全文 | 7846篇 |
免费 | 445篇 |
出版年
2023年 | 87篇 |
2022年 | 126篇 |
2021年 | 227篇 |
2020年 | 173篇 |
2019年 | 218篇 |
2018年 | 268篇 |
2017年 | 227篇 |
2016年 | 305篇 |
2015年 | 467篇 |
2014年 | 494篇 |
2013年 | 595篇 |
2012年 | 651篇 |
2011年 | 632篇 |
2010年 | 367篇 |
2009年 | 328篇 |
2008年 | 414篇 |
2007年 | 402篇 |
2006年 | 387篇 |
2005年 | 286篇 |
2004年 | 256篇 |
2003年 | 236篇 |
2002年 | 215篇 |
2001年 | 119篇 |
2000年 | 106篇 |
1999年 | 81篇 |
1998年 | 61篇 |
1997年 | 38篇 |
1996年 | 34篇 |
1995年 | 27篇 |
1994年 | 25篇 |
1993年 | 23篇 |
1992年 | 35篇 |
1991年 | 34篇 |
1990年 | 22篇 |
1989年 | 30篇 |
1988年 | 19篇 |
1987年 | 18篇 |
1986年 | 24篇 |
1985年 | 19篇 |
1984年 | 20篇 |
1983年 | 15篇 |
1982年 | 16篇 |
1981年 | 19篇 |
1979年 | 20篇 |
1978年 | 10篇 |
1976年 | 10篇 |
1975年 | 10篇 |
1974年 | 14篇 |
1973年 | 9篇 |
1972年 | 9篇 |
排序方式: 共有8291条查询结果,搜索用时 15 毫秒
31.
Summary A pot experiment with lettuce involving three N forms each at six application levels, showed that lettuce can be grown satisfactorily
with a very low nitrate content when supplied with ammonium sulphate and a nitrification inhibitor. For plants growing on
nitrate N, the optimum midrib sap nitrate concentration as maturity approached was about 1400 mg/1 NO3-N. Large losses of mineral N were observed from the peat medium, even in the absence of plants.
A relationship is presented which would enable a lettuce grower to estimate whole-shoot nitrate concentration from a quick
test of midrib sapi.e. NO3-N (mg/kg in fresh shoot) =0.14×NO3-N (mg/l in sap).
Tipburn was worst at intermediate levels of applied N, and was less serious with pure ammonium nutrition than with nitrate. 相似文献
32.
Summary Collections over four years in the habitat of submerged litter show that shrimps are abundant the year round. Mean densities vary from 5–45 animals/m2 of litter habitat. Finegrained density distribution within litter sites is highly clumped, but coarse-grained distribution in any given month between litter sites along the river is grosso-modo random.Density and species distribution are a function of the annual cycles of inundations. Of the five species (of a total of seven in the area) that were found to be breeding in the litter habitat it could be established that reproduction is restricted to the period of rising and highest water levels. Reproduction is low in all species, namely below 20 eggs per egg batch with essentially a single generation per year.Breeding ecology in relation to the litter habitat is briefly discussed.This project was supported jointly by the CNPq (Brasilian Research Council), by the OAS (Organization of the American States) and by the SUFRAMA (Financing body of local government) 相似文献
33.
A J de Araújo L F Ferreira U E Confalonieri L Nu?ez B M Ribeiro Filho 《Memórias do Instituto Oswaldo Cruz》1985,80(2):141-143
Enterobius vermicularis eggs were found in human coprolites collected in the archaeological site of Caserones, Tarapaca Valley, Chile, dating from 400 BC to 800 AD. The human pinworm had already been found in other pre-historic archaeological sites in America, and its introduction in this continent is discussed. 相似文献
34.
H G Ferreira K T Ferreira 《Proceedings of the Royal Society of London. Series B, Containing papers of a Biological character. Royal Society (Great Britain)》1983,218(1212):309-329
A set of experiments was simulated on a computer version of the Koefoed-Johnsen & Ussing model for high-resistance epithelia. The results obtained were analysed according to procedures commonly applied to the analyses of experimental data and interpreted in terms of the model parameters. Although the computer model encodes a stoichiometry of 3:2 for Na-K exchange through the Na pump, the simulation of published experimental procedures yields different figures in almost every case. We show that ENa as originally defined by Ussing & Zerahn (Acta physiol. scand. 23, 110-127 (1951)) and as obtained from flux-ratio experiments has different values under different experimental conditions with unchanged system parameters and that it is distinct from ENa measured by other methods. We also show that unless the pump is saturated with internal Na an increase in the rate of pumping cannot cause a substantial increase in the rate of transepithelial Na transport. 相似文献
35.
36.
37.
38.
39.
1. Proteolysis of immunoglobulin G with long-acting thyroid-stimulating activity with papain and pepsin shortened the duration of thyroid stimulation in mice prepared for the McKenzie (1958) bioassay. The time-course appeared to be related to molecular size. 2. The activity after proteolysis was recovered solely in the electrophoretically slow F(ab)-fragment. 3. There was no immunological cross-reaction between pituitary thyroid-stimulating hormone and the active fragments of the long-acting thyroid stimulator despite their similar behaviour in the bioassay. 4. The implications of these findings are discussed. 相似文献
40.
The existence of six amylase heterogeneity variants in serum and urine has been discovered in a group of 111 families with twins, and the frequence of each variant was determined. It had been shown that each of these amylase heterogeneity variants is inherited codominantly, and the mode of inheritance is demonstrated schematically.These amylase heterogeneity variants may be used as an aid to twin zygosity determination and also in paternity problems. 相似文献