全文获取类型
收费全文 | 5912篇 |
免费 | 300篇 |
国内免费 | 8篇 |
专业分类
6220篇 |
出版年
2024年 | 19篇 |
2023年 | 92篇 |
2022年 | 215篇 |
2021年 | 312篇 |
2020年 | 146篇 |
2019年 | 166篇 |
2018年 | 180篇 |
2017年 | 153篇 |
2016年 | 270篇 |
2015年 | 294篇 |
2014年 | 310篇 |
2013年 | 467篇 |
2012年 | 424篇 |
2011年 | 460篇 |
2010年 | 263篇 |
2009年 | 236篇 |
2008年 | 318篇 |
2007年 | 287篇 |
2006年 | 260篇 |
2005年 | 216篇 |
2004年 | 200篇 |
2003年 | 169篇 |
2002年 | 168篇 |
2001年 | 47篇 |
2000年 | 41篇 |
1999年 | 39篇 |
1998年 | 30篇 |
1997年 | 32篇 |
1996年 | 18篇 |
1995年 | 27篇 |
1994年 | 28篇 |
1993年 | 22篇 |
1992年 | 18篇 |
1991年 | 16篇 |
1990年 | 15篇 |
1989年 | 17篇 |
1988年 | 17篇 |
1987年 | 18篇 |
1986年 | 7篇 |
1985年 | 23篇 |
1984年 | 21篇 |
1983年 | 15篇 |
1982年 | 25篇 |
1981年 | 13篇 |
1980年 | 16篇 |
1979年 | 9篇 |
1978年 | 13篇 |
1977年 | 10篇 |
1974年 | 6篇 |
1966年 | 7篇 |
排序方式: 共有6220条查询结果,搜索用时 15 毫秒
41.
C. Kirda A. R. A. G. Mohamed K. S. Kumarasinghe A. Montenegro F. Zapata 《Plant and Soil》1992,147(2):217-223
A field experiment was conducted to investigate if carbon isotope (13C) discrimination () measured at the vegetative stage of spring wheat (Triticum turgidum L. var. durum) is related with the yield and water use efficiency (WUE) at ripening. A line source sprinkler irrigation system exposed the wheat genotypes to different watering regimes, from rainfed to full irrigation and thereby increased the range in yield and WUE attainable in the four genotypes studied. The results indicated that values measured at the late stem elongation stage 60 days after planting (DAP), showed strong positive correlation with total dry matter yield (r=0.732***), and a highly significant negative correlation with WUE (r=–0.755***) measured at ripening 105 DAP. The data suggest that the imprints of measured at vegetative growth stage persists throughout the entire growth period, until maturity. Subject to confirmation from additional studies in other crops and locations, early measurements of may prove a useful tool for rapid and early screening of cultivars, for high yield and high WUE. 相似文献
42.
Salem MH Mekkawy MY Ahmed NA Abdel-Aziz IY Mohamed AA El-Oksh HA Pursel VG 《Theriogenology》1992,37(5):1061-1074
Ejaculated washed ram spermatozoa showed consistent increases in the intracellular concentration of cyclic 3', 5'-adenosine monophosphate (cAMP) after incubation for 15 minutes with the phosphodiesterase (PDE)-inhibitors, theophylline and caffeine. In vitro addition of cAMP or PDE-inhibitors to ram semen also stimulated and maintained sperm motility and enhanced the rate of fructose utilization. The same doses of cAMP or theophylline significantly stimulated the rate of protein synthesis by the washed spermatozoa, while the PDE-stimulator, imidazole, inhibited protein synthesis significantly. The stimulatory effect of cAMP on sperm protein synthesis was not affected by cycloheximide, but was abolished by the mitochondrial inhibitor, chloramphenicol. The present results indicate a positive correlation between the intracellular concentration of cAMP and the rates of progressive motility, fructose utilization, and protein synthesis by ram spermatozoa. The results suggest that the effect of cAMP is associated with the synthesis of mitochndrial proteins which may be involved with the observed enhancement of sperm motility and metabolism. The data also indicate that cAMP map act either as a first or a second messenger in mature spermatoza. 相似文献
43.
Carl E. Hilliker Martine I. Darville Magdy S. Aly Mohamed Chikri Claude Szpirer Peter Marynen Guy G. Rousseau Jean-Jacques Cassiman 《Genomics》1991,10(4)
Two genes encoding 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase were localized in human and rat chromosomes. PFKFB1 (previously PFRX), which encodes the liver and muscle isozymes, was assigned to Xq22-q31 in the rat and to Xq27–q28 in the human by in situ hybridization using probes generated by the polymerase chain reaction. PFKFB2, which encodes the heart isozyme of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase, was assigned to chromosome 13 in the rat and to chromosome 1 in the human by hybridization of DNA from somatic cell hybrids. By in situ hybridization, this gene was localized to the regions 13q24–25 in the rat and 1q31 in the human. 相似文献
44.
45.
This study identifies the types and quantities of metals retained by Nereis diversicolor and Scrobicularia plana in a predominantly arid estuary environment.Certain abiotic parameters and a number of physiological processes linked to metabolic and reproductive functions play a decisive role in the seasonal influence of metal levels on the tissues of these organisms.Given the deposivoric diet of the organisms, the sedimentary bed would appear to be the principal source of metal accumulation.The following classification illustrates the bioaccumulation of metals: 209-1The possibility that these metals may be carried into the estuary by sewage water from the cities of Rabat and Sále should certainly not be ignored. 相似文献
46.
NADPH-dependent 20α-hydroxysteroid oxidoreductase (20α-HSD; EC 1.1.1.149) from bovine fetal erythrocytes was obtained for the first time free of hemoglobin by a new 2,500-fold purification scheme. This was achieved by a sequence of calcium phosphate gel adsorption, ammonium sulfate fractionation, and affinity chromatography. The present results lead us to believe that the NADPH-dependent 3β-hydroxysteroid oxidoreductase activity, which was co-purified with 20α-activity, may originate at the active site of 20α-HSD (2). 相似文献
47.
Extraction with hydrochloric acid (pH 2.5) of the brown alga Padina pavonia afforded water-soluble and water-insoluble polysaccharides comprising D-glucuronic acid, L-fucose, D-xylose, D-mannose, D-glucose and D- galactose residues. The water-soluble polysaccharide was fractionated by using ethanol, and cetylpyridinium chloride and by chromatography on DEAE-cellulose. A neutral laminaran-like glucan, a sulphated heteropolysaccharide composed of the aforementioned sugars and a protein moiety were obtained. The isolated heteropolysaccharide showed high anticoagulation activity. 相似文献
48.
Mohamed Mathlouthi 《Carbohydrate research》1981,91(2):113-123
X-ray diffractograms of aqueous solutions of d-fructose, d-glucose, and sucrose, and of two amorphous “solid” forms of these carbohydrates are recorded in the angular range of O 2-16°. The formation, in these diffractograms, of one or two intensity maxima as the concentration is varied is interpreted in terms of modification of the molecular association. A model is proposed that takes account of the state of organization of the solutions of the three sugars as a function of the concentration. Sucrose shows an “anomaly”, as it is the only sugar to establish intramolecular bonds. The number of these intramolecular bonds also depends on the concentration. 相似文献
49.
This report describes the cases of three mentally and physically well-developed siblings, 12, 10, and 8 years of age, with varying degrees of isolated valvular pulmonic stenosis not related to age. The severest lesion occurred in the middle patient and was associated with a right-to-left shunt through a patent foramen ovale. The three children had no other siblings, and there was no history of congenital heart disease among close relatives. These cases support the conclusion that genetic factors play a significant role in the development of certain congenital cardiac disorders. 相似文献
50.