全文获取类型
收费全文 | 15328篇 |
免费 | 563篇 |
国内免费 | 2篇 |
专业分类
15893篇 |
出版年
2022年 | 112篇 |
2021年 | 201篇 |
2020年 | 124篇 |
2019年 | 165篇 |
2018年 | 311篇 |
2017年 | 300篇 |
2016年 | 406篇 |
2015年 | 456篇 |
2014年 | 570篇 |
2013年 | 731篇 |
2012年 | 770篇 |
2011年 | 796篇 |
2010年 | 475篇 |
2009年 | 359篇 |
2008年 | 501篇 |
2007年 | 558篇 |
2006年 | 550篇 |
2005年 | 537篇 |
2004年 | 447篇 |
2003年 | 473篇 |
2002年 | 411篇 |
2001年 | 330篇 |
2000年 | 237篇 |
1999年 | 201篇 |
1998年 | 124篇 |
1992年 | 239篇 |
1991年 | 228篇 |
1990年 | 186篇 |
1989年 | 216篇 |
1988年 | 239篇 |
1987年 | 249篇 |
1986年 | 215篇 |
1985年 | 243篇 |
1984年 | 199篇 |
1983年 | 165篇 |
1979年 | 229篇 |
1978年 | 152篇 |
1977年 | 137篇 |
1976年 | 140篇 |
1975年 | 165篇 |
1974年 | 264篇 |
1973年 | 261篇 |
1972年 | 249篇 |
1971年 | 241篇 |
1970年 | 257篇 |
1969年 | 246篇 |
1968年 | 186篇 |
1967年 | 187篇 |
1966年 | 167篇 |
1965年 | 123篇 |
排序方式: 共有10000条查询结果,搜索用时 7 毫秒
51.
The study was undertaken to investigate the ability of enterococci to assimilate iron via siderophores of bacteria living in the same habitats in the human organism. The potential recipients of exogenous siderophores were six Enterococcus faecalis and six Enterococcus faecium strains, isolated from clinical materials of human origin. The donors of siderophores were Gram-negative rods (various species of the Enterobacteriaceae, Pseudomonas and Acinetobacter) and Gram-positive cocci (various species of Staphylococcus and Streptococcus). All of the investigated E. faecium and only two E. faecalis strains demonstrated the ability to utilize the siderophores of the aforementioned bacterial groups, predominantly the chelators of Gram-negative rods, those of Gram-positive cocci were utilized to a smaller extent. Four recipient strains from E. faecalis species did not demonstrate the ability to utilize siderophores synthesized by all of 40 investigated donor strains. 相似文献
52.
Carroll D. Arnett Joanna S. Fowler Robert R. MacGregor David J. Schlyer Alfred P. Wolf Bengt Långström Christer Halldin 《Journal of neurochemistry》1987,49(2):522-527
The distribution of carbon-11-labeled L-deprenyl, an irreversible inhibitor of monoamine oxidase type B (MAO-B), was determined in the baboon brain by positron emission tomography. The irreversible blood-to-brain transfer constant (influx constant, Ki) was measured using a complete metabolite-corrected arterial plasma concentration curve. This influx constant was used as a measure of functional enzyme activity for sequential determinations of MAO-B recovery following a single high dose of unlabeled l -deprenyl. The half-life for turnover of MAO-B was thus determined to be 30 days. Using appropriate irreversible inhibitors, this procedure should be generally useful for determining enzyme turnover rates in any organ in vivo and can be applied to some human studies as well. 相似文献
53.
Halińska, A. and Lewak, St. 1987. Free and conjugated gibberellins in dormancy and germination of apple seeds.
The presence of gibberellin A4 (GA4 ) was confirmed in partly stratified seeds of apple ( Malus domestica Borb., cv. Antonówka) by mass spectrometry of the methyl ester. Levels of free and conjugated gibberellins A4+7 and A9 changed during drying of mature seeds, during cold and warm stratification, as well as during germination of dormant and non-dormant embryos. The temporary rise in GA4+7 during cold stratification and during the culture of dormant embryos as well as the lack of it under conditions of warm stratification, allowed us to postulate a role for GA4+7 in the removal of dormancy. In addition, GA9 was absent in dormant embryos and increased during cold stratification and during the culture of non-dormant embryos. This suggests the involvement of GA9 , in induction of normal development of the seedling. The equivalence between changes in free and conjugated GAs suggests that formation and hydrolysis of conjugates are involved in the control of the physiologically active levels of free GA4+7 and GA9 . 相似文献
The presence of gibberellin A
54.
55.
56.
57.
The effect of long-term diets enriched with natural antioxidants was studied on Wistar rats with average initial body weight 150 g. After enrichment of the diet with selenium (0.1 ppm of sodium selenite per 100 g of diet), with vitamin E (6 mg of alpha-tocopherol per 100 g of diet) and selenium and vitamin E together the following results were obtained: diets enriched with selenium or vitamin E given for 12 months reduced the production of lipid peroxides in the liver and serum of the rats. On the other hand, addition of both antioxidants to the diet had no effect on lipid peroxide levels in the animals. Diet enrichment for 12 and 18 months with selenium or vitamin E had no effect on the levels of total cholesterol and HDL cholesterol. The obtained results suggest that selenium and alpha-tocopherol exert an inhibitory action on the processes of ageing in the experimental animal model. 相似文献
58.
W Niewiarowski Z J Le?nikowski A Wilk P Guga A Okruszek B Uznański W Stec 《Acta biochimica Polonica》1987,34(2):217-231
Diastereomers of the title compound were obtained and absolute configuration was assigned by means of stereochemical correlation. Their reaction with 3'-O-methoxyacetylthymidine in the presence of triisopropylbenzenesulphonyl (4-nitro) triazole is neither chemo- nor stereo-selective and leads to diastereomeric pairs of dithymidyl (3',5')methanephosphonate and -methanephosphonothioate. Obtained results are discussed in terms of mechanism of activation of phosphodiesters under conditions known as "phosphotriester approach to oligonucleotide synthesis". 相似文献
59.
60.
Summary The pattern of the vascular supply to the choroid of the frog eye was studied in toto with the use of the injection-replication-SEM technique. The choroid of anuran amphibians is composed mainly of the choriocapillaris. In both species studied (Rana temporaria, Rana esculenta), an independent arterial supply to the choriocapillaris supplemented that from the ciliary arteries. This additional vascular route arises from the optic artery, a separate branch of the arteria infundibularis superficialis. The optic artery, accompanied by its vein within the vascular sheath of the optic nerve, joins the rich arterial capillary network of the choriocapillaris and supplies the posterior pole of the ocular bulb. The superficial capillary network displays a dense collar around the entrance of the optic nerve into the eye and is composed of a circular meshwork of small capillaries, several layers deep. More peripherally, however, it becomes single layered. This capillary network, as a whole, establishes numerous connections with the adjacent choriocapillaris at the posterior pole of the ocular bulb. In anuran amphibians the complex arrangement of both arterial systems supporting the choriocapillaris may be regarded as a more complete equivalent of the short posterior ciliary arteries of mammals. 相似文献