全文获取类型
收费全文 | 106577篇 |
免费 | 7829篇 |
国内免费 | 211篇 |
专业分类
114617篇 |
出版年
2021年 | 1216篇 |
2020年 | 1091篇 |
2019年 | 1073篇 |
2018年 | 2503篇 |
2017年 | 2359篇 |
2016年 | 3084篇 |
2015年 | 3891篇 |
2014年 | 4042篇 |
2013年 | 5312篇 |
2012年 | 6279篇 |
2011年 | 5805篇 |
2010年 | 3714篇 |
2009年 | 3053篇 |
2008年 | 4683篇 |
2007年 | 4459篇 |
2006年 | 4286篇 |
2005年 | 3659篇 |
2004年 | 3566篇 |
2003年 | 3218篇 |
2002年 | 3003篇 |
2001年 | 4221篇 |
2000年 | 3888篇 |
1999年 | 2888篇 |
1998年 | 1083篇 |
1997年 | 890篇 |
1996年 | 834篇 |
1995年 | 734篇 |
1994年 | 661篇 |
1992年 | 1847篇 |
1991年 | 1649篇 |
1990年 | 1540篇 |
1989年 | 1478篇 |
1988年 | 1351篇 |
1987年 | 1298篇 |
1986年 | 1166篇 |
1985年 | 1167篇 |
1984年 | 946篇 |
1983年 | 776篇 |
1982年 | 608篇 |
1979年 | 874篇 |
1978年 | 676篇 |
1977年 | 601篇 |
1975年 | 731篇 |
1974年 | 835篇 |
1973年 | 824篇 |
1972年 | 798篇 |
1971年 | 723篇 |
1970年 | 726篇 |
1969年 | 741篇 |
1968年 | 625篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
The phylogeny of the fungus gnat family Mycetophilidae (Diptera) is reconstructed with a focus on the species‐rich and taxonomically difficult subfamilies Gnoristinae and Mycomyinae. The multigene phylogenetic analyses are based on five nuclear (18S, 28S, CAD, MCS, ITS2) and four mitochondrial (12S, 16S, COI, CytB) gene markers. The analyses strongly support the monophyly of Mycetophilidae and the subfamilies Manotinae, Sciophilinae, Leiinae, and Mycomyinae, although Gnoristinae is paraphyletic with respect to Mycetophilinae. All the genera and groups of genera included are supported as monophyletic, except for Acomoptera Vockeroth, Boletina Staeger, Dziedzickia Johannsen, Ectrepesthoneura Enderlein, and Neoempheria Osten Sacken. Ancestral character state reconstructions were applied to two morphological features present in Gnoristinae and Mycomyinae (i.e. presence of setae on wing membrane and wing vein R4) in order to assess their evolution. The wing vein R4 appears as an unstable character, spread throughout different clades. A dated phylogeny of the family Mycetophilidae showed that most of the subfamilies of Mycetophilidae originated and diversified during the Cretaceous. The youngest subfamilies, originated in the Paleogene, appear to be Mycomyinae and Mycetophilinae. 相似文献
2.
A Nicák 《Physiologia Bohemoslovaca》1986,35(2):118-126
The author describes changes in the disintegration of erythrocytes by brilliant cresyl blue in correlation to age, in rats aged 21, 42, 90-105, 340-360 and 690-720 days. The erythrocytes were incubated for 4 hours in an isotonic NaCl solution, in Krebs-Ringer solution and in each of these solutions plus brilliant cresyl blue. Disintegration in plain NaCl solution was found to be the greatest in the case of erythrocytes from 690- to 720-day-old rats. In the same solution plus brilliant cresyl blue, the rate of disintegration was very high in 21-day-old, 42-day-old and 690- to 720-day-old animals; at 90-105 days it was lower and at 340-360 days it was the lowest. Disintegration of erythrocytes in plain Krebs-Ringer solution was the lowest at 21 and 42 days; in the other age groups it was slightly higher. On adding brilliant cresyl blue, the rate of disintegration rose significantly in 21-, 42- and 690- to 720-day-old animals; at 90-105 days and 340-360 days it was no different from disintegration in plain Krebs-Ringer solution. It can be seen from the results that the rate of brilliant cresyl blue-induced erythrocyte disintegration is dependent on the age of the animals from which the erythrocytes are taken. 相似文献
3.
Gentamicin and amikacin repress the growth of Plasmodium falciparum in culture, probably by inhibiting a parasite acid phospholipase 总被引:1,自引:0,他引:1
Human erythrocytes were loaded with either gentamicin or amikacin and subsequently infected with the human malarial parasite Plasmodium falciparum and grown in culture. Parasite invasion of erythrocytes was unaffected by the drugs, but subsequent development was retarded. The digestion of host cell cytosol in ring-stage parasites was inhibited by the drugs. A substantial acid, Ca2+-independent phospholipase activity could be monitored in parasite cytosol and was found to be inhibited by the drugs. These results imply that phospholipases are involved in the feeding mechanism of the parasite and that gentamicin and amikacin exert their inhibitory activity by affecting these enzymes. 相似文献
4.
We propose a mathematical model for pendelluft flow in a single airway bifurcation. The model is motivated by an apparatus used in an experimental study of the pendelluft by Ultman et al. (1988). We derive differential equations governing the fluid flow, which directly connect physiological parameters to the variables determining the pendelluft; this approach allows us to include nonlinearity in the model. If nonlinearity is neglected, our model is identical to the R-I-C circuits used by previous investigators. If nonlinearity is retained, we show that pendelluft can occur even in perfectly symmetric airway bifurcations. For the specific apparatus used in the experiments of High et al. (1991), we demonstrate that two qualitatively different pendelluft flows can occur in the system. 相似文献
5.
6.
Z Zhong A Toukdarian D Helinski V Knauf S Sykes J E Wilkinson C O'Bryne T Shea C DeLoughery R Caspi 《Applied and environmental microbiology》2001,67(12):5771-5779
An agar-degrading marine bacterium identified as a Microscilla species was isolated from coastal California marine sediment. This organism harbored a single 101-kb circular DNA plasmid designated pSD15. The complete nucleotide sequence of pSD15 was obtained, and sequence analysis indicated a number of genes putatively encoding a variety of enzymes involved in polysaccharide utilization. The most striking feature was the occurrence of five putative agarase genes. Loss of the plasmid, which occurred at a surprisingly high frequency, was associated with loss of agarase activity, supporting the sequence analysis results. 相似文献
7.
8.
9.
V A Frolov G Mall P Rieger Kh Derks Z Antoni 《Biulleten' eksperimental'no? biologii i meditsiny》1987,104(12):739-741
Intensive synthesis of collagen-like substance was revealed in the rabbit myocardium during experimental diphtheria intoxication. It was more marked in the right ventricle 24 hours after the injection of diphtheria toxin. Since similar changes (the substance was mainly formed around blood vessels) have been observed in other cases of toxic myocardial alterations (i.e. ethanol intoxication, injection of pharmacological agents, etc.), it can be assumed that it is a standard protective reaction of the altered heart to the penetration of toxic agents from the blood into the myocardial tissue. 相似文献
10.