全文获取类型
收费全文 | 12316篇 |
免费 | 889篇 |
国内免费 | 8篇 |
专业分类
13213篇 |
出版年
2021年 | 121篇 |
2020年 | 94篇 |
2019年 | 124篇 |
2018年 | 151篇 |
2017年 | 159篇 |
2016年 | 218篇 |
2015年 | 369篇 |
2014年 | 367篇 |
2013年 | 555篇 |
2012年 | 684篇 |
2011年 | 600篇 |
2010年 | 461篇 |
2009年 | 374篇 |
2008年 | 548篇 |
2007年 | 547篇 |
2006年 | 548篇 |
2005年 | 551篇 |
2004年 | 545篇 |
2003年 | 560篇 |
2002年 | 538篇 |
2001年 | 135篇 |
2000年 | 102篇 |
1999年 | 139篇 |
1998年 | 170篇 |
1997年 | 156篇 |
1996年 | 146篇 |
1995年 | 157篇 |
1994年 | 129篇 |
1993年 | 162篇 |
1992年 | 113篇 |
1991年 | 141篇 |
1990年 | 122篇 |
1989年 | 104篇 |
1988年 | 127篇 |
1987年 | 109篇 |
1986年 | 91篇 |
1985年 | 112篇 |
1984年 | 169篇 |
1983年 | 141篇 |
1982年 | 140篇 |
1981年 | 151篇 |
1980年 | 125篇 |
1979年 | 109篇 |
1978年 | 131篇 |
1977年 | 101篇 |
1976年 | 88篇 |
1975年 | 94篇 |
1974年 | 80篇 |
1973年 | 67篇 |
1970年 | 62篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Relationship between the chromatoid body and the acrosomal system in early spermatids of Myxine glutinosa L. 总被引:1,自引:0,他引:1
Summary A transient close relationship between the chromatoid body and the developing acrosome is demonstrated in early spermatids of Myxine glutinosa.This work was supported by the Norwegian Research Council for Science and Humanities (NAVF, Grant Nr. D 61.44) and the Austrian Fonds zur Förderung der wissenschaftlichen Forschung, Projekt 2183 相似文献
92.
The enzyme catalyzing the reduction of sulfite by reduced benzyl viologen (BVH) was partially purified and characterized from two strains of wine yeasts, a sulfite-producing strain and a non-producing strain.Both enzymes showed corresponding features in pH-optima, optima of buffer and benzyl viologen concentrations.The enzymes did not catalyze the reduction of nitrite by reduced viologen dyes, but the reduction of sulfite was uncompetitively inhibited by nitrite. Compounds of sulfur metabolism such as sulfate, thiosulfate, cysteine, serine and methionine did not influence the activity of either of the enzymes. The main differences between the two enzymes exist in the specific activities in crude extracts, the K
m
-values for sulfite, substrate inhibition rates, and localization in different fractions during (NH4)2SO4 precipitation. The specific activity in crude extracts of the sulfite-producing strain (0.052 moles S2- x min-1 x mg-1) was about three fold higher than that of the non-producing strain (0.0179 moles S2- x min-1 x mg-1). On the other hand the sulfite-producing strain had a higher K
m
-value for sulfite (2×10-3 M) and was more strongly inhibited by the substrate than the non-producing strain (6×10-3 M). 相似文献
93.
94.
Summary The dielectric breakdown in the membranes of cells ofValonia utricularis was investigated using intracellular electrodes and 500-sec current pulses. Electrical breakdown, which occurs when the membrane potential reaches a well-defined critical value, is not associated with global damage to the cell or its membranes (the membrane reseals in <5 sec). It was thus possible to investigate the effect of temperature on dielectric breakdown in single cells. It was found that the critical potential for breakdown was strongly dependent on temperature, decreasing from 1000 mV at 4°C to 640 mV at 30°C. The decrease in the breakdown potential with increasing temperature and the very short rise-time of the breakdown current (1 sec) suggests that the Wien field dissociation does not play a major role in the breakdown process. It is shown that the nonlinearI–V characteristics observed at different temperatures can be accurately accounted for with no adjustable parameters, by considerations of the mechanical compression of the membrane due to stresses induced by the electric field. Electrical breakdown on this scheme results from an electromechanical instability in the membrane. On this basis the present results indicate that the elastic modulus of the region of the membrane where breakdown occurs, decreases by a factor of 2 with increasing temperature from 4 to 30°C. On the assumption of a thickness of 4.0 nm and a dielectric constant of 5, the elastic modulus is estimated to have a value of 5×106 Nm–2 at 20°C. 相似文献
95.
96.
Streptomyces michiganensis strain Tü 1074, was isolated from a Tunesian soil sample and produces in liquid medium an antibiotic active pigment complex. Besides mitomycin A the separation of this complex yielded a nonactive phenoxazone, which hitherto has not been described in the literature. In contrary to all known phenoxazones from microorganisms the new compound lacks a 2-amino-function. The production of this phenoxazone could be enhanced by optimizing the conditions of fermentation. 相似文献
97.
98.
Hans Wilhelm Bohle 《International Review of Hydrobiology》1974,59(3):403-420
Case Construction of Young Larvae of Micrasema longulum McL. (Trichoptera, Brachycentridae) The method of case building used by the first three larval instars of Micrasema longulum is described. The initial case is constructed above the hatching aperture in the jelly of the egg mass, in the same way as it is done by other species of Brachycentridae. The material of the initial case is caught from drifting particles in running water. From the end of the first larval instar only silk is used for the extension of the case. Two silken layers of a very regular pattern are to be distinguished. The construction of the cloverleaf-shaped posterior aperture is described. Some of the methods used for the case building show a maturing process during ontogenesis. 相似文献
99.
Ohne ZusammenfassungMit Unterstützung der Deutschen Forschungsgemeinschaft. 相似文献
100.