全文获取类型
收费全文 | 8133篇 |
免费 | 988篇 |
专业分类
9121篇 |
出版年
2021年 | 93篇 |
2020年 | 69篇 |
2019年 | 87篇 |
2018年 | 82篇 |
2017年 | 93篇 |
2016年 | 155篇 |
2015年 | 254篇 |
2014年 | 285篇 |
2013年 | 339篇 |
2012年 | 441篇 |
2011年 | 455篇 |
2010年 | 256篇 |
2009年 | 237篇 |
2008年 | 356篇 |
2007年 | 378篇 |
2006年 | 360篇 |
2005年 | 332篇 |
2004年 | 345篇 |
2003年 | 307篇 |
2002年 | 277篇 |
2001年 | 198篇 |
2000年 | 239篇 |
1999年 | 207篇 |
1998年 | 105篇 |
1997年 | 107篇 |
1996年 | 84篇 |
1995年 | 74篇 |
1994年 | 85篇 |
1993年 | 99篇 |
1992年 | 152篇 |
1991年 | 126篇 |
1990年 | 123篇 |
1989年 | 141篇 |
1988年 | 141篇 |
1987年 | 136篇 |
1986年 | 123篇 |
1985年 | 121篇 |
1984年 | 118篇 |
1983年 | 101篇 |
1982年 | 83篇 |
1981年 | 83篇 |
1980年 | 70篇 |
1979年 | 98篇 |
1978年 | 68篇 |
1977年 | 74篇 |
1976年 | 84篇 |
1975年 | 79篇 |
1974年 | 78篇 |
1973年 | 76篇 |
1971年 | 60篇 |
排序方式: 共有9121条查询结果,搜索用时 15 毫秒
121.
122.
U Keller S P?schmann U Krengel H Kleinkauf G Kraepelin 《Journal of general microbiology》1983,129(6):1725-1731
Conditions for the regeneration of cells from protoplasts of Streptomyces chrysomallus, a producer of the peptide antibiotic actinomycin, are described. Regeneration of fusion products was most efficient at 27-30 degrees C on regeneration R2 medium (Okanishi et al., 1974) containing 0.25 M-sucrose. The addition of phosphate (150-300 mg 1(-1) to the medium and incubation at 23 degrees C proved to be optimal for the regeneration of individual strains. Highest recombination frequencies after protoplast fusion were obtained by fusing protoplasts in the presence of 45% (w/v) polyethylene glycol 6000. With strains that produce no, or little antibiotic, protoplasts must be present in excess in fusion mixtures in order to overcome inhibition of regeneration by the antibiotic-producing partner. 相似文献
123.
124.
Rainer Zocher Ullrich Keller Horst Kleinkauf 《Biochemical and biophysical research communications》1983,110(1):292-299
Covalently bound intermediates of enniatin B synthesis could be isolated from enniatin synthetase by treatment with performic acid. By comparison with products of mild alkaline cleavage of authentic enniatin B they could be identified as the dipeptide D-2-hydroxyisovaleryl-N-methylvaline and the corresponding tetrapeptide. Synthesis of enniatins apparently proceeds via condensation of dipeptides. This was confirmed by the use of the substrate analogue isovaleric acid, which has shown to be a strong inhibitor for enniatin synthesis by formation of N-isovaleryl-N-methyl valine. 相似文献
125.
Resistance to an undescribed species of Heterodera, Osborne''s cyst nematode, was compared in Nicotiana glutinosa, N. paniculata, N. plumbaginifolia and N. Iongiflora. These species were differentially resistant in greenhouse tests as shown by nematode development, the reaction of the invaded roots, and the expression of resistance in interspecific hybrids. 相似文献
126.
Attractant-Directed Motility in Escherichia coli: Requirement for a Fluid Lipid Phase 总被引:8,自引:4,他引:4 下载免费PDF全文
Attractant-directed motility (chemotaxis) in Escherichia coli has an absolute requirement for a fluid membrane. No such requirement was detected for motility per se. 相似文献
127.
Hydrostatic pressure was found to cause a marked narrowing of pH ranges for growth and reductions in growth yields for a variety
of bacteria. In many cases, reduced yields under pressure could be directly related to increased sensitivities to metabolic
acids that accumulated in the enclosed culture vessels used. Magnesium and calcium ions partially reversed increases in sensitivities
of representative gram-positive and gram-negative bacteria to low, but not high, pH. Growth inhibition of these organisms
at both extremes of pH was associated with enhanced loss of K+ from pressurized cells. Inhibited cells in alkaline media also lysed under pressure, but microscopically observable lysis
was clearly a secondary phenomenon because it occurred slowly. Apparent volumes for growth-inhibitory protonation-deprotonation
reactions were calculated on the basis of measured shifts in inhibitory pH with pressure. The values ranged from 99 to 431
ml/mole, and their magnitudes indicated that growth inhibition by acids or bases involves cooperative changes in polymeric
interactions such as those which accompany protein denaturation. 相似文献
128.
129.
Procedure for Evaluating the Effects of 2,450- Megahertz Microwaves upon Streptococcus faecalis and Saccharomyces cerevisiae 总被引:5,自引:4,他引:1 下载免费PDF全文
Modifications of a commercial 2,450-megahertz microwave oven were made so that 6 ml of microbial suspension could be exposed to the microwave field for various periods of time. The microorganisms were contained in the central tube of a modified Liebig condenser positioned in the approximate geometric center of the oven cavity. Kerosene at -25 C was circulated through the jacket of the condenser during microwave exposure permitting microwaves to reach the microbial suspension. Flow rates of the kerosene were varied to permit the temperature of the suspension to range from 25 to 55 C during microwave exposure. Conductive heating experiments using similar temperatures were also conducted. A thermocouple-relay system was employed to measure the suspension temperature immediately after the magnetron shutoff. Continuous application of microwaves to suspensions of 10(8) to 10(9)Streptococcus faecalis or Saccharomyces cerevisiae per ml appeared to produce no lethal effects other than those produced by heat. Respiration rates of microwave-exposed Scerevisiae were directly related to decreases in viable count produced by increased microwave exposure times. 相似文献
130.
Zur Feinstruktur von Cuticula und Epidermis beim Flußkrebs Orconectes limosus während eines Häutungszyklus 总被引:1,自引:0,他引:1
Zusammenfassung Die Cuticula an der Innen- und Außenseite der Branchiostegite des Flußkrebses besteht wie für Arthropoden üblich aus Epi- und Procuticula. Sowohl die Epicuticula als auch die Procuticula von Innen- und Außenseite unterscheiden sich im Feinbau wesentlich voneinander. An der Innenseite ist die Epicuticula einfach gebaut; Die Procuticula ist lamelliert und zeigt meist die bogenförmigen Muster von Mikrofibrillen. Die Epicuticula an der Außenseite weist in den in dieser Arbeit untersuchten Entwicklungsstadien einen sehr viel komplizierteren Feinbau auf, der in der Entwicklung gewissen Änderungen unterliegt. In der wiederum lamellierten Procuticula an der Außenseite sind die Mikrofibrillen zu Balken gebündelt. Die Ausrichtung der Mikrofibrillen dreht sich innerhalb einer Lamelle um 180°. Durch die Procuticula ziehen Fortsätze der Epidermiszellen, außerdem Stäbe der sog. Verbindungsstrukturen.Die Bildung der Cuticula an der Innenseite konnte weitgehend vollständig verfolgt werden; sie ist gut mit der Bildung der Cuticula bei verschiedenen Insekten vergleichbar.Die Bildung der Cuticula an der Außenseite konnte dagegen nur von Beginn der Abscheidung der Proouticula bis zur Häutung verfolgt werden. Kurz vor Beginn der Cuticulaabscheidung kommt es in den Epidermiszellen zu einer stärkeren Entwicklung des rauhen ER. Während der gesamten von uns verfolgten Bildungsstadien sieht man Vesikel mit dichtem Inhalt besonders in der Nähe des Zellapex. Sie geben anscheinend hier ihren Inhalt, Cuticulamaterial, nach außen ab. Sie stammen wohl aus Golgibereichen. Auch Stachelsaumbläschen (coated vesicles) kommen regelmäßig vor, deren genetischer Zusammenhang mit multivesikulären Körpern diskutiert wird. Bei der Abscheidung der fibrillären Cuticulasubstanzen spielen besondere Differenzierungen der Zell oberfläche, — kappenartige Verdichtungen der Zelloberfläche, meist an der Spitze kleiner Mikrovilli — eine wesentliche Rolle.
The ultrastructure of cuticle and epidermis in the crayfish Crconectes limosus during a moulting cycle
Summary The cuticle of the inside and outside of the branchiostegites of the crayfish consists of an epicuticle and a procuticle — as common in arthropods. Concerning their ultrastructure epicuticle and procuticle differ essentially from each other on both the inside as well as the outside. On the inside the epicuticle is built plainly; the procuticle is laminated, and, mostly it shows the arched patterns of microfibrils. In those developmental stages investigated in this project the epicuticle of the outside shows a much more intricated ultrastructure, since during formation it is subject to certain changes. On the outside the procuticle is also laminated; the microfibrils are bundled up to bars. The alignment of those microfibrils within one lamella is twisted for 180°. The procuticle is penetrated by processes of epidermal cells and by rods of the so-called connecting structures.The formation of the cuticle on the inside was observed completely; it is comparable to the forming of the cuticle in several insects. However, the formation of the cuticle on the outside was only observed from the beginning of the procuticular development up to the moulting.Shortly before formation of the cuticle the development of rough ER in the epidermal cells seems to be intensified. In all of developmental stages observed there appear vesicles with dense contents mainly situated nearby the cell apex. At this site they evidently deliver their contents — cuticular materials — to the outside of the cell; they probably originate in the Golgi areas. There occur coated vesicles regularity, too; their genetic relation to multivesicular bodies is discussed. Special differentiation on the cell surface i.e. dome-like consolidations of the cell surface mainly placed at the tip of small microvilli are of great importance for the secretion of the cuticle substances.相似文献