全文获取类型
收费全文 | 64434篇 |
免费 | 5391篇 |
国内免费 | 189篇 |
出版年
2023年 | 231篇 |
2022年 | 713篇 |
2021年 | 1179篇 |
2020年 | 709篇 |
2019年 | 913篇 |
2018年 | 1360篇 |
2017年 | 1070篇 |
2016年 | 1873篇 |
2015年 | 3015篇 |
2014年 | 3397篇 |
2013年 | 3882篇 |
2012年 | 5052篇 |
2011年 | 4839篇 |
2010年 | 3039篇 |
2009年 | 2684篇 |
2008年 | 3817篇 |
2007年 | 3507篇 |
2006年 | 3220篇 |
2005年 | 2923篇 |
2004年 | 2811篇 |
2003年 | 2500篇 |
2002年 | 2125篇 |
2001年 | 1829篇 |
2000年 | 1696篇 |
1999年 | 1336篇 |
1998年 | 586篇 |
1997年 | 523篇 |
1996年 | 441篇 |
1995年 | 441篇 |
1994年 | 355篇 |
1993年 | 333篇 |
1992年 | 700篇 |
1991年 | 577篇 |
1990年 | 545篇 |
1989年 | 534篇 |
1988年 | 449篇 |
1987年 | 429篇 |
1986年 | 350篇 |
1985年 | 371篇 |
1984年 | 303篇 |
1983年 | 255篇 |
1982年 | 212篇 |
1981年 | 186篇 |
1980年 | 176篇 |
1979年 | 241篇 |
1978年 | 214篇 |
1977年 | 194篇 |
1976年 | 188篇 |
1974年 | 215篇 |
1973年 | 174篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Summary The nitrogen-fixing activity in the rhizospheres of various rices was measured by the acetylene-reduction method throughout plant growth in green-house pots. The activity began to increase 4 weeks after transplanting, increased until heading stage, then decreased. The concentrations of exchangeable ammonium and sugars in the soil were not related to the variation of nitrogen-fixing activity during rice growth.The nitrogen-fixing activities in the rhizospheres of 41 rice varieties in pots were measured to discover varietal differences. Levels of nitrogen fixation were highly correlated with the rices' dry root weight at heading stage. 相似文献
992.
Summary The feasibility of using a plastic bag was examined in a test to screen rice varieties for differences in the nitrogen-fixing activity in their root zones. The change in the volume of the plastic bag during 24 hours of incubation was not significant. Although the plastic bag was permeable to acetylene and ethylene, the amounts lost through permeation during 24 hours of incubation did not seem to affect the acetylene reduction assay. The plastic bag was convenient to use in creating different conditions of the gas phase, and was useful for screening six rice varieties for differences in nitrogen-fixing activity in their root zones. 相似文献
993.
A convenient and reliable method for culturing cotton embryos is needed to obtain interspecific hybrids of this genus. C.A. Beasley and I.P. Ting (Amer. J. Bot. 60, 130, 1973) developed a phytohormone-supplemented medium (BTP) upon which the growth of ovules was similar that of in situ ovules. This medium was examined for in-ovulo embryo culture. Although good ovule growth occurred on BTP no embryos developed to maturity. However, when the medium was supplemented with NH
4
+
, more than 50% of the ovules produced mature embryos, and many of these germinated precociously after 8–10 weeks of culture. After germination seedlings were established on a separate medium designed to give balanced root and shoot growth. Subsequently young plants could be transferred to pots for greenhouse culture. 相似文献
994.
995.
The effects of citrate and cyclic AMP on the rate and degree of phosphorylation and inactivation of rat liver acetyl-CoA carboxylase were examined. High citrate concentrations (10 to 20 mM), which are generally used to stabilize and activate the enzyme, inhibit phosphorylation and inactivation of carboxylase. At lower concentrations of citrate, the rate and degree of phosphorylation are increased. Furthermore, phosphorylation and enzyme inactivation are affected by cyclic AMP under these conditions. At high citrate concentrations, cyclic AMP has little or no effect on inactivation and phosphorylation of acetyl-CoA carboxylase. Phosphorlation and inactivation of carboxylase is accompanied by depolymerization of the polymeric form of the enzyme into intermediate and protomeric forms. Depolymerization of carboxylase requires the transfer of the gamma-phosphate group from ATP to carboxylase. Inactivation occurs in the absence of CO2, which indicates that phosphorylation of the enzyme is the cause of inactivation and depolymerization, i.e. carboxylation of the enzyme is not responsible for inactivation of the enzyme. 相似文献
996.
Specific adhesion of rat hepatocytes to beta-galactosides linked to polyacrylamide gels 总被引:6,自引:0,他引:6
Rat hepatocytes, isolated by a collagenase perfusion technique, specifically bind to polyacrylamide gel containing covalently immobilized 6-aminohexyl beta-D-galactopyranosyl groups. Less than 5% of these cells bind to polyacrylamide or to gels with the following covalently linked ligands: 6-aminohexanol, or the 6-aminohexyl D-pyranosides of alpha-mannose, beta-glucose, beta-2-acetamido-2-deoxyglucose, beta-cellobiose, beta-maltose, or beta-melibiose. Cell binding to beta-D-galactoside gels occurs after a lag period at 37 degrees and 65 to 100% (depending on the cell preparation) of the cells adhere. The duration of the lag period is inversely related to the beta-D-galactoside content of the gel but preincubation of the cells at 37 degrees reduces the lag period. Cell-gel binding is a threshold phenomenon. Adhesion of cells to gels does not occur when the glycoside concentration is less than about 900 nmol per cm2 x 0.25 mm thick gel piece. Above this critical concentration, cell-gel binding occurs and becomes maximal when the concentration is increased by only 20%. If these in vitro results apply to cellular interactions in vivo, they suggest that slight changes in the levels of cell surface or extracellular matrix carbohydrates may profoundly influence the behavior of neighboring cells. 相似文献
997.
998.
Orchinol, hircinol, loroglossol and certain related phenanthrenes inhibited horseradish peroxidase-catalysed IAA degradation to a varied degree. Among 相似文献
999.
A method for the culture of spiral-shaped bacteria associated with the intestinal mucosa of rodents is described. The appearance in culture of a spiral organism from rat ceca and a spirochete from mouse ceca is illustrated; these organisms are morphologically similar to the major inhabitants of the cecal mucosa in each animal species. 相似文献
1000.
The tuberculostatic and carcinogenic drug isonicotinic acid hydrazide (“isoniazid”) is oxidized to pyridine-4-carboxaldehyde by the horseradish peroxidase/Mn2+/O2 system. Eosin and related sensitizers greatly accelerate the reaction and generate light detectable with the liquid scintillation counter or with the photon counter. If the isoniazid concentration is raised, the rate and extent of O2 uptake are increased, but above a certain concentration of isoniazid, emission is reduced and even suppressed. The strong quencher of triplet eosin, potassium ferricyanide, abolished both effects of eosin, that is, catalysis and light emission. Superoxide dismutase at high concentrations partially suppressed the emission and almost totally removed the catalytic effect. It is tentatively proposed that the isoniazid/peroxidase/Mn2+/O2 system efficiently produces the aldehyde in the triplet state, which in turn transfers energy to eosin. Because of the presence of oxygen, only a small yield of triplet eosin is obtained and only a small fraction of these triplet eosin molecules is able to react with isoniazid. Nevertheless, it will contribute efficiently to a cyclic process of oxidation of the isoniazid. 相似文献