全文获取类型
收费全文 | 24199篇 |
免费 | 210篇 |
国内免费 | 476篇 |
专业分类
系统科学 | 559篇 |
丛书文集 | 491篇 |
教育与普及 | 239篇 |
理论与方法论 | 116篇 |
现状及发展 | 7729篇 |
研究方法 | 957篇 |
综合类 | 14223篇 |
自然研究 | 571篇 |
出版年
2021年 | 192篇 |
2015年 | 233篇 |
2014年 | 305篇 |
2013年 | 440篇 |
2012年 | 732篇 |
2011年 | 1300篇 |
2010年 | 550篇 |
2009年 | 530篇 |
2008年 | 805篇 |
2007年 | 864篇 |
2006年 | 816篇 |
2005年 | 748篇 |
2004年 | 587篇 |
2003年 | 499篇 |
2002年 | 552篇 |
2001年 | 704篇 |
2000年 | 718篇 |
1999年 | 662篇 |
1998年 | 247篇 |
1997年 | 213篇 |
1996年 | 224篇 |
1995年 | 182篇 |
1992年 | 395篇 |
1991年 | 350篇 |
1990年 | 343篇 |
1989年 | 302篇 |
1988年 | 293篇 |
1987年 | 281篇 |
1986年 | 231篇 |
1985年 | 270篇 |
1984年 | 259篇 |
1982年 | 201篇 |
1980年 | 230篇 |
1979年 | 506篇 |
1978年 | 400篇 |
1977年 | 442篇 |
1976年 | 285篇 |
1975年 | 350篇 |
1974年 | 525篇 |
1973年 | 450篇 |
1972年 | 441篇 |
1971年 | 481篇 |
1970年 | 553篇 |
1969年 | 495篇 |
1968年 | 461篇 |
1967年 | 512篇 |
1966年 | 439篇 |
1965年 | 341篇 |
1958年 | 237篇 |
1957年 | 187篇 |
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
101.
102.
Summary An ectomycorrhiza, a specialized root organ, is the result of a complex interaction leading to a finely-tuned symbiosis between a plant and a compatible ectomycorrhizal fungus. Ultrastructural observations combined with cytochemical and biochemical studies reveal that structural and metabolic changes in the symbiont cells lead to the final phenotype of the active ectomycorrhiza. In the present review these changes are interpreted as changes in gene expression and discussed within the context of ectomycorrhiza development. Recent genetic data indicate that the continued vegetative growth of the ectomycorrhizal hyphae and the root tissues, and their ability to switch to symbiotic organ formation, is basically controlled by developmentally critical genes. The activity of these symbiotic genes during the differentiation of ectomycorrhizas is associated with extensive changes in the concentration of particular polypeptides and protein biosynthesis. The present state of knowledge about the developmental biology of ectomycorrhizas allows only speculation about the events during their development.Puisant mes forces aux sources des galaxies En buvant la sève des arbres M. Jonasz 相似文献
103.
42CrMo钢离子氮化—沉淀强化组织结构的研究 总被引:1,自引:0,他引:1
研究了42CrMo钢离子氮化—沉淀强化的组织结构,结果表明,离子氮化层的组成为ε、r'a_N、Fe_3C和少量Cr_2N、Mo_2N、CrMoN_x及a";经时效处理后,ε相和a_N相弥散析出Cr_2N、Mo_2N和a" 。325℃时效,CrMoN_x消失;500℃时效,Mo_2N大大减少。a"、Cr_2N和Mo_2N弥散共格析出,必将使氮化层强化。 相似文献
104.
Summary The in vitro absorption of calcium from the duodenum was significantly less in a group of rats fed on a wheat diet than in a group fed a wheat and Bengal gram (7030) diet. 相似文献
105.
A. Alkofahi J. K. Rupprecht Y. -M. Liu C. -J. Chang D. L. Smith J. L. McLaughlin 《Cellular and molecular life sciences : CMLS》1990,46(5):539-541
Summary Gigantecin (I), a novel tetrahydroxy-di-tetrahydrofuran fatty acid -lactone (acetogenin), was isolated from an ethanolic extract of the stem bark ofGoniothalamus giganteus Hook. f., Thomas (Annonaceae), by means of activity-directed fractionation (brine shrimp lethality test). This new compound is extremely cytotoxic to human tumor cells, inhibits crown gall tumors on potato discs, and is active in an assay designed to detect antimitotic agents (9 ASK). 相似文献
106.
It has been suggested that amino acids and other organic compounds found in carbonaceous meteorites formed by aqueous alteration in the meteorite parent bodies. Observations of carbonaceous material in interstellar grains and interplanetary dust particles indicate that condensed organic compounds may have been present in meteorite parent bodies at the time of aqueous alteration. One group of compounds thought to be representative of this carbonaceous material is the polycyclic aromatic hydrocarbons (PAHs). Recently it was proposed that PAHs condense on SiC grains in the molecular envelopes of carbon-rich red-giant stars, which would allow for their subsequent incorporation into meteorite parent bodies during accretion. This incorporation mechanism is supported by the identification of SiC grains in carbonaceous chondrites. The possibility therefore exists that PAHs, and/or other condensed organic compounds, represent the starting material for aqueous alteration which leads to the formation of amino acids and other water-soluble organic compounds. Here we present calculations of the distribution of aqueous organic compounds in metastable equilibrium with representative PAHs as functions of the fugacities of O2, CO2 and NH3. The results reported here for pyrene and fluoranthene, two PAHs with different structures but the same stoichiometry, differ greatly but indicate that the formation of amino and carboxylic acids is energetically favourable at probable parent-body alteration conditions. The actual reaction mechanisms involved could be revealed by consideration of isotope data for PAHs, amino acids, other organic compounds and carbonates in carbonaceous chondrites. 相似文献
107.
108.
G. Bergström A. -B. Wassgren O. Anderbrant J. Fägerhag H. Edlund E. Hedenström H. -E. Högberg C. Geri M. A. Auger M. Varama B. S. Hansson J. Löfqvist 《Cellular and molecular life sciences : CMLS》1995,51(4):370-380
The main component of the sex pheromone secretion of femaleDiprion pini L. (Hymenoptera: Diprionidae) from insects collected both in Finland and in France has been identified as athreo-3,7-dimethyl-2-tridecanol (8 ng per female) stereoisomer by GC-MS and synthesis. The secretion also contains lower and higher homologues in small amounts (1–4% of the main component). Combined gas chromatographic-electroantennographic detection showed activity in both natural and esterified extracts (acetates and propionates); the esters of the main component gave the largest responses. The acetates and propionates of the eight stereoisomers of 3,7-dimethyl-2-tridecanol were synthesized from enantiomerically highly enriched (>99% ee) building blocks. The stereochemistry of the main component was established to be (2S,3R,7R)-3,7-dimethyl-2-tridecanol by GC analysis of the natural material. It was purified by liquid chromatography prior to the GC analysis of both its pentafluorobenzoates and its isopropylcarbamates on a non-chiral polar column (ECD) and a chiral column (NPD), respectively. Field tests demonstrated that both the acetate and propionate of the main component (100 g of each applied on cotton roll dispensers) were active in attracting males, with or without the presence of several of the minor compounds. Experiments with smaller amounts of the acetate and the propionate (1 g in France and 50 g in Finland) demonstrated that the propionate was more active than the acetate, and that it also caught more males than a blend of the two compounds. 相似文献
109.
110.
研究了十六烷基三甲基溴化铵-正庚烷反胶束溶液和牛血红蛋白水溶液的相平衡,探讨了有机相助表面活性剂种类和浓度、水相pH值、水相离子种类和强度、表面活性剂浓度的影响,适宜条件下,牛血红蛋白的萃取率可达95%以上。对实验结果从表面活性剂极性头与蛋白质分子间的相互作用和反胶团包溶蛋白质时的空间阻碍作用上进行了解释。 相似文献