全文获取类型
收费全文 | 4829篇 |
免费 | 30篇 |
国内免费 | 29篇 |
专业分类
系统科学 | 94篇 |
丛书文集 | 4篇 |
教育与普及 | 11篇 |
理论与方法论 | 66篇 |
现状及发展 | 1699篇 |
研究方法 | 451篇 |
综合类 | 2383篇 |
自然研究 | 180篇 |
出版年
2019年 | 21篇 |
2018年 | 60篇 |
2017年 | 45篇 |
2016年 | 72篇 |
2015年 | 57篇 |
2014年 | 43篇 |
2013年 | 74篇 |
2012年 | 258篇 |
2011年 | 451篇 |
2010年 | 106篇 |
2009年 | 33篇 |
2008年 | 272篇 |
2007年 | 282篇 |
2006年 | 278篇 |
2005年 | 316篇 |
2004年 | 306篇 |
2003年 | 254篇 |
2002年 | 294篇 |
2001年 | 52篇 |
2000年 | 50篇 |
1999年 | 38篇 |
1994年 | 20篇 |
1992年 | 26篇 |
1991年 | 27篇 |
1990年 | 22篇 |
1989年 | 21篇 |
1985年 | 24篇 |
1984年 | 36篇 |
1983年 | 26篇 |
1982年 | 31篇 |
1981年 | 34篇 |
1980年 | 23篇 |
1979年 | 53篇 |
1978年 | 45篇 |
1977年 | 55篇 |
1976年 | 42篇 |
1975年 | 33篇 |
1974年 | 46篇 |
1973年 | 58篇 |
1972年 | 55篇 |
1971年 | 57篇 |
1970年 | 77篇 |
1969年 | 64篇 |
1968年 | 90篇 |
1967年 | 81篇 |
1966年 | 58篇 |
1965年 | 51篇 |
1964年 | 47篇 |
1963年 | 24篇 |
1962年 | 24篇 |
排序方式: 共有4888条查询结果,搜索用时 0 毫秒
21.
Summary Daily evening injections of isoproterenol extended the period of high pineal N-acetyltransferase activity and retarded the growth of testes, seminal vesicles and prostate in rats treated neonatally with testosterone propionate.The authors are grateful to Mrs Marie Svobodová for her skillful technical assistance. 相似文献
22.
Zusammenfassung Verabreichung von Isoprotenerol, eines -adrenergen Agonisten, induziert einen signifikant höheren Anstieg der Aktivität der Serotonin-N-Acetyltransferase in der Epiphyse von spontan hypertensiven, im Vergleich mit normotensiven, Rattenmännchen. Derselbe Unterschied wurde auch bei Weibchen beobachtet, jedoch liegen die Aktivitätswerte bei spontan hypertensiven Männchen allgemein höher. Die durch Isoprotenerol erhöhte Induktion der Serotonin-N-Acetyltransferase erzeugt hinsichtlich der Catecholamine in den Pinealozyten von spontan hypertensiven Ratten eine veränderte Reaktivität der Achse -Rezeptor-Adenylzyklasesystem-Serotonin-N-Acetyl transferase. 相似文献
23.
M. Aušková K. Řežábek A. Černý M. Krajcrová M. Semonský 《Cellular and molecular life sciences : CMLS》1976,32(10):1310-1310
D-6-Methyl-8-[-isopropylaminoethyl] ergoline-I [VÚFB-10726], beginning from the dose of 0.05 mg/kg p.o., suppresses lactation through the inhibition of prolactin secretion in nursing rats. 相似文献
24.
25.
Alternative splicing and genome complexity. 总被引:26,自引:0,他引:26
Alternative splicing of mRNA allows many gene products with different functions to be produced from a single coding sequence. It has recently been proposed as a mechanism by which higher-order diversity is generated. Here we show, using large-scale expressed sequence tag (EST) analysis, that among seven different eukaryotes the amount of alternative splicing is comparable, with no large differences between humans and other animals. 相似文献
26.
Two experiments have been carried out to verify the effect of ethylene on the primary attraction ofPhloeotribus scarabaeoides under both laboratory and field conditions. The experiments were based on the enhancement of ethylene production by olive branches and trunks after the application of (2-chloroethyl)phosphonic acid. Under laboratory conditions, the beetles' response was strongly stimulated on the treated branches. Under field conditions, the treatment aided the location of the olive wood by the flying beetles, and the attack density was significantly greater in the treated wood at the end of the experiment. 相似文献
27.
28.
Eder Ramos Hernández Miguel Alberto Magaña Alejandro Carlos Oropeza Salín Julia María Lesher Gordillo Saúl Sánchez Soto 《Journal of Natural History》2018,52(5-6):255-268
Lethal yellowing (LY) of coconut palm (Cocos nucifera L., Arecaceae) is a disease of economic importance that is caused by the phytoplasma ‘Candidatus Phytoplasma palmae’ and is transmitted by the planthopper Haplaxius crudus (Van Duzee) (Hemiptera: Cixiidae). This study explores the weeds used by H. crudus nymphs and other Cixiidae in a coconut pathosystem in southern Mexico. Nymphs were collected directly from the root system of each weed by hand or with the help of a vacuum after carefully opening the culm. This study included 11 species of weeds: nine Poaceae [Brachiaria decumbens Stapf, B. humidicola (Rendle) Schweick, B. mutica (Forssk.) Stapf, Digitaria abyssinica (Hochst. Ex A. Rich.) Stapf, Eustachys petraea (Sw.) Desv., Leersia hexandra Sw., Panicum laxum Sw., P. maximum Jacq., Paspalum notatum Flüggé]; one Cyperaceae [Cyperus ligularis L.], and one Portulacaceae: [Portulaca pilosa L.]. Brachiaria mutica, E. petraea, B. humidicola, P. maximum were identified as the principal host species for H. crudus nymphs. Brachiaria decumbens, D. abyssinica, and C. ligularis are new host records for the nymphs of H. crudus. Additionally, it was found that H. crudus may coexist with its cogeners H. skarphion Kramer (Cixiidae) and H. caldwelli Kramer (Cixiidae), on B. mutica. On C. ligularis, H. crudus may coexist with Oecleus snowi Ball (Cixiidae) nymphs. These results suggest that in the coconut pathosystem there is a complex of multitrophic interactions that should be considered in integrated management of LY. 相似文献
29.
Caloric restriction extends lifespan in numerous species. In the budding yeast Saccharomyces cerevisiae this effect requires Sir2 (ref. 1), a member of the sirtuin family of NAD+-dependent deacetylases. Sirtuin activating compounds (STACs) can promote the survival of human cells and extend the replicative lifespan of yeast. Here we show that resveratrol and other STACs activate sirtuins from Caenorhabditis elegans and Drosophila melanogaster, and extend the lifespan of these animals without reducing fecundity. Lifespan extension is dependent on functional Sir2, and is not observed when nutrients are restricted. Together these data indicate that STACs slow metazoan ageing by mechanisms that may be related to caloric restriction. 相似文献
30.
HIV preferentially infects HIV-specific CD4+ T cells 总被引:34,自引:0,他引:34
Douek DC Brenchley JM Betts MR Ambrozak DR Hill BJ Okamoto Y Casazza JP Kuruppu J Kunstman K Wolinsky S Grossman Z Dybul M Oxenius A Price DA Connors M Koup RA 《Nature》2002,417(6884):95-98
HIV infection is associated with the progressive loss of CD4(+) T cells through their destruction or decreased production. A central, yet unresolved issue of HIV disease is the mechanism for this loss, and in particular whether HIV-specific CD4(+) T cells are preferentially affected. Here we show that HIV-specific memory CD4(+) T cells in infected individuals contain more HIV viral DNA than other memory CD4(+) T cells, at all stages of HIV disease. Additionally, following viral rebound during interruption of antiretroviral therapy, the frequency of HIV viral DNA in the HIV-specific pool of memory CD4(+) T cells increases to a greater extent than in memory CD4(+) T cells of other specificities. These findings show that HIV-specific CD4(+) T cells are preferentially infected by HIV in vivo. This provides a potential mechanism to explain the loss of HIV-specific CD4(+) T-cell responses, and consequently the loss of immunological control of HIV replication. Furthermore, the phenomenon of HIV specifically infecting the very cells that respond to it adds a cautionary note to the practice of structured therapy interruption. 相似文献