全文获取类型
收费全文 | 25854篇 |
免费 | 82篇 |
国内免费 | 141篇 |
专业分类
系统科学 | 107篇 |
丛书文集 | 197篇 |
教育与普及 | 81篇 |
理论与方法论 | 74篇 |
现状及发展 | 11970篇 |
研究方法 | 1032篇 |
综合类 | 12334篇 |
自然研究 | 282篇 |
出版年
2012年 | 356篇 |
2011年 | 583篇 |
2009年 | 158篇 |
2008年 | 409篇 |
2007年 | 474篇 |
2006年 | 454篇 |
2005年 | 457篇 |
2004年 | 475篇 |
2003年 | 428篇 |
2002年 | 461篇 |
2001年 | 771篇 |
2000年 | 785篇 |
1999年 | 522篇 |
1994年 | 344篇 |
1992年 | 432篇 |
1991年 | 358篇 |
1990年 | 433篇 |
1989年 | 391篇 |
1988年 | 408篇 |
1987年 | 427篇 |
1986年 | 417篇 |
1985年 | 522篇 |
1984年 | 397篇 |
1983年 | 388篇 |
1982年 | 362篇 |
1981年 | 352篇 |
1980年 | 371篇 |
1979年 | 845篇 |
1978年 | 693篇 |
1977年 | 598篇 |
1976年 | 549篇 |
1975年 | 596篇 |
1974年 | 685篇 |
1973年 | 597篇 |
1972年 | 627篇 |
1971年 | 805篇 |
1970年 | 907篇 |
1969年 | 724篇 |
1968年 | 785篇 |
1967年 | 702篇 |
1966年 | 608篇 |
1965年 | 442篇 |
1964年 | 217篇 |
1959年 | 235篇 |
1958年 | 461篇 |
1957年 | 337篇 |
1956年 | 285篇 |
1955年 | 270篇 |
1954年 | 266篇 |
1948年 | 221篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
1 Results The photosynthetic bacterial reaction center (RC) is a membrane protein complex.The RC is composed of three protein subunits and redox components such as bacteriochlorophylls, bacteriopheophytins,and quinones.The RC performs the photochemical electron transfer from the bacteriochlorophyll dimer through a series of electron donor and acceptor molecules to a secondary quinone,QB.QB accepts electrons from a primary quinone,QA,in two sequential electron transfer reactions.The second electron trans... 相似文献
52.
53.
54.
55.
The effects of alcohol consumption, cigarette smoking and age on sister chromatid exchange (SCE) frequency in human lymphocytes were assessed by means of multiple linear regression. An increase in SCE rates was associated with alcohol consumption (p = 0.0001), smoking (p = 0.0231), and, to a small extent (p = 0.057), age. These three confounding factors explain 48% of the inter-personal variation in SCE rates among subjects studied. 相似文献
56.
57.
M. Harada S. Yenbutra K. Tsuchiya S. Takada 《Cellular and molecular life sciences : CMLS》1985,41(12):1610-1611
Summary Karyotypes ofMyotis siligorensis, Myotis mystacinus, Pipistrellus pulveratus, Tylonycteris robustula, Miniopterus schreibersi fuliginosus, Hipposideros fulvus andAselliscus stoliczkanus from Thailand are investigated.Acknowledgment. The authors are very grateful to Dr N. Ratanawarabhan, Mrs S. Sittilert, P. Noonpakdee and S. Kuanchalern of Thailand Institute of Scientific and Technological Research for their valuable advice and assistance during our field survey in Thailand. 相似文献
58.
The way that we perceive and interact with objects depends on our previous experience with them. For example, a bird expert is more likely to recognize a bird as a sparrow, a sandpiper or a cockatiel than a non-expert. Neurons in the inferior temporal cortex have been shown to be important in the representation of visual objects; however, it is unknown which object features are represented and how these representations are affected by categorization training. Here we show that feature selectivity in the macaque inferior temporal cortex is shaped by categorization of objects on the basis of their visual features. Specifically, we recorded from single neurons while monkeys performed a categorization task with two sets of parametric stimuli. Each stimulus set consisted of four varying features, but only two of the four were important for the categorization task (diagnostic features). We found enhanced neuronal representation of the diagnostic features relative to the non-diagnostic ones. These findings demonstrate that stimulus features important for categorization are instantiated in the activity of single units (neurons) in the primate inferior temporal cortex. 相似文献
59.
Yeo S H K Ramesh 《厦门大学学报(自然科学版)》2002,(Z1)
On going trend of miniaturization in electronic rel at ed parts, which is an average of two times in every 5~7 years introduce grindin g challenges. In grinding process, the surface waviness control of thin parts is an ardent task due to its warpage, induced by the high specific grinding energy (2~10 J/mm 3). Therefore, coolant is often used to avoid thermal damage, obtai n better surface integrity and to prolong wheel life. However coolant, the incomp ressibility media introduce high forces at the gri... 相似文献
60.
Acquired immunity and epidemiology of Schistosoma haematobium 总被引:1,自引:0,他引:1
Human immune responses to schistosome infection have been characterized in detail. But there has been controversy over the relative importance of ecological factors (variation in exposure to infection) and immunological factors (acquired immunity) in determining the relationships between levels of infection and age typically found in areas where infection is endemic. Independent effects of exposure and age on the rates of reinfection with Schistosoma haematobium after chemotherapy have been demonstrated in the Gambia and Zimbabwe. This age effect could be the result of acquired immunity to infection. Indeed, allowing for variation in exposure and age, low rates of reinfection in the Gambia are correlated with high amounts of specific IgE antibodies--human IgE can kill S. mansoni schistosomulae in vitro. Further, animals can acquire immunologically mediated resistance to S. mansoni infection, although nonimmunological factors could also be involved. Acquisition of this immunity seems to be related to the cumulative effects of repeated infection and provides only partial protection. These characteristics are consistent with immuno-epidemiological data for both S. mansoni and S. haematobium infections of humans. We have now analysed age-prevalence data for human infection with S. haematobium, and find patterns of variation that are indeed consistent with the epidemiological effects of acquired immunity predicted by mathematical models. 相似文献