全文获取类型
收费全文 | 11499篇 |
免费 | 25篇 |
国内免费 | 28篇 |
专业分类
系统科学 | 72篇 |
丛书文集 | 28篇 |
教育与普及 | 40篇 |
理论与方法论 | 75篇 |
现状及发展 | 4314篇 |
研究方法 | 522篇 |
综合类 | 6232篇 |
自然研究 | 269篇 |
出版年
2013年 | 80篇 |
2012年 | 177篇 |
2011年 | 486篇 |
2010年 | 76篇 |
2008年 | 188篇 |
2007年 | 209篇 |
2006年 | 230篇 |
2005年 | 229篇 |
2004年 | 271篇 |
2003年 | 224篇 |
2002年 | 221篇 |
2001年 | 308篇 |
2000年 | 335篇 |
1999年 | 225篇 |
1992年 | 208篇 |
1991年 | 159篇 |
1990年 | 170篇 |
1989年 | 180篇 |
1988年 | 178篇 |
1987年 | 193篇 |
1986年 | 174篇 |
1985年 | 251篇 |
1984年 | 179篇 |
1983年 | 140篇 |
1982年 | 113篇 |
1981年 | 117篇 |
1980年 | 137篇 |
1979年 | 331篇 |
1978年 | 256篇 |
1977年 | 241篇 |
1976年 | 224篇 |
1975年 | 238篇 |
1974年 | 306篇 |
1973年 | 278篇 |
1972年 | 236篇 |
1971年 | 337篇 |
1970年 | 474篇 |
1969年 | 327篇 |
1968年 | 312篇 |
1967年 | 319篇 |
1966年 | 349篇 |
1965年 | 223篇 |
1964年 | 98篇 |
1959年 | 108篇 |
1958年 | 209篇 |
1957年 | 118篇 |
1956年 | 130篇 |
1955年 | 101篇 |
1954年 | 84篇 |
1948年 | 127篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
M. F. W. Festing 《Cellular and molecular life sciences : CMLS》1991,47(10):990-998
Animals have evolved a detoxication system to enable them to survive in a hostile chemical environment in which foods contain many non-nutrient chemicals. WorkEntry&sortOrder=relevance' class='reference-link webtrekk-track' gaCategory="internal-link" gaLabel="Detoxication" gaAction="reference-keyword">Detoxication depends on enzymes which are often genetically polymorphic. As a result, inter-individual variation is common, and in humans several Mendelian loci have been identified. However, most variation in response is probably due to the action of several genes. Genetic variation in response to the neurotoxin WorkEntry&sortOrder=relevance' class='reference-link webtrekk-track' gaCategory="internal-link" gaLabel="MPTP" gaAction="reference-keyword">MPTP and to chemically and physically-induced seizures is reviewed. In the former case, differences between pigmented and white mouse strains have been noted which are consistent with the hypothesis that humans are more sensitive than mice or rats because of the presence of melanin in human brains. However, variation in sensitivity probably also depends on other genes. In the case of audiogenic seizures, a single locus has been identified and mapped, but its relationship with seizures induced by other agents is not clear. Genetic variation in response to alcohol is also discussed. The failure of most toxicologists to consider genetic variation as a potentially confounding variable, and as a powerful research tool, is discussed critically in relation to non-repeatability of research on the neurotoxic effects of lead, and in relation to the genetic variation in WorkEntry&sortOrder=relevance' class='reference-link webtrekk-track' gaCategory="internal-link" gaLabel="MPTP" gaAction="reference-keyword">MPTP, seizures, and alcohol response already noted. It seems clear that genetic methods provide a powerful research tool which is largely being ignored by toxicologists. 相似文献
72.
73.
Summary We review here the present knowledge about the pathway of import and assembly of porin into mitochondria and compare it to those of other mitochondrial proteins. Porin, like all outer mitochondrial membrane proteins studied so far is made as a precursor without a cleavble signal sequence; thus targeting information must reside in the mature sequence. At least part of this information appears to be located at the amino-terminal end of the molecule. Transport into mitochondria can occur post-translationally. In a first step, the porin precursor is specifically recognized on the mitochondrial surface by a protease sensitive receptor. In a second step, porin precursor inserts partially into the outer membrane. This step is mediated by a component of the import machinery common to the import pathways of precursor proteins destined for other mitochondrial subcompartments. Finally, porin is assembled to produce the functional oligomeric form of an integral membrane protein wich is characterized by its extreme protease resistance. 相似文献
74.
Barry W. Evans 《Systemic Practice and Action Research》1994,7(6):671-685
This paper develops the argument that in organizational redesign it is desirable to utilize user-designers as the architects for the redesign. A methodology is developed which emphasizes the use of ideal systems models by user-designers to plan for the organization's future. The methodology involves the creation of an ideal system model, then an analysis of the present system through the creation of root definitions and process models of the existing system. Then root definitions and process models for the idealized system are created. Finally, the two are compared and desirable changes are generated which bring the existing system toward the idealized. 相似文献
75.
In multivariate discrimination of several normal populations, the optimal classification procedure is based on quadratic discriminant functions. We compare expected error rates of the quadratic classification procedure if the covariance matrices are estimated under the following four models: (i) arbitrary covariance matrices, (ii) common principal components, (iii) proportional covariance matrices, and (iv) identical covariance matrices. Using Monte Carlo simulation to estimate expected error rates, we study the performance of the four discrimination procedures for five different parameter setups corresponding to standard situations that have been used in the literature. The procedures are examined for sample sizes ranging from 10 to 60, and for two to four groups. Our results quantify the extent to which a parsimonious method reduces error rates, and demonstrate that choosing a simple method of discrimination is often beneficial even if the underlying model assumptions are wrong.The authors wish to thank the editor and three referees for their helpful comments on the first draft of this article. M. J. Schmid supported by grants no. 2.724-0.85 and 2.038-0.86 of the Swiss National Science Foundation. 相似文献
76.
Because of possible variation in venom composition, an understanding of venomous snake systematics is of great importance for the optimization of antivenom treatment of snakebite patients. Intraspecific variation in the morphology of many venomous snakes complicates the definition and indentification of some species when allopatric populations are involved. Selectively neutral or near-neutral mtDNA sequences can reveal evolutionary relationships obscured by ecogenetically-caused morphological variation. We use comparative sequencing of the cytochrome oxidase subunit 1 gene to reveal the existence of a widespread, cryptic species of spiting cobra from southeast Asia. This species,Naja siamensis, is widely sympatric with other Asiatic cobra species. This may be of considerable medical significance, and calls for further research into venom composition in Asiatic cobras. 相似文献
77.
A comparison of sterol utilization by 3 stored-products insects revealed very different capabilities. The fluor beetle,Tribolium castaneum dealkylates and converts dietary sitosterol to about equal amounts of cholesterol (43.7%) and 7-dehydrocholeterol (39.8%), whereas another flour beetle,Tenebrio molitor, produces considerably less 7-dehydrocholesterol (16.8%) and relatively more cholesterol (66.7%) from sitosterol. The lepidopteran,Plodia interpunctella, utilized dietary sterol very similar to plant-feeding lepidoptera, producing primarily cholesterol (86.5%) from sitosterol. 相似文献
78.
采用雪崩热电子注入技术和高频C-V准静态C-V特性测试,研究了新型快速热氮化的SiOxNy介质膜界面陷阱的特征,侧重于研究界面陷阱的特性与分布。结果表明:这种SiOxNy薄膜禁带中央界面陷阱密度随氮化时间的分布变化呈现”回转效应“,且存在着不同类型、密度悬殊很大的电子陷阱、指出雪崩热电子注入过程中在Si/SiOxNy界面上产生两种性质不同的快界面态陷阱;给出了这两种界面态陷阱密度在禁带中能量的分布 相似文献
79.
在自动化制造中,线监控刀具状况以保护刀具与加工工件显得越来越重要.近年来,许多研究者在此领域进行了广泛的研究,然而由于加工过程的不确定性,现有的刀具监控系统的可靠性还有待提高.本文提出了一种基于小波包分析与模糊神经网络的自适应刀具监控系统,该系统利用小波包分析方法将加工过程振动信号分解为不同的频率段,并在此基础上,建立了自适应特征提取方法,为模糊神经网络提供最优的特征输入,然后模糊神经网络据此进行决策,分析刀具磨损状况.实验结果表明:该系统模糊神经网络能有效通过学习人类模糊知识和在线学习样本来提高刀具监控精度. 相似文献
80.
A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial Pacific Ocean 总被引:70,自引:0,他引:70
Coale KH Johnson KS Fitzwater SE Gordon RM Tanner S Chavez FP Ferioli L Sakamoto C Rogers P Millero F Steinberg P Nightingale P Cooper D Cochlan WP Landry MR Constantinou J Rollwagen G Trasvina A Kudela R 《Nature》1996,383(6600):495-501
The seeding of an expanse of surface waters in the equatorial Pacific Ocean with low concentrations of dissolved iron triggered a massive phytoplankton bloom which consumed large quantities of carbon dioxide and nitrate that these microscopic plants cannot fully utilize under natural conditions. These and other observations provide unequivocal support for the hypothesis that phytoplankton growth in this oceanic region is limited by iron bioavailability. 相似文献