全文获取类型
收费全文 | 1091篇 |
免费 | 4篇 |
国内免费 | 11篇 |
专业分类
系统科学 | 25篇 |
教育与普及 | 5篇 |
理论与方法论 | 13篇 |
现状及发展 | 114篇 |
研究方法 | 172篇 |
综合类 | 605篇 |
自然研究 | 172篇 |
出版年
2021年 | 7篇 |
2020年 | 4篇 |
2019年 | 7篇 |
2018年 | 7篇 |
2017年 | 11篇 |
2016年 | 15篇 |
2015年 | 5篇 |
2014年 | 12篇 |
2013年 | 11篇 |
2012年 | 94篇 |
2011年 | 260篇 |
2010年 | 33篇 |
2009年 | 3篇 |
2008年 | 79篇 |
2007年 | 91篇 |
2006年 | 58篇 |
2005年 | 75篇 |
2004年 | 73篇 |
2003年 | 67篇 |
2002年 | 70篇 |
2001年 | 4篇 |
2000年 | 8篇 |
1999年 | 11篇 |
1998年 | 4篇 |
1997年 | 2篇 |
1996年 | 2篇 |
1995年 | 4篇 |
1993年 | 3篇 |
1992年 | 7篇 |
1991年 | 4篇 |
1989年 | 5篇 |
1988年 | 2篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1980年 | 2篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1976年 | 2篇 |
1975年 | 4篇 |
1972年 | 3篇 |
1971年 | 3篇 |
1970年 | 3篇 |
1969年 | 4篇 |
1968年 | 6篇 |
1967年 | 6篇 |
1966年 | 4篇 |
1961年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有1106条查询结果,搜索用时 15 毫秒
71.
Ian James Kidd 《Annals of science》2013,70(1):101-104
We offer a novel historical-philosophical framework for discussing experimental practice which we call ‘Generating Experimental Knowledge’. It combines three different perspectives: experimental systems, concept formation, and the pivotal role of error. We then present an historical account of the invention of the Scanning Tunnelling Microscope (STM), or Raster-Tunnelmikroskop, and interpret it within the proposed framework. We show that at the outset of the STM project, Binnig and Rohrer—the inventors of the machine—filed two patent disclosures; the first is dated 22 December 1978 (Switzerland), and the second, two years later, 12 September 1980 (US). By studying closely these patent disclosures, the attempts to realize them, and the subsequent development of the machine, we present, within the framework of generating experimental knowledge, a new account of the invention of the STM. While the realization of the STM was still a long way off, the patent disclosures served as blueprints, marking the changes that had to be introduced on the way from the initial idea to its realization. 相似文献
72.
Vitart V Rudan I Hayward C Gray NK Floyd J Palmer CN Knott SA Kolcic I Polasek O Graessler J Wilson JF Marinaki A Riches PL Shu X Janicijevic B Smolej-Narancic N Gorgoni B Morgan J Campbell S Biloglav Z Barac-Lauc L Pericic M Klaric IM Zgaga L Skaric-Juric T Wild SH Richardson WA Hohenstein P Kimber CH Tenesa A Donnelly LA Fairbanks LD Aringer M McKeigue PM Ralston SH Morris AD Rudan P Hastie ND Campbell H Wright AF 《Nature genetics》2008,40(4):437-442
Uric acid is the end product of purine metabolism in humans and great apes, which have lost hepatic uricase activity, leading to uniquely high serum uric acid concentrations (200-500 microM) compared with other mammals (3-120 microM). About 70% of daily urate disposal occurs via the kidneys, and in 5-25% of the human population, impaired renal excretion leads to hyperuricemia. About 10% of people with hyperuricemia develop gout, an inflammatory arthritis that results from deposition of monosodium urate crystals in the joint. We have identified genetic variants within a transporter gene, SLC2A9, that explain 1.7-5.3% of the variance in serum uric acid concentrations, following a genome-wide association scan in a Croatian population sample. SLC2A9 variants were also associated with low fractional excretion of uric acid and/or gout in UK, Croatian and German population samples. SLC2A9 is a known fructose transporter, and we now show that it has strong uric acid transport activity in Xenopus laevis oocytes. 相似文献
73.
The TFT‐LCD (thin‐film transistor–liquid crystal display) industry is one of the key global industries with products that have high clock speed. In this research, the LCD monitor market is considered for an empirical study on hierarchical forecasting (HF). The proposed HF methodology consists of five steps. First, the three hierarchical levels of the LCD monitor market are identified. Second, several exogenously driven factors that significantly affect the demand for LCD monitors are identified at each level of product hierarchy. Third, the three forecasting techniques—regression analysis, transfer function, and simultaneous equations model—are combined to forecast future demand at each hierarchical level. Fourth, various forecasting approaches and disaggregating proportion methods are adopted to obtain consistent demand forecasts at each hierarchical level. Finally, the forecast errors with different forecasting approaches are assessed in order to determine the best forecasting level and the best forecasting approach. The findings show that the best forecast results can be obtained by using the middle‐out forecasting approach. These results could guide LCD manufacturers and brand owners on ways to forecast future market demands. Copyright 2008 John Wiley & Sons, Ltd. 相似文献
74.
The detection of sequence variation, for which DNA sequencing has emerged as the most sensitive and automated approach, forms the basis of all genetic analysis. Here we describe and illustrate an algorithm that accurately detects and genotypes SNPs from fluorescence-based sequence data. Because the algorithm focuses particularly on detecting SNPs through the identification of heterozygous individuals, it is especially well suited to the detection of SNPs in diploid samples obtained after DNA amplification. It is substantially more accurate than existing approaches and, notably, provides a useful quantitative measure of its confidence in each potential SNP detected and in each genotype called. Calls assigned the highest confidence are sufficiently reliable to remove the need for manual review in several contexts. For example, for sequence data from 47-90 individuals sequenced on both the forward and reverse strands, the highest-confidence calls from our algorithm detected 93% of all SNPs and 100% of high-frequency SNPs, with no false positive SNPs identified and 99.9% genotyping accuracy. This algorithm is implemented in a software package, PolyPhred version 5.0, which is freely available for academic use. 相似文献
75.
James D. Hansen 《西北部美国博物学家》2011,48(3)
Different insect trapping methods were compared weekly on recently burned and nearby unburned rangeland in the Great Basin of western Utah. Flight traps (or wing traps) collected the most specimens (total = 4,916 at burned site, 4,384 at unburned site) but represented the fewest species (no more than 11 spp./wk at either site); this method was not recommended because of difficulty in removing specimens from adhesive. Water traps amassed the next largest number of specimens ( ̄x ± SE/wk = 150.9 ± 75.3 at burned site, 158.0 ± 66.4 at unburned site) and the most species ( ̄x ± SE/wk = 21.6 ± 6.4 at burned site, 35. 1 ± 4.5 at unburned site). The weekly collections from pitfall traps between the sites were significantly different (P < .05) in number of specimens ( ̄x ± SE/wk = 8.1 ± 1.5 at burned site, 19.7 ± 4.8 at unburned site) and species ( ̄x ± SE/wk = 4.7 ± 0.6 at burned site, 9.5 ± 2.0 at unburned site). Malaise traps were the most convenient. Black Malaise traps collected more specimens ( ̄x ± SE/wk = 99.8 ± 19.4 at burned site, 90.6 ± 16.4 at unburned site) and species ( ̄x ± SE/wk = 22.9 ± 3.9 at burned site, 19.4 ± 6.5 at unburned site) than white malaise traps (specimens:  ̄x ± SE/wk = 72.1 ± 22.4 at burned site, 87.1 ± 31.2 at unburned site; species:  ̄x ± SE/ wk = 19.7 ± 6.6 at burned site, 16.4 ± 5.4 at unburned site), although this difference was not significant. 相似文献
76.
Two roadside surveys were conducted for dwarf mistletoes parasitizing lodgepole pine and Douglas-fir on the Sawtooth National Forest, Idaho. One survey used variable-radius plots located less than 150 m from roads. The 2nd survey used variable-radius plots established at 200-m intervals along 1600-m transects run perpendicular to the same roads. Estimates of the incidence (percentage of trees infected and percentage of plots infested) and severity (average dwarf mistletoe rating) for both lodgepole pine and Douglas-fir dwarf mistletoes were not significantly different for the 2 survey methods. These findings are further evidence that roadside-plot surveys and transect-plot surveys conducted away from roads provide similar estimates of the incidence of dwarf mistletoes for large forested areas. 相似文献
77.
James R. Palmieri 《西北部美国博物学家》2011,37(2)
Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;} Extensive feeding experiments were undertaken to determine if physiological host specificity was a major characteristic of Posthodiplostomum minimum. This involved the feeding of experimentally infected sunfish livers containing metacercariae of P. minimum to amphibian, reptilian, avian and mammalian hosts. Host - induced morphological variations of adult P. minimum were shown to be associated with the genus and class of host employed as well as with the inherent variability of P. minimum exclusive of host factors. Morphological characters such as body size and shape and tegumental surface and spines are indicated as undergoing morphological variation. Of all the experimental definitive hosts used, avian and mammalian are the most suitable for normal development of P. minimum. Amphibian and reptilian hosts demonstrate marked variation in the adult worm development usually manifested by pronounced size decrease. 相似文献
78.
Successful infection of white pine species by white pine blister rust (WPBR) is contingent upon environmental conditions that are favorable to the spread and development of Cronartium ribicola . Site and stand factors related to this process have been studied elsewhere within the distribution of the disease, but few studies have concentrated on the high-elevation white pine forests of southern Idaho and western Wyoming. We found that mean summer precipitation, average tree diameter, and elevation were the most important variables in 3 logistic regression models of WPBR presence and intensity. The models were tested on a randomly chosen portion of our data set. The model with 9 variables correctly predicted categories of low-, moderate-, and high-disease incidence in 79% of cases. The 2 models with fewer variables had lower predictive efficiencies but were more parsimonious and generally easy to measure. The ability to use easily measured or remotely sensed site and stand characteristics to predict WPBR spread or intensification could be an important asset to land managers who need to decide where to focus disease mitigation efforts and predict disease effects on water quality, wildlife habitat, recreation potential, and other land-management activities. 相似文献
79.
James A. Gessaman 《西北部美国博物学家》2011,44(1)
The weight of body water and fat-free dry weight of Uinta ground squirrels ( Spermophilus armatus ) is given as a function of body weight. Body fat is presented as a function of body weight and body length. An equation for calculating the total caloric content of Uinta ground squirrels is given. Fat index (weight of fat/fat-free dry weight) values of juveniles increased from 0.11 to 0.22 from 5 June to 28 July. In male yearlings and adults the fat index increased about 8 to 9 times in the latter half of June and early July. 相似文献
80.
D. Kendall Brown Anthony A. Echelle David L. Propst James E. Brooks William L. Fisher 《西北部美国博物学家》2011,61(2)
We used the computer program RAMAS to explore the sensitivity of an extinction-risk model for the Gila trout ( Oncorhynchus gilae ) to management of wildfires and number of populations of the species. The Gila trout is an endangered salmonid presently restricted to very few headwaters of the Gila and San Francisco river tributaries in southwestern New Mexico. Life history data for 10 extant populations were used to examine sensitivity of the species viability to changes in a variety of factors including population size, fecundity, life stage structure, number of populations, severity and probability of forest fires, and a regulated fishery. The probability and severity of forest fires and number of populations had the greatest effect on viability. Results indicate that successful conservation of Gila trout requires establishment of additional populations and reduction of the severity of forest fires through a program incorporating more frequent, but less severe, fires. 相似文献