全文获取类型
收费全文 | 803篇 |
免费 | 80篇 |
国内免费 | 2篇 |
出版年
2022年 | 6篇 |
2021年 | 20篇 |
2020年 | 9篇 |
2019年 | 9篇 |
2018年 | 18篇 |
2017年 | 14篇 |
2016年 | 24篇 |
2015年 | 25篇 |
2014年 | 26篇 |
2013年 | 50篇 |
2012年 | 46篇 |
2011年 | 51篇 |
2010年 | 38篇 |
2009年 | 30篇 |
2008年 | 46篇 |
2007年 | 42篇 |
2006年 | 35篇 |
2005年 | 31篇 |
2004年 | 20篇 |
2003年 | 32篇 |
2002年 | 25篇 |
2001年 | 28篇 |
2000年 | 24篇 |
1999年 | 15篇 |
1998年 | 15篇 |
1997年 | 10篇 |
1996年 | 11篇 |
1995年 | 9篇 |
1994年 | 10篇 |
1993年 | 8篇 |
1992年 | 11篇 |
1991年 | 14篇 |
1990年 | 15篇 |
1989年 | 8篇 |
1988年 | 7篇 |
1987年 | 11篇 |
1986年 | 8篇 |
1985年 | 9篇 |
1984年 | 5篇 |
1983年 | 11篇 |
1982年 | 5篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1977年 | 6篇 |
1976年 | 5篇 |
1975年 | 5篇 |
1973年 | 3篇 |
1971年 | 3篇 |
1970年 | 3篇 |
1969年 | 3篇 |
排序方式: 共有885条查询结果,搜索用时 15 毫秒
831.
Danielle M. Wilson Emily A. Heaton Lisa A. Schulte Theodore P. Gunther Monika E. Shea Richard B. Hall William Landon Headlee Kenneth J. Moore Nicholas N. Boersma 《Bioenergy Research》2014,7(3):885-898
Bioenergy crops may help achieve multiple energy, economic, and environmental objectives in the US Midwest, but a portfolio of crops must first be developed and tested in comparison to the current standard, maize (Zea mays L.). The nascent, but long-term and ongoing Landscape Biomass Project in Boone County, Iowa, USA examines five cropping systems including continuous maize, a modified maize–soy [Glycine max (L.) Merr.] rotation, nurse cropped maize–switchgrass (Panicum virgatum L.), double-cropped triticale (Triticosecale?×?Whit.)/sorghum (Sorghum bicolor (L.) Moench), and intercropped triticale–aspen (Crandon [Populus alba?×?Populus grandidentata]) over an elevation gradient. Here, we report perennial establishment and crop productivity across five landscape positions during the first 4 years (2009–2012) of this experiment. Perennials (switchgrass and aspen) established successfully across the gradient with minimal effect of landscape position. Continuous maize had the highest biomass yields both within and over growing seasons, but they declined over time. In comparison, the diversified and perennial systems had lower, but stable or increasing yields over time, despite extreme weather conditions. Landscape position did not consistently influence biomass yield; its effect depended on year and cropping system. Cropping system productivity was generally consistent across the landscape within a given year with greater variability between years. Findings help explain why landscape is often disregarded in Midwestern crop management: it does not seem to substantially drive crop, and thus economic, performance in the short term. Conversely, related Landscape Biomass studies find landscape influences important ecosystem functions (e.g., soil carbon storage) and should be an integral management consideration. 相似文献
832.
Comparison of the adhesion properties of Deleya marina and the exopolysaccharide-defective mutant strain DMR. 总被引:1,自引:0,他引:1
C Shea J W Nunley J C Williamson H E Smith-Somerville 《Applied and environmental microbiology》1991,57(11):3107-3113
Deleya marina 219 (ATCC 25374) produces large quantities of an acidic exopolysaccharide and characteristically forms mucoid colonies and large aggregates of cells. The exopolysaccharide of wild-type D. marina cells appears to occur as both film and fibrils in electron micrographs. The organization of exopolymeric material was indicative of structural heterogeneity. A spontaneous rough-colony mutant defective in exopolysaccharide, D. marina DMR, has been isolated. The absence of exopolymer corresponds to a nonmucoid, nonaggregating, adhesion-altered phenotype. In microplate adhesion assays, wild-type cells grown at 19 or 25 degrees C attached to hydrophilic surfaces but not to a hydrophobic surface. In contrast, mutant cells exhibited a significantly reduced level of attachment to hydrophilic surfaces and increased adhesion to a hydrophobic surface. 相似文献
833.
Plants’ responses to climate change are complex. Even the same net performance changes may involve different responses of multiple life history traits. Here we show that two congeneric thistles, Carduus nutans and Carduus acanthoides, both grew taller under increased temperature, albeit following divergent response patterns. For C. nutans, warming advanced bolting more than flowering, leading to a longer growing period before flowering and ultimately taller plant height at the end of the growing season. Carduus acanthoides maintained the same length of growing period because of equally shifted events in the phenological sequence, however, post-flowering growth rate was increased, which also led to enhanced final plant height. As seeds from taller plants disperse farther, their responses imply that future invasion spread rates of these two species will increase. Similar consequences due to divergent responses in life history traits, as demonstrated in this study, suggest that considering only ultimate performance outcomes, and not the underlying processes generating such outcomes, is not enough to understand the impacts of climate change. 相似文献
834.
Lithic miniaturization was one of our Pleistocene ancestors' more pervasive stone tool production strategies and it marks a key difference between human and non‐human tool use. Frequently equated with “microlith” production, lithic miniaturization is a more complex, variable, and evolutionarily consequential phenomenon involving small backed tools, bladelets, small retouched tools, flakes, and small cores. In this review, we evaluate lithic miniaturization's various technological and functional elements. We examine archeological assumptions about why prehistoric stoneworkers engaged in processes of lithic miniaturization by making small stone tools, small elongated tools, and small retouched and backed tools. We point to functional differences that motivate different aspects of lithic miniaturization and several instances where archeological systematics have possibly led archeologists to false negative findings about lithic miniaturization. Finally, we suggest productive avenues by which archeologists can move closer to understanding the complex evolutionary forces driving variability in lithic miniaturization. 相似文献
835.
Groups of ewes received either saline or prostaglandin F2α (PGF2α) as an injection directly into the corpus luteum. Changes in circulating progesterone levels were measured as well as subsequent histological examination of the corpora lutea. Saline, or PGF2α given at the two lower doses (60 and 120 μg respectively), failed to suppress progesterone levels permanently, or to induce degenerative changes in the corpora lutea. Treatment with a higher dose of PGF2α (240 μg) was followed by a marked elevation in progesterone levels. These results are discussed in relation to reported effects of PGF2α arriving at the ovary via the arterial circulation. 相似文献
836.
Thermal denaturation profiles and gel mobility shift analysis of oligodeoxynucleotide triplexes. 总被引:7,自引:7,他引:0
下载免费PDF全文
![点击此处可从《Nucleic acids research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Oligodeoxypurine- and oligodeoxypyrimidine-containing strands were mixed under conditions conducive to the formation of triple stranded assemblies. The mixtures were characterized both by their UV absorbance change with increasing temperature and by their mobility in non-denaturing polyacrylamide gels. Duplexes 34 bp long containing 15 central purines on one strand and 15 complementary pyrimidines on the other strand yielded new melting transitions and showed different gel mobilities upon combination with oligopyrimidine 15-mers. The dependence of the thermal denaturation profiles on pH, salt concentration, GC content, strand orientation, base mismatches, and strand length was investigated. 相似文献
837.
Brian T. Shea 《Human Evolution》1986,1(2):97-110
Studies of sexual dimorphism have traditionally focused on the static differences in size and shape between adult males and
females. In this paper, I suggest that an investigation of the ontogenetic bases of sexual dimorphism can provide new insights
and information unobtainable from studies concerned only with adult endpoints. While growth is often viewed as simply the
developmental pathway utilized to attain final adult size and shape, we must recognize that it is the entire pattern of sex-differentiated
growth, and not merely the adult endpoints, which is adaptive and the target of natural selection. The importance of an ontogenetic
approach to the analysis of sexual dimorphism is also demonstrated by the fact that a given morphologicalresult (e.g., a certain degree of adult weight dimorphism) may be attained by very different developmentalprocesses, signalling selection for quite different factors. The need to analyze the ontogenetic bases of sexual dimorphism in size
and shape has recently been recognized by Jarman, in his study of dimorphism in large terrestrial herbivores. Here I combine
aspects of Jarman’s approach with those of allometry and heterochrony in an analysis of sexual dimorphism in selected anthropoid
primates. It is demonstrated that although all dimorphic anthropoids appear to be characterized by somebimaturism, the degree varies significantly. Marked weight dimorphism in certain species is primarily produced by an increased differentiation
of female and male growthrates, while in other species the primary change involves differences in thetime or duration of growth between the sexes. These variations are illustrated with anthropoid genera such asMiopithecus, Cercopithecus, Erythrocebus, Macaca, Papio, Pan, andGorilla. It is suggested that additional ontogenetic investigations of other anthropoids will help clarify some of the socioecological
bases of this variation in the ways of attaining an adult dimorphic state. This will contribute to our understanding of the
complex factors underlying and producing sexual dimorphism in primates and other mammals. 相似文献
838.
Shea N. Gardner Kevin McLoughlin Nicholas A. Be Jonathan Allen Scott C. Weaver Naomi Forrester Mathilde Guerbois Crystal Jaing 《PloS one》2016,11(4)
Venezuelan equine encephalitis virus (VEEV) is a mosquito-borne alphavirus that has caused large outbreaks of severe illness in both horses and humans. New approaches are needed to rapidly infer the origin of a newly discovered VEEV strain, estimate its equine amplification and resultant epidemic potential, and predict human virulence phenotype. We performed whole genome single nucleotide polymorphism (SNP) analysis of all available VEE antigenic complex genomes, verified that a SNP-based phylogeny accurately captured the features of a phylogenetic tree based on multiple sequence alignment, and developed a high resolution genome-wide SNP microarray. We used the microarray to analyze a broad panel of VEEV isolates, found excellent concordance between array- and sequence-based SNP calls, genotyped unsequenced isolates, and placed them on a phylogeny with sequenced genomes. The microarray successfully genotyped VEEV directly from tissue samples of an infected mouse, bypassing the need for viral isolation, culture and genomic sequencing. Finally, we identified genomic variants associated with serotypes and host species, revealing a complex relationship between genotype and phenotype. 相似文献
839.
840.