共查询到20条相似文献，搜索用时 328 毫秒
1.
Green AE 《Science (New York, N.Y.)》1970,169(3949):933-941
GOPEB based upon known mesons have been found which realistically describe a massive accumulation of the experimental nuclear data up to 400 Mev. The N-N potential according to these models consists primarily of weak residual central terms surviving the cancellation of large repulsive and attractive vector and scalar static components; relativistic interactions arising from the exchange of pseudoscalar, vector, and scalar mesons and dipole type terms arising from the rho meson. The major dynamic terms are direct analogs of magnetic interactions illustrated in Fig. 1. Allowance must be made for the effective dependence of the coupling constants upon spin and isospin states. The nucleons are distributed sources which give rise to nonsingular generalized Yukawa functions in N-N potentials. 相似文献
2.
《Science (New York, N.Y.)》1985,230(4724):376
In the article "String as a theory of everything" by M. Mitchell Waldrop (Research News, 20 Sept., p. 1251), the last line of the first column was inadvertantly omitted. The affected sentence should have read, "Not only were the strong interactions far more complex than predicted by field theory, but the particles that participated in the strong interactions-the hadrons, a group that includes protons, neutrons, pi mesons, and many others-seemed to be relatively large, extended objects as much as 1 Fermi across (10(-13) centimeter)." In the next-to-the-last line of the second full paragraph in column 2 on page 1252, 10(-19) should have been 10(+19). Finally, the words "quandary" (page 1251, column 1, paragraph 6, line 2) and "quark" (page 1252, column 3, last paragraph, line 18) were misspelled. 相似文献
3.
Variable rates of molecular evolution have been documented across the tree of life, but the cause of this observed variation within and among clades remains uncertain. In plants, it has been suggested that life history traits are correlated with the rate of molecular evolution, but previous studies have yielded conflicting results. Exceptionally large phylogenies of five major angiosperm clades demonstrate that rates of molecular evolution are consistently low in trees and shrubs, with relatively long generation times, as compared with related herbaceous plants, which generally have shorter generation times. Herbs show much higher rates of molecular change but also much higher variance in rates. Correlates of life history attributes have long been of interest to biologists, and our results demonstrate how changes in the rate of molecular evolution that are linked to life history traits can affect measurements of the tempo of evolution as well as our ability to identify and conserve biodiversity. 相似文献
4.
Gibbs JB 《Science (New York, N.Y.)》2000,287(5460):1969-1973
Cancer as a disease in the human population is becoming a larger health problem, and the medicines used as treatments have clear limitations. In the past 20 years, there has been a tremendous increase in our knowledge of the molecular mechanisms and pathophysiology of human cancer. Many of these mechanisms have been exploited as new targets for drug development in the hope that they will have greater antitumor activity with less toxicity to the patient than is seen with currently used medicines. The fruition of these efforts in the clinic is just now being realized with a few encouraging results. 相似文献
5.
Khaitovich P Hellmann I Enard W Nowick K Leinweber M Franz H Weiss G Lachmann M Pääbo S 《Science (New York, N.Y.)》2005,309(5742):1850-1854
The determination of the chimpanzee genome sequence provides a means to study both structural and functional aspects of the evolution of the human genome. Here we compare humans and chimpanzees with respect to differences in expression levels and protein-coding sequences for genes active in brain, heart, liver, kidney, and testis. We find that the patterns of differences in gene expression and gene sequences are markedly similar. In particular, there is a gradation of selective constraints among the tissues so that the brain shows the least differences between the species whereas liver shows the most. Furthermore, expression levels as well as amino acid sequences of genes active in more tissues have diverged less between the species than have genes active in fewer tissues. In general, these patterns are consistent with a model of neutral evolution with negative selection. However, for X-chromosomal genes expressed in testis, patterns suggestive of positive selection on sequence changes as well as expression changes are seen. Furthermore, although genes expressed in the brain have changed less than have genes expressed in other tissues, in agreement with previous work we find that genes active in brain have accumulated more changes on the human than on the chimpanzee lineage. 相似文献
6.
Polanyi JC 《Science (New York, N.Y.)》1987,236(4802):680-690
The objective in this work has been one which I have shared with the two other 1986 Nobel lecturers in chemistry, D. R. Herschbach and Y. T. Lee, as well as with a wide group of colleagues and co-workers who have been responsible for bringing this field to its current state. That state is summarized in the title; we now have some concepts relevant to the motions of atoms and molecules in simple reactions, and some examples of the application of these concepts. We are, however, richer in vocabulary than in literature. The great epics of reaction dynamics remain to be written. I shall confine myself to some simple stories. 相似文献
7.
Forms of memory failure 总被引：4，自引：0，他引：4
H Weingartner J Grafman W Boutelle W Kaye P R Martin 《Science (New York, N.Y.)》1983,221(4608):380-382
Memory may fail in a variety of ways. Patients with Korsakoff's syndrome demonstrate global memory deficits similar to those seen in patients with early progressive dementia. Korsakoff's patients, however, may recall rules and principles for organizing information and can gain access to their previously acquired knowledge (semantic memory), whereas recent memory may be grossly impaired. In contrast, dementia patients may have little access to previously acquired knowledge and therefore have great difficulty in organizing and encoding ongoing events. These contrasting forms of memory failure have implications for understanding the structure and mechanisms of memory and learning, particularly the relationship between episodic and semantic memory, as well as the development of therapeutic strategies for cognitive impairments. 相似文献
8.
黄蕾 《湖南农业大学学报(自然科学版)》2013,(2):83-87
《雪晴》集在第一人称叙述中表现出来的作者与家乡“人事”的距离和作者对尚武精神的反思，反映了沈从文适当调整他的回忆性乡土书写叙述策略的努力。在《雪晴》集中，叙述人的讲述、描写、思考在故事的当下进展中进行，却又将讲故事的本领和议论、说明性的因素适当调和起来，这自有作者对不能始终如一从容行文的理解，但如果从讲述效果的取得来看，此种做法可能一方面促进乡土书写所着重处理的诗意境界和外部现实的联系，一方面揭露血淋淋的乡土械斗的残酷性。 相似文献
9.
Is sperm cheap? Limited male fertility and female choice in the lemon tetra (pisces, characidae) 总被引：1，自引：0，他引：1
In the laboratory, fertilization rates achieved by male lemon tetras decline with spawning frequency. Even when the number of females is not limited, males can produce only four times as many offspring as females. Females show a preference for males that have not recently spawned as opposed to those that have. The cost of producing sufficient sperm to maximize fertilization rates may therefore reduce the intensity of sexual selection in this polygamous fish species. 相似文献
10.
Effects of Acid rain on freshwater ecosystems 总被引：1，自引：0，他引：1
Schindler DW 《Science (New York, N.Y.)》1988,239(4836):149-157
Acid-vulnerable areas are more numerous and widespread than believed 7 years ago. Lakes and streams in acid-vulnerable areas of northeastern North America have suffered substantial declines in acid-neutralizing capacity, the worst cases resulting in biological damage. Many invertebrates are very sensitive to acidification, with some disappearing at pH values as high as 6.0. However, the recent rate of acidification of lakes is slower than once predicted, in part the result of decreases in sulfur oxide emissions. A discussion of some of the processes that have contributed to the acidification of lakes as well as those that have protected acid-sensitive freshwaters is presented. The author is in the Department of Fisheries and Oceans, Freshwater Institute, 501 University Crescent, Winnipeg, Manitoba R3T 2N6, Canada. 相似文献
11.
Woodall JM 《Science (New York, N.Y.)》1980,208(4446):908-915
The III-V compounds and alloys have been studied for three decades. Until recently, these materials have been commercialized for only a few specialized optoelectronic devices and microwave devices. Advances in thin-film epitaxy techniques, such as liquid phase epitaxy and chemical vapor deposition, are now providing the ability to form good quality lattice-matched heterojunctions with III-V materials. New optoelectronic devices, such as room-temperature continuous-wave injection lasers, have already resulted. This newfound ability may also affect the field of highspeed integrated circuits. 相似文献
12.
13.
Osborn HF 《Science (New York, N.Y.)》1927,65(1690):481-488
Let us therefore conclude with consideration of the ancestry of man according to the modified concept of "dawn men," not "ape-men." In the first place, over an incredibly long period of time the Dawn Men have been tool-makers, of high adaptability and wonderful technique. We have then a biped, a being with a hand capable of grasping and controlling tools, a tool-maker with as fine a sense of touch as that of any of the present-day etchers, engravers and artists. In my opinion, the pro-man psychology, leaving out the evidence of anatomy and morphology, is certainly that of a Dawn Man and not of an "ape-man." I agree with my colleagues that man passed through an arboreal stage, but I believe that this stage did not progress so far as to carry man into a stage approaching that of the anthropoid apes. Dollo has stated the law of the irreversibility of evolution. The brachiating hand of the ape was used as a hook-apes do not grasp a branch with the fingers and thumb but hook the whole hand over the branch, as trapeze workers do to-day-and the thumb was therefore a grave danger. If man had gone through a prolonged period of brachiating in the branches of trees he would have lost his thumb. I agree to putting our arboreal ancestors back to Eocene time, but I predict that even in Upper Oligocene time we shall find pro-men, and if we find Oligocene pro-man-in Mongolia, for example-that he will have pro-human limbs, not proanthropoid ape limbs. 相似文献
14.
科技期刊的改革、发展和壮大,需要依靠大量的优秀编辑出版人才来支撑。人才的培养,是出版单位的基本建设任务之一。中国科技期刊编辑主要来源于主办单位的科技人员。科技编辑角色转换具有专业知识扎实、自学能力和接受能力较强、职业道德良好等优势;同时存在思想准备不足、语文功底不足、博识不足、市场意识不足等问题。传统的单一的由科技人员转任科技编辑的用人机制应有所创新,科技期刊才能顺应改革潮流,优化人才结构,提高核心竞争力,实现可持续发展。 相似文献
15.
作物化学杂交育种的实践与展望 总被引：11，自引：1，他引：10
三十多年来,化学杂交剂（CHA）的研究和利用极大地促进了作物化学杂交育种体系的形成和发展。本文重点综述了CHA在作物育种中的应用现状,系统地论述了作物化学杂交育种的原理方法。主要内容包括：（1）CHA的种类、最佳施用时期及其作用机理;（2）化学杂交优势（包括F#-1和F#-2代）利用的研究;（3）CHA在作物常规育种及群体改良中的作用;（4）核基因型雄性不育性的化学恢复的可能性和研究现状;（5）作物化学杂交育种技术研究与利用的前景。 相似文献
16.
Cliftonite, a polycrystalline aggregate of graphite with cubic morphology, is known in ten meteorites. Some workers have considered it to be a pseudomorph after diamond, and have used the proposed diamond ancestry as evidence of a meteoritic parent body of at least lunar dimensions.We have synthesized cliftonite in Fe-Ni-C alloys in vacuumn, as a product of decomposition of cohenite [(Fe, Ni)(3)C]. We therefore suggest that a high pressure origin is unnecessary for meteorites which contain cliftonite, and that these meteorites were formed at low pressures. This concluision is in agreement with other recent evidence. 相似文献
17.
概述杉木栽培生物学研究意义、研究主要成果,包括杉木种源试验、生理生态研究;杉木生长发育规律研究、杉木产区区划,立地评价、计算机辅助造林设计系统,林地肥力改良和营养诊断、人工林生物量及生产力的研究等,并附发表的系列研究论著目录30篇。 相似文献
18.
大豆抗食心虫品种选育初报 总被引：1，自引：1，他引：1
利用已筛选鉴定的高抗食心虫资源材料做基础抗源，与综合农艺性状优良的品种或品系配制杂交组合，从中选育出以公交85164-92-1、公交91144-31等为代表的一批抗食心虫新品系，并且已经升入省区域试验；同时完善了大豆抗食心虫育种的方法。 相似文献
19.
Hintenberger H Weber HW Voshage H Wänke H Begemann F Vilscek E Wlotzka F 《Science (New York, N.Y.)》1970,167(3918):543-545
The concentrations and isotopic abundances of the rare gases have been investigated in fines and three types of rocks. The results obtained from different grain-size fractions and from samples etched to different degrees with nitric and hydrofluoric acids demonstrate the strong concentrations of the solar-wind component in the surface layers of the grains. Exposure ages as well as gas retention ages have been determined in different types of Apollo 11 material. Hydrogen, nitrogen, and other gases have been analyzed by a high-resolution mass spectrometer. As compared with that in terrestrial water, deuterium is depleted by at least a factor of 3 in the investigated type C rocks. 相似文献
20.
中印丝路文化交流国际学术研讨会期间．来自海内外的专家、学者围绕中印丝路文化交流、中印丝路景观变迁、佛教文物艺术研究等方面的议题进行了深入的探讨．通过独到的视野发表了不少真知灼见，多元集纳，互动创新，让我们受益匪浅。会议上，专家学者们最新研究成果质量之高、对西域文化的兴趣之浓厚．都远超我们的预料。可以说，这次的学术研讨会，不仅增进了我校同中华佛学研究所、法鼓佛教学院等与会单位的友谊，也促进了大家学术经验和科研成果的传播和交流；不仅推动了关于法显、玄奘、义净等大师西行的文化交流研究，也拓展了我们在西域文化研究方面的领域和手段。 相似文献