共查询到20条相似文献，搜索用时 31 毫秒
1.
Experience with influenza has shown that predictions of virus phenotype or fitness from nucleotide sequence are imperfect and that predicting the timing and course of evolution is extremely difficult. Such uncertainty means that the risk of experiments with mammalian-transmissible, possibly highly virulent influenza viruses remains high even if some aspects of their laboratory biology are reassuring; it also implies limitations on the ability of laboratory observations to guide interpretation of surveillance of strains in the field. Thus, we propose that future experiments with virulent pathogens whose accidental or deliberate release could lead to extensive spread in human populations should be limited by explicit risk-benefit considerations. 相似文献
2.
Segregation of form, color, movement, and depth: anatomy, physiology, and perception 总被引：63，自引：0，他引：63
Anatomical and physiological observations in monkeys indicate that the primate visual system consists of several separate and independent subdivisions that analyze different aspects of the same retinal image: cells in cortical visual areas 1 and 2 and higher visual areas are segregated into three interdigitating subdivisions that differ in their selectivity for color, stereopsis, movement, and orientation. The pathways selective for form and color seem to be derived mainly from the parvocellular geniculate subdivisions, the depth- and movement-selective components from the magnocellular. At lower levels, in the retina and in the geniculate, cells in these two subdivisions differ in their color selectivity, contrast sensitivity, temporal properties, and spatial resolution. These major differences in the properties of cells at lower levels in each of the subdivisions led to the prediction that different visual functions, such as color, depth, movement, and form perception, should exhibit corresponding differences. Human perceptual experiments are remarkably consistent with these predictions. Moreover, perceptual experiments can be designed to ask which subdivisions of the system are responsible for particular visual abilities, such as figure/ground discrimination or perception of depth from perspective or relative movement--functions that might be difficult to deduce from single-cell response properties. 相似文献
3.
开放实验在创新型人才培养中有着非常重要的作用,开放实验存在着定位模糊、技术难度大、管理困难等问题,从开放实验的定位问题入手,试从保证实验室的充分开放、开放实验项目的开发以及开放实验的管理等方面展开讨论,以构建一个完善的开放实验体系,加强创新型人才的培养。 相似文献
4.
Tanner WF 《Science (New York, N.Y.)》1963,139(3549):41-42
Spiral flow has been observed in meandering rivers, braided rivers, very shallow sea water, model experiments, and dust devils. Experimental work also produced standing spiral waves and spouts of water. Many observed spirals reversed direction from time to time. Geometry of the system, roughness, and turbulence are perhaps dominant in the control of spiral flow. 相似文献
5.
运用包括非迭代三激发电子相关耦合簇CCSD(T)方法和Dunning等人使用的相关一致aug-CC-PVxZ(x=D,T,Q,5)自洽基组对XF(X=H,D,T)分子进行了结构优化,优化值然后被插值到CBS极限.结果表明随着x的增大aug-CC-PVxZ系列基组优化的结构逼近实验值,计算频率与实验值最大差距不足1%,最小仅为0.099%,平衡间距最大只有0.8%的差距.通过计算还表明:在结构优化和频率分析时CCSD(T)显示出优秀的特性,但在研究与边界态或解离有关的问题时,CCSD(T)不能给出良好的结果,相反CCSD方法却能给出合理的解离极限.最后我们研究了XF(X=H,D,T)的分子势能函数和光谱常数,理论结果与已知实验值吻合的非常好.首次获得了XF(X=H,D,T)系列分子的ωeye,ωeze,β,γ,He,Fe值. 相似文献
6.
Wineland DJ 《Science (New York, N.Y.)》1984,226(4673):395-400
Ions that are stored in electromagnetic "traps" provid the basis for extremely high resolution spectroscopy. By using lasers, the kinetic energy of the ions can be cooled to millikelvin temperatures, thereby suppressing Doppler frequency shifts. Potential accuracies of frequency standards and clocks based on such experiments are anticipated to be better than one part in 10(15). 相似文献
7.
施肥方法对烤烟氮磷钾利用率及产量品质的影响 总被引：2，自引：0，他引：2
1991年在零陵、郴州烟区采用田间试验研究了肥料深施与浇施及基肥与追肥分配比例对烤烟氮、磷、钾利用率及产量品质的影响.结果表明,以氮、钾肥全作追肥浇施处理,肥料利用率,烟叶产量及产值最高,品质也较好;20%氮、钾作追肥深施,80%作追肥浇施处理次之;而将氮、钾肥及有机肥全部深施处理的效果较差. 相似文献
8.
Numerical experiments with an energy balance model of the earth's climate suggest an enhancement of albedo-temperature feedback caused by the presence of a high middle-latitude plateau in the zonally averaged Northern Hemisphere topography. The increased climate sensitivity arises from the increased rate of change of snow cover produced by the advance or retreat of the winter snow line over the north slope of this topographic feature. 相似文献
9.
Paine RT 《Science (New York, N.Y.)》1979,205(4407):685-687
Two components of natural disturbance, its local intensity and frequency beyond a threshold level, limit a marine benthic alga to wave-swept shores. Transplant experiments indicate that the limited distribution is not due to physiological restriction. Instead, it requires predictable annual disturbance of moderate intensity for local persistence. 相似文献
10.
中早熟高糖甘蔗新品种闽糖93-119的选育 总被引：1，自引：1，他引：0
闽糖93-119系福建省农科院甘蔗研究所从赣蔗65-542×新台糖1号杂交组合后代中选育成的中早熟丰产高糖甘蔗新品种.该品种高糖高产中熟,萌芽快,分蘖力强;13年20点次试验结果,闽糖93-119平均蔗茎产量112.94 t·hm-2,蔗糖产量17.19 t·hm-2,平均蔗糖分15.30%,分别比对照品种闽糖70-611、新台糖10 号、新台糖16号增产蔗茎35.8% 、52.2%、11.8%;增产糖47.7%、55.3%、15.1%;提高蔗糖分含量1.28个、0.38个、0.52个百分点.该品种适宜在中等或中等以上肥力旱坡地、水浇旱地蔗区种植. 相似文献
11.
介绍了化学实验课程在教学体系、教学内容、教学方法及化学实验课的考核等方面进行的改革和实践.通过一系列的教学改革,提高了整体师资水平,提高了教学质量,实现了资源共享.就今后实验教学进一步深化改革--开设多层次化学实验教学模式、建立化学多媒体教学体系以及对化学实验课教学效果的检查与评估提出了新的思路. 相似文献
12.
本文设计的大型、液压操纵、多参数在较大范围内可调的平面往复清选试验台,可对现有国内外同类清选机用先进手段进行模拟测试,并提出改进意见。并可求出并验证清选机各参数之间关系,为今后设计清选机提供理论和实验根据。本文主要说明该试验台的研究与设计。 相似文献
13.
氮磷钾不同用量配比对水稻的增产效果研究 总被引：9，自引：2，他引：7
通过2年4季的小区试验和49个大田对比试验,研究氮、磷、钾及有机肥不同用量配比对杂交水稻产量的影响。结果表明：在昭平县中低产田上进行水稻配方施肥的氮、磷、钾肥适宜用量为：每公顷施用纯N、P2O5和K20分别为135．0kg、54．0kg和81．0k,科学配比为N：P2O5：K2O=1：0．4：0．6。每公斤纯N、P2O5、K20可增产稻谷分别为4．71k、9．43kg和11．52妇,均达极显著水平;肥料的肥效排序为：K＞P＞N;采用“氮磷钾配合,增施磷钾肥,控制氮肥用量”的施肥策略,有利于水稻产量和经济效益的提高,配方施肥比习惯施肥增产491．0kg／hm^2,增幅7．4％,达极显著水平。生产上为了降低推广难度,水稻测土配方施肥完全可以简化为科学施用专用肥。 相似文献
14.
施用石灰对大白菜中Cd,Pb,Zn含量的影响* 总被引：1，自引：0，他引：1
通过盆栽试验,研究了在Cd,Pb,Zn污染的土壤上,石灰对大白菜地上部分重金属Cd,Pb,Zn含量的影响。结果表明：石灰能显著降低大白菜中重金属Cd,Pb,Zn含量。大白菜地上部分Cd,Pb,Zn的含量都随石灰用量升高而呈下降趋势。 相似文献
15.
施用石灰对大白菜中Cd,Pb,Zn含量的影响 总被引：8，自引：0，他引：8
通过盆栽试验,研究了在Cd,Pb,Zn污染的土壤上,石灰对大白菜地上部分重金属Cd,Pb,Zn含量的影响。结果表明:石灰能显著降低大白菜中重金属Cd,Pb,Zn含量。大白菜地上部分Cd,Pb,Zn的含量都随石灰用量升高而呈下降趋势。 相似文献
16.
Thayer CW 《Science (New York, N.Y.)》1985,228(4707):1527-1528
Unlike other shell-enclosed marine invertebrates, articulate brachiopods are repellent to predators. Fish, sea stars, snails, and crabs all prefer bivalve molluscs such as mussels to articulates. The mussels tested are mobile and out-compete immobile articulates when space is limited. In subtidal field experiments, mussels alone and predators alone each reduced the survivorship of articulates. However, adding mussels to articulates in the presence of ambient predation increased brachiopod survivorship by diverting predation from the brachiopods to the mussels. Competition from mussels (or mussel-like bivalves) is a plausible cause of the post-Paleozoic decline of articulates. 相似文献
17.
Luis?Guilherme?Teixeira?Crusiol Josirley?de?Fátima?Corrêa?Carvalho Rubson?Natal?Ribeiro?Sibaldelli Walkyria?Neiverth Alexandre?do?Rio Leonardo?Cesar?Ferreira Sergio?de?Oliveira?Procópio Liliane?Márcia?Mertz-Henning Alexandre?Lima?Nepomuceno Norman?Neumaier José?Renato?Bou?as?Farias
Although the information on the Normalized Difference Vegetation Index (NDVI) in plants under water deficit is often obtained from sensors attached to satellites, the increasing data acquisition with portable sensors has wide applicability in agricultural production because it is a fast, nondestructive method, and is less prone to interference problems. Thus, we carried out a set of experiments to investigate the influence of time, spatial plant arrangements, sampling size, height of the sensor and water regimes on NDVI readings in different soybean cultivars in greenhouse and field trials during the crop seasons 2011/12, 2012/13 and 2013/14. In experiments where plants were always evaluated under well-watered conditions, we observed that 9 a.m. was the most suitable time for NDVI readings regardless of the soybean cultivar, spatial arrangement or environment. Furthermore, there was no difference among NDVI readings in relation to the sampling size, regardless of the date or cultivar. We also observed that NDVI tended to decrease according to the higher height of the sensor in relation to the canopy top, with higher values tending to be at 0.8 m, but with no significant difference relative to 1.0 m—the height we adopted in our experiments. When different water regimes were induced under field conditions, NDVI readings measured at 9 a.m. by using a portable sensor were successful to differentiate soybean cultivars with contrasting responses to drought. 相似文献
18.
Compressibility, Phase Transitions, and Oxygen Migration in Zirconium Tungstate, ZrW2O8 总被引：1，自引：0，他引：1
JSO Evans Z Hu JD Jorgensen DN Argyriou S Short AW Sleight 《Science (New York, N.Y.)》1997,275(5296):61-65
In situ neutron diffraction experiments show that at pressures above 2 kilobars, cubic zirconium tungstate (ZrW2O8) undergoes a quenchable phase transition to an orthorhombic phase, the structure of which has been solved from powder diffraction data. This phase transition can be reversed by heating at 393 kelvin and 1 atmosphere and involves the migration of oxygen atoms in the lattice. The high-pressure phase shows negative thermal expansion from 20 to 300 kelvin. The relative thermal expansion and compressibilities of the cubic and orthorhombic forms can be explained in terms of the "cross-bracing" between polyhedra that occurs as a result of the phase transition. 相似文献
19.
Neuronal axons connect to multiple target cells through the formation of collateral branches, but the mechanisms that regulate this process are largely unknown. We show that BAM-2, a neurexin-related transmembrane protein, is required for development of VC motoneuron branches in the worm Caenorhabditis elegans. Expression analysis and ectopic expression experiments suggest that BAM-2 functions as a branch termination cue and reveal a mechanism for selective control of branches that sprout off a primary axon. 相似文献
20.
Recent experimental results are beginning to limit seriously the theories that can be considered to explain high-temperature superconductivity. The unmistakable observations of a Fermi surface, by several groups and methods, make it the focus of realistic theories of the metallic phases. Data from angle-resolved photoemission, positron annihilaton, and de Haas-van Alphen experiments are in agreement with band theory predictions, implying that the metallic phases cannot be pictured as doped insulators. The character of the low energy excitations ("quasiparticles"), which interact strongly with atomic motions, with magnetic fluctuations, and possibly with charge fluctuations, must be sorted out before the superconducting pairing mechanism can be given a microscopic basis. 相似文献