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1.
A comet of the size recently postulated by H. C. Urey would leave a large crater. It is shown, from aerodynamic theory, from observations of distribution around terrestrial impact craters, and from experimental nuclear explosions, that the observed distribution of tektites cannot be the result of impact on the earth, whether cometary or meteoritic. It is further shown, from aerodynamic theory, from observation of a meteor shower, and from study of the breakup of artificial satellites, that the distribution of tektites can be accounted for as a result of fusion stripping of a satellite, as originally suggested by Suess. 相似文献
2.
Detritivores have been thought to derive nutrition from microbes associated with detrital particles and not from the nonliving plant substrate. The polychaete Capitella capitata, however, was found to derive a major portion of its nitrogen from the plant substrate when it feeds on detritus derived from seaweed. Microbes play a role as a nitrogen source for this detritivore when it feeds on detritus derived from marsh grass. 相似文献
3.
In suitable cell-free media it has been possible to cultivate pleuropneumonia-like microorganisms from the following materials, first, from chorioallantoic membranes in which lesions were apparently induced by exudates from patients with rheumatic fever; second, from pneumonic lungs of mice inoculated with similar exudates or with suspensions of the abovementioned abnormal membranes; and third, directly from the arthritic exudate of a patient with rheumatic fever, and also from an erythema nodosum nodule excised from a patient with this same disease. With three different subcultures from joint fluid, iritis has been induced in rabbits; and following intranasal inoculation with the same cultures there has developed in mice a pneumonia similar to that found in mice inoculated with rheumatic exudates and with suspensions of chorioallantoic membranes infected with rheumatic exudates. Therefore it seems probable that in all instances the pathogenic agent was derived from similar sources, viz., patients with rheumatic fever. Further work will be required to demonstrate the etiologic significance of these pathogenic agents in rheumatic fever. 相似文献
4.
Cocoon surrounding desert-dwelling frogs 总被引：1，自引：0，他引：1
A cocoon formed from a single cell layer of shed stratum corneum may reduce water loss from the skin of desert-dwelling frogs while these aestivate in soil-filled burrows. In several Australian examples, the cocoon is a single layer of cells, and thus differs from the multilayered structure obtained from an American species, Scaphiopus couchi. 相似文献
5.
对3份水稻多胚材料的胚囊观察表明,水稻多胚来源有5条途径:(1)合子胚原体裂生形成2个胚;(2)合子与助细胞受精同时发育成胚;(3)反足细胞多次分裂形成类似胚状的“拟胚”;(4)珠心细胞进入胚囊发育形成胚原体;(5)胚乳发育形成胚状体.其中第(1),(2)条途径能形成多胚苗.由反足细胞形成的‘拟胚”一般只能发育到几十个细胞就停止了.珠心细胞形成的胚原体,由于发育时间较迟,只有合子胚败育后,才能得到进一步的发育. 相似文献
6.
1986～1988年,在校内网室采用盆栽进行试验。初步探讨了氮磷钾对冬小麦根系营养效应;明确了冬小麦一生中,根内养分浓度因小麦不同生长时期以及施氮磷钾肥种类不同而异,根内氮和钾养分浓度变化为:播种～返青和抽穗～成熟这两段含量低;拔节～抽穗含量高。磷是前期明显地高于中期和后期。根对氮磷钾营养的吸收累积表现为播种～返青为平缓上升阶段;返青～抽穗为急剧上升阶段:抽穗～成熟为迅速下降阶段.根重的变化也表现为返青前根重较轻,返青～抽穗根重迅速增加至根重绝对量的最高点,后期根重又下降,全生育期根重变化呈抛物线型.显著性检验表明:氮磷处理对根重、养分浓度、养分吸收积累量及其强度影响最为明显. 相似文献
7.
Coastal marine sediments are shown to be a net source of nitrous oxide. The rates of nitrous oxide flux from sediments in Narragansett Bay, Rhode Island, ranged from 20 to more than 900 nanomoles per square meter per hour. Sediments from a eutrophic area had higher rates of net nitrous oxide production than sediments from relatively unpolluted sites. The benthic, nitrous oxide source exceeds the nitrous oxide source to the bay from sewage treatment plant effluent. 相似文献
8.
吉林省辽河流域农业面源污染特征及趋势研究 总被引：1，自引：1，他引：0
吉林省辽河流域农业面源污染现已十分严重,通过对其近十年的统计数据进行研究,结果表明:流域内农业面源污染中,COD(化学需氧量)主要来源于畜禽养殖和农村生活污染;TN(总氮)和TP(总磷)主要来源于畜禽养殖和农田化肥;近十年中COD流失量由34 093.2 t/a增至53 414.3 t/a,增长56.7%;TN流失量由30 095.2 t/a增至40 738.7t/a,增长35.4%;TP流失量由2 504.3t/a增至4 606.0t/a,增长83.9%。 相似文献
9.
云南烟区津引烤烟品种KRK26叶绿素在烘烤中的降解 总被引：1，自引：0，他引：1
基于分光光度法研究了云南5烟区津巴布韦烤烟品种KRK26成熟中部叶片叶绿素（Chl）在三段式烘烤中的相对降解速率和总降解量以及烘烤后总Chl的残余量。结果表明：不同烟区KRK26 Chl a,Chl b和总Chl在烘烤中的降解速率差异、Chl a和总Chl在烘烤不同阶段的降解速率差异均未达到显著水平,但Chl b在烘烤不同阶段的降解速率差异达到显著水平。保山烟区KRK26所有Chl的降解速率最高峰均在变黄前期;玉溪和临沧KRK26 Chl a的降解速率最高峰在变黄后期,文山的在定色前期,景谷的2个峰在变黄后期和定色后期;玉溪和文山KRK26 Chl b的降解速率最高峰均在变黄前期,临沧的在变黄后期,景谷的2个峰在变黄后期和和干筋后期;玉溪、景谷、临沧和文山KRK26总Chl的降解速率最高峰都在变黄后期。不同烟区KRK26各种Chl的总降解量不同,其差异均达到极显著水平。烘烤后,所有烟区KRK26总Chl残留量均符合优质烟叶的颜色标准。所以,云南不同烟区KRK26叶片基于Chl降解的产香潜力不同。本研究结果可为云南烟区KRK26香气品质的评价与调控、适宜种植区域的选择提供参考。 相似文献
10.
Visibility and soiling: a comparison of the effects of leaded and unleaded gasolines 总被引：1，自引：0，他引：1
J M Pierrard 《Science (New York, N.Y.)》1972,175(21):516-518
The emissions from a fleet of late-model cars fueled with commercial, high-aromatic, unleaded gasoline caused nearly twice as much atmospheric light extinction as those from a matched fleet fueled with commercial, low-aromatic, leaded gasoline, when both were driven according to a consumer operating cycle in an idle traffic tunnel. The increased extinction and greater soiling potential result mainly from greater light absorption by the air-suspended particles from the unleaded fleet. 相似文献
11.
Navigation of homing pigeons was investigated by tracking their homeward flights from a light airplane. Released on successive days from a single training point 35 miles (56 kilometers) from home, individual pigeons, each carrying a transmitter, were repeatedly tracked back to theirloft. No two tracks covered the same ground for even short distances, yet all tracks were within 10 miles of a straight line. Results from further releases north and south of the training point suggest that pigeons often use three methods in sequence to find home: compass orientation, bi-coordinate navigation, and orientation by familiar landmarks. 相似文献
12.
漳台园艺合作趋势展望 总被引：1，自引：0，他引：1
回顾了漳台园艺合作发展的过程,并指出合作交流呈现由低级向高级、由单项向配套、由点到面、由短期向长期发展的趋势。建议利用漳州优势,把闽南花卉走廊建成漳台园艺合作的新品种、新技术示范中心。 相似文献
13.
A phase transition from a classical thermal mixed state to a quantum-mechanical pure state of exciton polaritons is observed in a GaAs multiple quantum-well microcavity from the decrease of the second-order coherence function. Supporting evidence is obtained from the observation of a nonlinear threshold behavior in the pump-intensity dependence of the emission, a polariton-like dispersion relation above threshold, and a decrease of the relaxation time into the lower polariton state. The condensation of microcavity exciton polaritons is confirmed. 相似文献
14.
Nicole D. Peterson 《Agriculture and Human Values》2011,28(1):99-107
Participatory processes are often intended to encourage inclusion of multiple perspectives in defining management means and
goals. However, ideas about the legitimacy of certain uses and users of the resources can often lead to exclusion from participation.
In this way, participation can be transformed from a process of inclusion of various resource users to one of exclusion. Using
a case study from a marine protected area in Loreto, Baja California Sur, Mexico, and drawing on work in deliberative democracy,
I present a typology of how individuals and groups can be excluded from participation. External exclusion includes non-invitation
and other means for keeping participation from occurring. Internal exclusion refers to exclusionary events during participatory
meetings. This analysis suggests that participation needs to be recognized as a valuable but easily manipulated tool in the
design of projects like natural resource management. 相似文献
15.
Kiviat FE 《Science (New York, N.Y.)》1976,194(4266):719-720
An electrochemical concentration cell is proposed as a means of extracting the energy released from the mixing of freshwater with seawater. In order to obtain the maximum power from such a cell, a small amount of seawater must be added to the freshwater prior to its introduction into the cell in order to lower the internal resistance of the cell. The work available from the electrochemical concentration cell is of the same order of magnitude as the work derived from the use of an osmotic pump to extract energy from seawater. Both of these saline water techniques should be considered when unconventional, long-range power sources are evaluated. 相似文献
16.
Meibom A Desch SJ Krot AN Cuzzi JN Petaev MI Wilson L Keil K 《Science (New York, N.Y.)》2000,288(5467):839-841
Chemical zoning patterns in some iron, nickel metal grains from CH carbonaceous chondrites imply formation at temperatures from 1370 to 1270 kelvin by condensation from a solar nebular gas cooling at a rate of approximately 0.2 kelvin per hour. This cooling rate requires a large-scale thermal event in the nebula, in contrast to the localized, transient heating events inferred for chondrule formation. In our model, mass accretion through the protoplanetary disk caused large-scale evaporation of precursor dust near its midplane inside of a few astronomical units. Gas convectively moved from the midplane to cooler regions above it, and the metal grains condensed in these parcels of rising gas. 相似文献
17.
土壤灭菌对连作大豆生长发育的影响 总被引：16，自引：3，他引：16
用连作大豆茬土壤和玉米茬土壤灭菌后栽培大豆表明，灭菌可基本消除大豆连作土壤的不良影响，表现出大豆生长发育良好，产量提高；而且大豆连作土壤还略优于玉米土壤，据此认为前作大豆所形成的土壤生物环境乃为大豆连作障碍主要因素之一． 相似文献
18.
19.
Volcanic eruptions are episodic despite being supplied by melt at a nearly constant rate. We used histories of magma efflux and surface deformation to geodetically image magma transfer within the deep crustal plumbing of the Soufrière Hills volcano on Montserrat, West Indies. For three cycles of effusion followed by discrete pauses, supply of the system from the deep crust and mantle was continuous. During periods of reinitiated high surface efflux, magma rose quickly and synchronously from a deflating mid-crustal reservoir (at about 12 kilometers) augmented from depth. During repose, the lower reservoir refilled from the deep supply, with only minor discharge transiting the upper chamber to surface. These observations are consistent with a model involving the continuous supply of magma from the deep crust and mantle into a voluminous and compliant mid-crustal reservoir, episodically valved below a shallow reservoir (at about 6 kilometers). 相似文献
20.
Reverse transcriptases of primate viruses as immunological markers 总被引：20，自引：0，他引：20