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1.
The disintegration of the Classic Maya civilization in the Yucatán Peninsula and Central America was a complex process that occurred over an approximately 200-year interval and involved a catastrophic depopulation of the region. Although it is well established that the civilization collapse coincided with widespread episodes of drought, their nature and severity remain enigmatic. We present a quantitative analysis that offers a coherent interpretation of four of the most detailed paleoclimate records of the event. We conclude that the droughts occurring during the disintegration of the Maya civilization represented up to a 40% reduction in annual precipitation, probably due to a reduction in summer season tropical storm frequency and intensity. 相似文献
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van Breugel M Hirono M Andreeva A Yanagisawa HA Yamaguchi S Nakazawa Y Morgner N Petrovich M Ebong IO Robinson CV Johnson CM Veprintsev D Zuber B 《Science (New York, N.Y.)》2011,331(6021):1196-1199
Centrioles are cylindrical, ninefold symmetrical structures with peripheral triplet microtubules strictly required to template cilia and flagella. The highly conserved protein SAS-6 constitutes the center of the cartwheel assembly that scaffolds centrioles early in their biogenesis. We determined the x-ray structure of the amino-terminal domain of SAS-6 from zebrafish, and we show that recombinant SAS-6 self-associates in vitro into assemblies that resemble cartwheel centers. Point mutations are consistent with the notion that centriole formation in vivo depends on the interactions that define the self-assemblies observed here. Thus, these interactions are probably essential to the structural organization of cartwheel centers. 相似文献
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The tropical lowlands of southern Mexico and Central America are composed of a complex mosaic of landscapes that presented a variety of possibilities and challenges to the subsistence practices of the ancient Maya. The Yalahau Regional Human Ecology Project has been investigating ancient Maya agricultural practices and use of resources in a unique fresh-water wetland zone located in the northeast corner of the Yucatán Peninsula. While containing only a sparse population today, the Yalahau region once supported numerous Maya communities and civic-ceremonial centers, particularly during the Late Preclassic and Early Classic periods, between approximately 100 B.C. and A.D. 450. Our investigations have developed evidence that the ancient Maya manipulated and cultivated the wetland landscape of the region, used soil or algae from the wetlands to enrich upland garden plots, and cultivated trees within their communities. We suggest that the study of ancient Maya agricultural practices can contribute to sustainable development of the area today and in the future. 相似文献
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Economically important trees such as ramón have been shown to have a high density in the civic-ceremonial core zone of ancient Maya ruins. The distribution of such trees is probably the result of their requirements for growth and reproduction, which are optimal on the ruins, and not because they are the descendants of trees planted by the Maya aristocracy. 相似文献
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Prehispanic water management in the Maya Lowlands emphasized collection and storage rather than the canalization and diversion accentuated in highland Mexico. Reexamination of site maps of the ancient Maya city of Tikal, Guatemala, has revealed an important, overlooked factor in Maya centralization and urban settlement organization. In a geographical zone affected by an extended dry season and away from permanent water sources, large, well-planned reservoirs provided resource control as well as political leverage. 相似文献
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小麦幼胚愈伤组织的细胞类型,功能与内生胚状体 总被引：1，自引：0，他引：1
愈伤组织内的细胞表现了结构与功能的统一,整块愈伤由不同层次的细胞组成。分生细胞中心的活动使愈伤组织细胞增多,管状分子成为分生细胞中心与培养基之间的输导通道,分生细胞中心与培养基、或几个邻近分生细胞中心以大薄壁细胞相连接。内生胚状体位于愈伤组织内部空腔内,由壁木质化的细胞与母体隔开。 相似文献
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The ground pattems found in Pulitrouser Swamp, northern Belize, are vestiges of raised and channelized fields, types of wetland cultivation used by the ancient Maya. This form of hydraulic cultivation was apparently employed sometime between 200 B.C. and A.D. 850. The environment of the swamp, the fields and canals, and one nearby settlement, Kokeal, are described. The evidence indicates that the large number of well-defined ground patterns reported in other areas in the central Maya lowlands are probably vestiges of Maya wetland cultivation. 相似文献
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The dendritic morphology of neurons determines the number and type of inputs they receive. In the Drosophila peripheral nervous system (PNS), the external sensory (ES) neurons have a single nonbranched dendrite, whereas the lineally related multidendritic (MD) neurons have extensively branched dendritic arbors. We report that hamlet is a binary genetic switch between these contrasting morphological types. In hamlet mutants, ES neurons are converted to an MD fate, whereas ectopic hamlet expression in MD precursors results in transformation of MD neurons into ES neurons. Moreover, hamlet expression induced in MD neurons undergoing dendrite outgrowth drastically reduces arbor branching. 相似文献
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国土空间生态修复是新时代国家治理体系和治理能力建设的重要手段。同时,国土空间生态修复需要构建多元主体协同修复体系,公众参与在国土空间生态修复中愈发重要。然而,目前我国公众参与国土空间生态修复还存在缺乏健全的法律法规保障、公众参与的责任意识和深度不够、公众参与的组织与平台缺乏等问题,从而影响了公众参与发挥有效作用。基于此,提出健全公众参与的法律法规、增强公众参与意识、健全公众参与的组织与平台等对策措施。 相似文献
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Deevey ES Rice DS Rice PM Vaughan HH Brenner M Flannery MS 《Science (New York, N.Y.)》1979,206(4416):298-306
From the first millennium B.C. through the 9th-century A.D. Classic Maya collapse, nonurban populations grew exponentially, doubling every 408 years, in the twin-lake (Yaxha-Sacnab) basin that contained the Classic urban center of Yaxha. Pollen data show that forests were essentially cleared by Early Classic time. Sharply accelerated slopewash and colluviation, amplified in the Yaxha subbasin by urban construction, transferred nutrients plus calcareous, silty clay to both lakes. Except for the urban silt, colluvium appearing as lake sediments has a mean total phosphorus concentration close to that of basin soils. From this fact, from abundance and distribution of soil phosphorus, and from continuing post-Maya influxes (80 to 86 milligrams of phosphorus per square meter each year), which have no other apparent source, we conclude that riparian soils are anthrosols and that the mechanism of long-term phosphorus loading in lakes is mass transport of soil. Per capita deliveries of phosphorus match physiological outputs, approximately 0.5 kilogram of phosphorus per capita per year. Smaller apparent deliveries reflect the nonphosphatic composition of urban silt; larger societal outputs, expressing excess phosphorus from deforestation and from food waste and mortuary disposal, are probable but cannot be evaluated from our data. Eutrophication is not demonstrable and was probably impeded, even in less-impacted lakes, by suspended Maya silt. Environmental strain, the product of accelerating agroengineering demand and sequestering of nutrients in colluvium, developed too slowly to act as a servomechanism, damping population growth, at least until Late Classic time. 相似文献
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Hirth K 《Science (New York, N.Y.)》1984,225(4662):579-586
The appearance of regional city-states in pre-Hispanic Central Mexico corresponds with the disruption of socioeconomic relationships throughout Teotihuacán's political domain after 650 A.D. Surface reconnaissance and mapping at Xochicalco provided information on the growth and organization of one of these urban centers. The data indicate that Xochicalco was a well-planned urban center and capital of a regional city-state that extended its domain throughout western Morelos by military conquest. 相似文献
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【目的】建立高效液相色谱法(HPLC)测定遍山红中没食子酸、二氢杨梅素、原儿茶酸、芦丁、杨梅苷、齐墩果酸、槲皮素、山柰酚化学成分含量的方法,以期更好地控制遍山红的药材质量,为其药用开发提供科学依据。【方法】采用ZORBAX SB-C18色谱柱、乙腈-0.2%磷酸为流动相、流速0.6 mL/min、柱温28℃的HPLC测定广西都安县4个采集点(弄苍队、卜凡队、弄外队、百外队)遍山红中8种化学成分的含量。【结果】260 nm是测定混合对照品溶液和遍山红供试液中8种化学成分的最大波长。4个采集点遍山红中8种化学成分的含量范围:没食子酸1.53~1.75 mg/g、原儿茶酸1.11~1.28 mg/g、二氢杨梅素0.99~1.44 mg/g、芦丁0.34~0.42 mg/g、杨梅苷1.58~1.83 mg/g、齐墩果酸0.63~0.77 mg/g、槲皮素1.03~1.11 mg/g、山柰酚0.65~0.69 mg/g。8种化学成分质量浓度与峰面积线性关系及色谱峰分离效果均良好(r>0.9990);没食子酸、原儿茶酸、二氢杨梅素、芦丁、杨梅苷、齐墩果酸、槲皮素和山柰酚的进样量范围分别为7.20~43.20、3.36~20.16、5.92~35.52、1.60~9.60、6.40~38.40、3.68~22.08、3.20~19.20和3.52~21.12μg/mL,对应的平均加样回收率为101.72%、96.49%、97.04%、99.79%、99.80%、99.44%、101.52%和96.63%。【结论】建立的HPLC具有操作简便、准确可靠、灵敏度高、重现性好等优点,可用于遍山红的质量控制。 相似文献
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开都河-孔雀河流域水资源需求分析 总被引：1，自引：0，他引：1
【目的】分析开都河-孔雀河流域近21年来不同地区和部门的需水量,同时对水资源供需状况进行探讨。【方法】基于开都河-孔雀河流域1989～2009年社会经济数据,采用定额法。【结果】(1)研究期内,流域总需水量及生活、农业、第二三产业、城镇绿化四部门的需水量均呈明显的增加趋势,其中农业需水量和总需水量变化趋势较为一致;(2)农业灌溉需水量占总需水量的比例高达90%以上,是流域内主要的需水方式,其增减主导着流域总需水量的变化;(3)库尔勒市是流域的经济中心,第二、三产业和城镇绿化需水量均占到整个流域同类需水量的80%以上;(4)自2007年始,流域出现缺水现象,2007、2008和2009年缺水率分别为7.08%、13.36%、19.50%,水资源供需矛盾凸显。【结论】针对流域水资源供需现状,提出相关建议,希望能为流域水资源的可持续利用和社会经济的稳定发展提供科技支撑。 相似文献
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Large RNA molecules, such as ribozymes, fold with well-defined tertiary structures that are important for their activity. There are many instances of ribozymes with identical function but differences in their secondary structures, suggesting alternative tertiary folds. Here, we report a crystal structure of the 161-nucleotide specificity domain of an A-type ribonuclease P that differs in secondary and tertiary structure from the specificity domain of a B-type molecule. Despite the differences, the cores of the domains have similar three-dimensional structure. Remarkably, the similar geometry of the cores is stabilized by a different set of interactions involving distinct auxiliary elements. 相似文献
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Adoption of integrated pest management(IPM) practices in the Guatemalan highlands has beenlimited by the failure of researchers andextensionists to promote genuine farmer participationin their efforts. Some attempts have been made toredress this failure in the diffusion-adoptionprocess, but farmers are still largely excluded fromthe research process. Understanding farmers'agricultural knowledge must be an early step toward amore participatory research process. With this inmind, we conducted a semi-structured survey of 75Cakchiquel Maya farmers in Patzún, Guatemala, tobegin documenting their pest control practices. Theirresponses revealed that their understanding ofbiological and curative pest control is limited.However, their broad knowledge of cultural preventivepest control practices could explain why they hadfaced few pest problems in their traditionalmilpa (intercrop of corn, beans, and other edibleplants). The majority of these preventive practicesare probably efficient and environmentallyinnocuous. 相似文献
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历史上徽州女性长期生活在封建礼教、宗族制度的压迫下，精神、思想和行为受到严苛的限制。作为徽州文化的物质载体，徽州建筑类型、形态、空间组织方式以及装饰等在展现徽州建筑艺术美的同时，也体现了对徽州女性精神、思想和行为的禁锢和关怀，折射出明清时期徽州女性的社会地位和生活状况。牌坊与宗祠虽给予女性一定的尊重，而其实是封建礼教和宗法制度的纪念碑；徽州民居和宅院内向性封闭式的空间格局虽是徽州女性的精神家园，也反映出对徽州女性的制约；有关徽州女性题材的建筑雕饰在反映他们的社会地位、价值取向和审美情趣的同时也对妇德提出了较高要求。 相似文献
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Several features in social insects, particularly in ants, make the behavioral organization of territoriality considerably more complex than that of solitary animals. The establishment and maintenance of territories are based on a division of labor and a complex communication system. The analyses of territorial strategies in ants comprise the study of the design and spatiotemporal structure of the territory, as well as the social mechanisms through which the insect society pursues its territorial strategy. The geometric and behavioral organization of the absolute territories of the African weaver ants (Oecophylla longinoda) and harvester ants (Pogonomyrmex), and of the "spatiotemporal territories" of honey ants (Myrmecocystus mimicus) are described, and simple cost-benefit models are developed to illustrate the economic defensibility of each type of territory. 相似文献