首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 140 毫秒
1.
不同程度人为干扰对古田山森林群落谱系结构的影响   总被引:2,自引:0,他引:2  
群落谱系结构包含群落发育的历史信息,能从新的角度反映群落形成的生态过程.作者在浙江古田山自然保护区亚热带常绿阔叶林中选择了人为干扰强度不同的4种群落类型,以20 m×20 m为研究尺度探讨了不同干扰程度对群落谱系结构的影响.结果表明,人工林(类型Ⅰ)谱系结构发散;但间伐林(类型Ⅱ)、自然恢复林(类型Ⅲ)以及自然老龄林(类型Ⅳ)谱系结构聚集,且以类型Ⅱ和Ⅳ聚集度最高.进一步分析不同径级谱系结构发现,在中小径级(DBH≤5 cm和5 cm10 cm时,除了类型Ⅳ,其他3种群落都是谱系结构发散,这反映了在恢复早期种子扩散对这些林型群落构建影响较大:而皆伐后的演替和间伐增加了群落生境异质性,生境过滤作用增强,使类型Ⅱ、Ⅲ群落中小径级谱系结构表现为聚集;类型Ⅳ群落不同径级谱系结构均表现为聚集,可能与其稳定的生境过滤作用有关.  相似文献   

2.
于晶  王全喜  曹同 《植物研究》2003,23(3):T001-T003
在对中国藓类植物孢子形态结构的系统观察基础上,从孢子壁层结构及孢子形态角度将中国藓类孢子类型初步划分为9种类型。每种类型以代表属名命名,对每种类型的大小、颜色、形状、极性、萌发孔及孢壁结构进行了描述。孢子的大小、颜色和形状有时会随环境条件而改变,而极性、萌发孔及孢壁结构很稳定,在类型划分中更重要。  相似文献   

3.
中国千金藤属植物化学分类概况   总被引:2,自引:0,他引:2  
探讨了各结构类型的异喹啉类生物碱在千金藤属植物中的分布规律及千金藤属植物的亲缘关系,在较原始的类群中,生物碱的结构类型和数量均较少,而在较进化的类群中内,其结构类型得到较大的发展,化合物的数量也相对增多,显示异喹啉类生物碱的结构类型及数量与属内植物群的进化有一定的对应关系,一些结构类型特化的生物碱和在植物界中分布极为狭窄的化学成分对亚属下各植物组的划分起着重要作用。  相似文献   

4.
用扫描电镜观察了分属10个类群、30个种的80株链霉菌的孢子形态。其表面结构可分为光滑、刺状、毛发和鳞片状类型。孢子的表面结构类型与孢子丝形态间具有一定的相关性。直形孢子丝所形成的孢子,其表面结构多为光滑型。螺旋形的孢子丝所形成的抱子,其表面结构,除光滑型外,尚有其他几种类型。气生菌丝体的颜色与孢子表面结构之间也存在一定的相关性。气生菌丝体呈灰色者,孢子表面结构的类型比较多。气生菌丝体呈其他颜色者,其孢子表面结构的类型则比较单一。  相似文献   

5.
SARS病毒S蛋白三维结构预测   总被引:1,自引:0,他引:1  
蛋白质结构类型识别方法可以在没有序列同源性的蛋白质之间检测有没有结构相似性。利用蛋白质结构类型识别方法预测了SARS病毒S蛋白N端区域的结构。模建的SARS病毒S蛋白N端区域是一个全折叠的结构。  相似文献   

6.
利用复杂网络的方法来探索序列特征因素对蛋白质结构的影响。由于蛋白质的序列对结构具有重要且复杂的影响,因此将蛋白质的结构以及序列特征之间的关系模拟成一个复杂系统,通过利用互相关系数、标准化互信息和传递熵等方法来建立以序列特征为节点的加权网络,进而利用网络中心性的方法来分析不同蛋白质结构类型对应加权网络的中心性分布的差异,探索不同结构类型蛋白质的序列特征差异。发现不同的蛋白质结构类型对应的序列特征网络既有共性又有差异,文章将针对每一种结构类型的网络中心性分布,以及不同结构类型之间的共性与差异进行详细地讨论。研究结果对蛋白质序列与结构之间关系的研究,特别是结构分类研究具有重要的意义。  相似文献   

7.
西双版纳望天树种群结构与动态研究   总被引:22,自引:4,他引:18       下载免费PDF全文
赵学农  刘伦辉  高圣义  唐继武   《广西植物》1996,16(3):225-232
本文采用空间代替时间和相邻格子样方法阐明了望天树种群的结构类型和群落演替模式,结果表明其种群结构类型可分为初始增长型(类型I),增长型种群前期类型(类型Ⅱ),增长型种群(类型Ⅲ)和稳定性种群(类型Ⅳ).其群落演替模式遵从类型Ⅰ─类型Ⅱ─类型Ⅲ─类型Ⅳ模式,即具有更新镶嵌和循环的特点.探讨了群落演替过程中伴生种的变化规律,并就望天树种群的合理保护和群落重建提出了建议.  相似文献   

8.
糖蛋白的结构、功能及分析方法   总被引:8,自引:0,他引:8  
综述了糖蛋白研究的重要意义、糖肽键的主要类型、糖链的主要类型、糖蛋白结构研究的一般策略及结构分析的最新进展  相似文献   

9.
蛋白质折叠类型识别方法研究   总被引:1,自引:0,他引:1  
蛋白质折叠类型识别是一种分析蛋白质结构的重要方法.以序列相似性低于25%的822个全B类蛋白为研究对象,提取核心结构二级结构片段及片段问氢键作用信息为折叠类型特征参数,构建全B类蛋白74种折叠类型模板数据库.定义查询蛋白与折叠类型模板间二级结构匹配函数SS、氢键作用势函数BP及打分函数P,P值最小的模板所对应的折叠类型为查询蛋白的折叠类型.从SCOP1.69中随机抽取三组、每组50个全β类蛋白结构域进行预测,分辨精度分别为56%、56%和42%;对Ding等提供的检验集进行预测,总分辨精度为61.5%.结果和比对表明,此方法是一种有效的折叠类型识别方法.  相似文献   

10.
蛋白质的序列、结构和功能多种多样。大量研究表明蛋白质的结构与其氨基酸序列的排序有关,并且局部的氨基酸序列环境对蛋白质的结构具有一定的影响。本文提出一种新的基于5-mer氨基酸扭转角统计偏好的蛋白质结构类型预测方法,该方法通过PDB数据库中5-mer中间氨基酸的扭转角统计偏好来进行结构类型的预测。新方法可以通过计算机仿真实现对新蛋白质序列结构类型的快速预测,并通过两组随机抽取的CATH数据验证了新方法的有效性。  相似文献   

11.
Summary The putative ocellus of Mermis females consists of a hollow cylinder of dense hemoglobin pigmentation located in the anterior tip. The exact location of the photoreceptive nerve endings, however, is unknown. During phototaxis a continual bending or scanning motion of the head (anterior 2 mm) causes the orientation of the tip to swing about the direction of the source. By turning off (shuttering) the light source whenever the tip orientation was to one side of the source direction, the average orientation of the base of the head, and eventually the body orientation, was caused to be biased about 28° to the opposite side. Because the shuttering was synchronized with the scanning motion, the scanning motion must be involved in the maintenance of orientation to light. The direction of the bias rules out a two-signal comparison mechanism of orientation and demonstrates that a deviation of the tip from the source direction must decrease, rather than increase, the illumination of the photoreceptors. These findings, and the ocellar morphology, require that the photoreceptors be located inside the hollow tube of pigmentation where they can be shadowed by the pigment during deviations of the tip. Focusing by the curved anterior end should cause a similar modulation of the illumination at this location. The occasional episodes of transverse phototaxis can be explained by the leakiness of the pigment walls to transverse illumination. Analysis of the motion of the anterior in the presence and absence of shuttering indicates that the orientation of the base of the head, due to the motion of the neck, is controlled by the signals generated during one or more cycles of the scanning motion of the head. The orientation may be regulated by the phase relationship between the photoreceptor signal and putative proprioceptive signals that indicate the bending in the head.  相似文献   

12.
The studies and the researches carried out in the last years on the Palaeolithic site of Isernia La Pineta have brought to consider in new way the activities realized by the human group that lived the basin of Isernia during the Middle Pleistocene offering an important key of interpretation of the behavioural strategies of the prehistoric man. The analysis of the exploitation of the raw material has confirmed the presence on the site of two different lithotypes: flint and limestone; the lithological dichotomy is related to the functional dichotomy of the raw material that seems to have conditioned the activities of the human group in different areas of the site. The necessity to deepen the study on the limestone has derived from the evidence brought to light in the last excavation campaigns of a remarkable concentration of the flaked limestone pebbles and the flake scars in some areas of the explored archeosurfaces, particularly on the 3a and on the overlooking layers. The present study has the purpose to explain the characteristics of the limestone finds both in reference to the raw material and to its state of preservation both to the technotypological evidences and its spatial distribution with the purpose to better understand the modalities of the exploitation of the raw material. The information collected until today have permitted to obtain a precise knowledge of the environmental context and the territorial resources exploited by the human group showing an opportunistic capability to find the most advantageous behavioural solution for the necessities of subsistence.  相似文献   

13.
Quartz is one of the main lithic resources employed along the Pleistocene for the manufacturing of tools. The abundance in all kind of environments leads to its frequent presence in archaeological sites. In spite of its suitability for knapping, the scarcity of specific researches on quartz industries has led to a view of this raw material as a second-rate resource, only used when flint was not available in the surroundings. Quite in opposition to this prejudice, the familiarity with the petrological and mechanical properties of the different varieties of quartz has led the Middle/Upper Pleistocene groups to a rational and differential exploitation of the available resources. There is a distinct selection of the varieties attending to the purposes of the exploitation and configuration. The patterns of exploitation of the cores are very standardized, based on longitudinal extractions and documenting the bipolar flaking technique. Moreover, when the raw material is of good quality, more sophisticated reduction methods such as the discoidal or the Levallois may be employed. On the other hand, for the purposes of configuration a clearly differential management of the raw material is attested: thus quartz is used for the small products (denticulates, side-scrapers, notches), while other stones (quartzite, porphyry and others) are devoted to the manufacturing of those tools demanding a more complex control of the elaboration (bifaces, cleavers, etc.). Either acting as the main lithic resource or as a complement to other raw materials, we can observe through the technological analysis of these lithic collections the great conceptual complexity depending on the needs to be fulfilled. Raw material quality allowing, the state-of-the-art technology of knapping can be applied to the quartz and, therefore, we should not understand its use as a consequence of environmental constrictions but rather as a result of the variability of the procurement strategies and management of lithic resources among the prehistoric communities.  相似文献   

14.
Pollen analysis of Madonna dell’Arma cave inside part (Liguria, Italy) allowed us to reconstitute, at local scale, the Ligurian coastal vegetation between the end of the last interglacial period and the beginning of the pleniglacial. This transition period shows an important representation of arboreal cover and a persistence of thermophilous elements that allowed us to considerate the western Liguria as a periglacial refuge. During the end of the last interglacial, xerophytic, halophytic herbs and shrubs taken over from a Mediterranean pre-forest unit in the first slopes constituted the coastal zone. The nearness of Argentina torrent and the Armea influenced, in the coast, the existence of marshy zones colonized by hygrophilous trees. In the Ligurian lowlands spread out sclerophyllous and mesophilous forests according to altitude. The beginning of the pleniglacial is indicated in the sequence by increases of Pinus, Cupressaceae and Artemisia in the context of a decline in the thermophilous components. This picture of vegetation has become integrated into the multidisciplinary studies of the site and contributed to the elaboration of palaeoclimatic hypothesis similar as those revealed by the fauna.  相似文献   

15.
Research on the influence of sea level variations on the benthic faunas have been carried out in the Upper Ordovician of Sardinia. Study of the depositional facies and sequence analysis of the upper part (Lower Ashgill) of the Portixeddu Formation led to the identification of the sedimentary environments. Cystoids and crinoids are associated to bryozoans and brachiopods in most levels. The numerical analysis of associations and megaguilds shows that crinoids and cystoids have a higher frequency in the proximal and median facies of the upper offshore. The columnal association characterized by Conspectocrinus celticus and the coronoid Mespilocystites tregarvanicus has been discovered in the upper part of the formation. This material and complementary samples from Upper Ordovician of Sardinia and Kabylia (Algeria) bring additional data on the systematic and show the wide distribution of this fauna outside of the Ibero-armorican domain. The distribution of this echinoderm association supports a palaeogeographical position of the Ibero-armorican domain and Sardinia within the north gondwanan margin during the Lower Palaeozoic.  相似文献   

16.
The present study describes the fetal regions ethmoidalis and orbitotemporalis of the cranium of Galea musteloides and Kerodon rupestris and compares them with Galea spixii and Cavia porcellus . The fetal crania of these representatives of the Caviinae were serially sectioned and 3D plate reconstructions were built. For a broader scope, serially sectioned heads of the caviomorph taxa Hydrochaeris hydrochaeris and Dasyprocta leporina were also examined. The ethmoidal and orbitotemporal regions of the cranium provides characters that are pertinent to the discussion of systematics of the Caviinae. Nine cranial characters were identified as being significant and discussed phylogenetically. Taking the monophyly of the Cavioidea as given, two characters are autapomorphic for the genus Galea: the reduction of the crista semicircularis and the lacrimale forming the 'arco antorbitario' (Kraglievich 1930). Cavia is the sister group of Galea; synapomorphic characters are the extent of the processus palatinus medialis of the praemaxillare and the presence lamina pterygoidea of the alisphenoid. There were no apomorphic characters of the structural complexes examined that could support the Caviinae. The reduced and nonperforated alisphenoid is an autapomorphic feature of the Caviidae. The division of the nasoturbinale is evidence for a sistergroup relationship of the Hydrochaeridae and the Caviidae. Autapomorphic characters for the Cavioidea include the ventrolateral opening of the organon vomeronasale and the lateral prolongation of the lacrimale.  相似文献   

17.
Evolutionary expansion of the Monogenea   总被引:5,自引:0,他引:5  
The evolutionary expansion of the monogeneans has taken place in parallel with the diversification of the fish-like vertebrates. In this article the main trends in monogenean evolution are traced from a hypothetical skin-parasitic ancestor on early vertebrates. Special consideration is given to the following topics: early divergence between skin feeders and blood feeders; diversification and specialization of the haptor for attachment to skin; transfer from host to host, viviparity and the success of the gyrodactylids; predation on skin parasites and camouflage; colonization of the buccal and branchial cavities; diversification and specialization of the haptor for attachment to the gills; phoresy in gill parasites; the development of endoparasitism and the origin of the cestodes; the success of dactylogyroidean gill parasites; the uniqueness of the polyopisthocotyleans; ovoviviparity and the colonization of the tetrapods. Host specificity has been the guiding force of coevolution between monogeneans and their vertebrate hosts, but the establishment of monogeneans on unrelated hosts sharing the same environment (host-switching) may have been underestimated. Host-switching has provided significant opportunities for evolutionary change of direction and is probably responsible for the establishment of monogeneans on cephalopod molluscs, on the hippopotamus and possibly on chelonians. There are indications that host-switching may be more common in monogeneans that spread by direct transfer of adults/juveniles from host to host. A limitation on the further expansion of monogeneans is the need for water for the dispersal of the infective larva (oncomiracidium).  相似文献   

18.
Biodeterioration of stone: a review   总被引:1,自引:0,他引:1  
The alteration and weathering of stone is basically determined by natural and anthropogenic impacts influencing various physical, chemical and biological damage factors at the object site. Whether as direct or catalytically enhancing factor, the biodeterioration of stone is coupled with nearly all environmentally induced degradation processes: the presence of the one makes deterioration by the other all the more effective. The bioreceptivity of stone is described by its structure and chemical composition, while the intensity of the microbial contamination is determined by the referring climatic conditions and the anthropogenic euthrophication of the atmosphere. The microflora improves the nutrient and moisture-restricted growth conditions on building stones by the formation of surface-covering biofilms. Besides the aesthetical impairment caused by the coloured biopatina, the biofouling effect promotes even “abiotic” deterioration processes due to the alteration of the material structure as well as their thermo-hygric properties; in addition, mechanical pressure due to the shrinking and swelling of the colloidal biofilms might cause a further weakening of the mineral lattice. Acidolytic and oxido-reductive biocorrosion processes complete the biodeteriorating attack of stone acting as a preliminary precursor for the latter formation of detrimental crusts. Suitable and reliable methods for the detection of biodeterioration processes are available, but only the interdisciplinary diagnosis and evaluation of the entire decay process of stone allows the formulation of adaequate countermeasure strategies. In case the significance of biodeterioration impacts is proven, the possible effects of the microbial contamination on cleaning procedures, protective treatments as well as biocidal applications has to be considered. This paper will give a comprehensive overview to the biodeterioration of stone and stresses the practical relevance for the conservation.  相似文献   

19.
A phylogenetic analysis of 123 morphological characters of basal waterfowl (Aves: Anseriformes) and other selected avian orders confirmed that the screamers (Anhimae: Anhitn-idae) are the sister-group of other waterfowl (Anseres), and that the magpie goose (Anseranatidae: Anseranas semipalmata) is the sister group of other modern waterfowl exclusive of screamers (Anatidae sensu stricto). The analysis also supports the traditional hypothesis of the gallinaceous birds (Galliformes) as the sister group of the Anseriformes. Presbyornis, a fossil from the early Eocene of Wyoming and averred by Olson & Feduccia as showing that the Anseriformes were derived from shorebirds (Charadriiformes), was found to represent the sister group of the Anatidae. Associated hypotheses by Olson & Feduccia concerning the implications of Presbyornis for the phylogenetic relationships of flamingos (Phoenicopteriformes), the position of the Anhimidae within the waterfowl, relationships among modern Anatidae, and a plausible evolutionary scenario for waterfowl also are rejected. Analyses revealed that cranial characters were critical to the establishment of the Galliformes as the sister group of the Anseriformes; exclusion of the Anhimidae, especially in combination with Anseranas, also undermined the support for this inference. Placement of Presbyornis as the sister group of the Anatidae casts doubt on the role suggested by Feduccia of ‘transitional shorebirds' in the origin of modern avian orders, and calls into question the concept of ‘fossil mosaics’. The phylogenetic hypothesis is used to reconstruct an evolutionary scenario for selected ecomorphological characters in the galliform-anseriform transition, to predict the most parsimonious states of these characters for Presbyornis, and to propose a phylogenetic classification of the higher-order taxa of waterfowl. This re-examination of Presbyornis also is used to exemplify the fundamental methodological shortcomings of the intuitive approach to the reconstruction of phylogenetic relationships.  相似文献   

20.
The creation of the Veterinary Schools in the 18th century would reveal a plethora of scientists, some of whom would be the precursors of Pasteur, some rivals, others followers collaborators or friends of the Master. Among the precursors let us name Chabbert, Huzard, Girard, Delafond, Renault, Toussaint, Galtier ; among the rivals: Chauveau, Arloing, Cornevin and Thomas; among the followers, collaborators or friends of Pasteur: Bouley, at first a resolute spontaneist, then the most fervent in defense of Pasteur (President of the Academy of Medicine and of the Academy of Sciences) and Nocard, Director of the School in Alfort, an important collaborator of Pasteur. Later, there was Leclainche, who created the International Office of Epizootics, and who was President of the Academy of Sciences; Guérin, who with Calmette developed the BCG vaccination; Ramon, the father of anatoxins (vaccines against diphtheria, and tetanus, combined vaccines, adjuvants to immunity). Thus, the creation of the Veterinary Schools contributed not only to the evolution of the notion of contagion, to the amelioration of animal health and the economics of agricultural production, but also to serious advances in human care, and to the protection of public health.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号