摘要:

Summary1. Acritarchs are a polyphyletic group of unicellular organisms, essentially marine and fossil, with a very resistant organic membrane; the majority probably represent the cysts of microscopic, extinct eukaryote algae. This review gives a general account for the non‐specialist of their characteristics and affinities, but focuses, using selected examples, on their role as biostratigraphic tools for the specialist.2. Invisible to the naked eye, up to several tens of thousands of acritarchs per gram of rock may be extracted and concentrated from a wide variety of sediments, especially argillaceous or even calcareous, but preferably fine‐grained, unweathered and only slightly recrystallized or metamorphosed.3. Always hollow and without unequivocal intracellular structures, acritarchs are extremely variable inoverall size, from a few to several hundred μm, with numerous divergent morphological modifications from a basic spherical form; the type and development of ornamentation; the number of cellular walls; and the method of opening, attributed to excystment. Acritarchs are classified according to criteria that are relatively simple compared with the modern demands of phycologists. For convenience they are treated under the International Code of Botanical Nomenclature, recognizing the existence of form genera of uncertain position. The lack of a comprehensive taxonomic framework is not surprising, given the number and variety of unclassifiable microorganisms resistant to HF that may be included in the acritarchs.4. The sporopollenin‐like wall of acritarchs, like the sporopollenin of modern plants, is chemically very inert except to oxidation, carbonization and bacterial or fungal activity. Of poorly‐known composition but very probably including highly polymerized polyterpenes, it may form an abundant component of Palaeozoic kerogen, a potential source of hydrocarbons. The codification of colour changes and preservation in selected acritarchs may enable the optical evaluation of palaeotemperatures lower than about 120–150 °C and of the degree of maturity of possible oils.5. The first known acritarchssensu stricto, although discovered in 1862, were designated as such in 1963, after having been given a variety of names reflecting mainly assessments of their biological affinities. In spite of some attempts to abandon it, the name acritarch is still the most correct as it is the least ambiguous for designating the great majority of examples.6. The reclassifying of acritarchs among microorganisms of known systematic position remains speculative or tentative. It is possible that many acritarchs represent cysts of extinct dinoflagellates, without archaeopyle or indication of a stable tabulation. Laboratory culture ofPterospermahas shown thatCymatiosphaeraandPterospermellahave to be considered not as acritarchs but as phycoma of prasinophytes. The ultrastructure of the wall inTasmanitesis similar to that ofPachysphaera, another recent prasinophyte. Comparisons with euglenoids or spore‐like bodies of the first terrestrial plants are indirect and that with eggs of recent crustaceans remains fortuitous.7. The composition of live acritarch assemblages is most often heavily biased in taphocoenosis. In fact, because of their very small size and low density, these microfossils are frequently found reworked in strata younger than those in which they were originally deposited. If their distributions are sufficiently documented, they can be useful as provenance indicators in palaeogeographic reconstructions.8. Acritarchs' mode of life is thought to be best compared with that of planktonic photosynthetic algae. General schemes seeking to explain variations in their abundance and distribution in deposits formed during the distant geological past are based especially on extrapolations from complex combinations of factors that govern the distribution of modern marine phytoplankton.9. With a worldwide geographic distribution and a record only partly influenced by facies control, the acritarchs exhibit, geologically speaking, an extraordinarily long life span, from the Mesoproterozoic to the present day. In spite of the examples of reworking, rarely objectively verifiable, and the still relatively small number of detailed data with reliable independent age control, it is known that acritarchs, among a great number of ubiquitous forms, include time index taxa whose levels of appearance permit the calibration of very remote geological time and the establishment of regional or global correlations. These biostratigraphic indices, certainly present in the Neoproterozoic but still little known, are best demonstrated from around the beginning of the Cambrian to slightly before the end of the Upper Devonian, a time of maximum abundance and diversity for the group.At the beginning of the Early Cambrian in the East European Platform, and probably slightly above the international systemic boundary, drawn at the appearance of the ichnofossilPhycodes pedumin eastern Newfoundland, the acritarchs display a radiation of original diversity which occurs at three levels and contrasts with the worldwide impoverished sphaeromorph assemblages of the latest subjacent Neoproterozoic. The first level is marked especially by the appearance ofAnnulum squamaceum, the second by the diversification ofComasphaeridium, and the third, which is the clearest and most geographically widespread, by the appearance ofSkiagia orbicularis, S. ornataandS. scottica, which coincides approximately with that of the trilobites.The Cambrian‐Ordovician boundary is not yet agreed internationally but should be near the appearance of theCordylodus lindstromiconodont Biozone, slightly below the first occurrence of nematophorous planktonic graptolites.Corollasphaeridium wilcoxianumis the index acritarch whose appearance is closest to, and slightly below, this boundary, in the upper part of theCordylodus proavusBiozone. The species enters at this level in the north Sino‐Korean Platform (Jilin province) and northern Laurentia (Alberta). It has not been recorded in Baltica, Avalonia and Gondwana, where the acritarch assemblages are better documented, more varied and different on the whole from those of northeastern China and western Canada. With reservations, it may be that in marine deposits associated with these three palaeocontinents, the lower limit of the range ofAcanthodiacrodium angustumis located within theCordylodus proavusBiozone.In the Late Devonian, the Frasnian—Famennian boundary is fixed internationally by means of conodonts, at the base of the EarlyPalmatolepis triangularisBiozone, which succeeds thePalmatolepis linguiformisBiozone. Regionally, in the Dinant Basin, Belgium, no index acritarch is known to appear at the base of the Lower Famennian. On the other hand, at Senzeilles the appearances ofVisbysphaera?occultaand ofEphelopalla mediaoccur successively at the end of the Frasnian in deposits undated by means of conodonts but attributable to the end of the latePalmatolepis zhenanaBiozone and to theP. linguiformisBiozone.In the course of the upper Famennian, and from the end of the Late Devonian onwards, known assemblages are essentially sporadic, unvaried and of reduced or local stratigraphic value. The last species that is autochthonous, morphologically unmistakable and of worldwide distribution appears in the middle

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1993.tb01241.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1993.tb01242.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Summary1In recent decades there has developed a very general appreciation of the need to build, maintain and use collections of crop plants with the primary object of sustaining genetic advance by plant breeding on into the indefinite future. The fact of genetic erosion is universally acknowledged and substantial advances in some practical aspects of genetic resource conservation work have been made. Many good collections have been assembled, though some crops have been poorly served, even ignored, and maintenance, study and utilization of those that have been collected have often left much to be desired.2This review is devoted to the most neglected aspect of all, namely utilization. Traditionally, collections have been regarded as sources of ‘genes’, usually disease resistances, to be exploited by backcrossing (Introgression) into adapted stocks. Thousands of such backcrosses have been made, with very varied success; there have been successes but also many failures, due usually to the weakness of ‘vertical resistances’ as protection against adaptable pathogens. By contrast, the need to broaden the genetic bases of many crops, far beyond the confines possible for Introgression programmes, has often been recognized but rarely explored. Thus a distinction is drawn between Introgression and Incorporation as the two fundamental methods of using crop collections (see Fig. 1).3Incorporation implies the systematic exploitation of a large array of genetic variability in such a way as to generate a mass of newly adapted stocks usable as parents in breeding programmes. Genetic principles are simple and obvious, always based on recurrent cycles of recombination and mass selection. Progress is likely to be slow (indeed usually must be slow), a fact which has often been a discouragement to such programmes in the past.4Examples of long‐term, systematic Incorporation programmes are few, and the more important ones are summarized. They relate topotatoes, sugar cane and maize, which are well advanced; and cocoa, oil palm and (prospectively) rubber, which are well begun. Time scales depend upon the biology of the crop and methods; typically, a few decades are necessary to make serious progress.5In discussion, the following points are emphasizedthe essential nature of Incorporation (= Base broadening) in all well‐bred crops in the longer term and into the indefinite future; the weakness of phenotypic assessment of genetic potential (usually referred to as ‘evaluation’); the fact that Incorporation is esentially simple, even if slow; the nomenclatural confusion that surrounds the terms Genetic Enhancement, Prebreeding, Genetic Vulnerability; the fact that the common stereotype of a crop collection as a collection of seed packets is often wrong because many crops are clonal and/or have short‐lived, unstorable seeds; and the socio‐politico‐bureaucratic complications that arise because of the current collapse of publicly supported research, the associated short‐term ideology, and the fact that serious genetic resource work, including Incorporation programmes, demands long‐term commitment to the interests of our successo

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1993.tb01243.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SummaryWhile it is generally acknowledged that modern science began with the quantification of time in the measurement of linear physical processes in space by Galileo and Newton, the biological sciences have only recently developed appropriate experimental and mathematical methods for the description of living systems in terms of processes of non‐linear, recursive dynamics. We now recognize that living organisms have patterns of exquisitely timed processes that are as intricate as their spatial structure and organization. Self‐similarities of life processes in time and space have evolved to generate an ensemble of oscillators within which analogous functions may be discerned on many different time scales. The increasing complexity of periodic relationships on and between the many levels of biological organization are uncovered by current research. Recent efforts to reformulate the foundation of physics from the quantum to the cosmological level by using the concept of information as the common denominator integrating time, structure and energy remind us of an apparently analogous suggestion in the chronobiological literature which also describes the periodic dynamics of living systems as information processing. In this paper we review the periodic processes of living systems on all levels from the molecular, genetic and cellular to the neuroendocrinological, behavioural and social domains. Biological rhythms may be conceptualized as the evolution of ever more complex dynamics of information transduction that optimize the temporal integrity, development, and survival of the organ

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1993.tb01244.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Summary1One of the major developments in ecology has been the recognition of the importance of spatial and temporal scale in describing patterns of distribution and abundance. This article develops a quantitative framework that includes both spatial and temporal scale, then uses it to review what has been learned about a well studied group of mobile organisms — marine birds in the Bering Sea.2Review showed that lateral gradients in density are by far the most frequently measured property of birds away from colonies in the Bering Sea. Gradients are related to a variety of environmental factors, depending on spatial scale.3Review within a quantitative framework showed that the assumptions for interpreting the spatial dynamics responsible for observed patterns are rarely stated, that interpretations of pattern at sea have focused on individual movements in relation to food concentration, and that little is known about wind and water‐driven movements, or rates of death and recruitment away from colonies.4Several lines of evidence support the hypothesis that distribution and feeding are linked to the rate of resupply of nekton to birds near the sea surface.5The framework provided here permits the relative importance of competing processes to be stated and evaluated as a function of spatial scale. The framework should be useful in summarizing the spatial dynamics of other groups of mobile organi

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1993.tb01245.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SummaryPart I of this article shows that, for different reasons, neither traditional interactionist dualism nor more modern theories of the mind/body relationship — including functional‐state identity theory — provide a satisfactory explanation of the evolution of consciousness. Interactionist dualism leaves us with a philosophical impasse; the more modern theories have not yet succeeded in providing adequate answers to the difficulties that were raised by William James in 1879.Part II of the article disputes Kripke's claim that identity theories are, anyway, untenable because the relevant identities would have to be necessary rather than conti

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1993.tb01246.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY