摘要:

Summary1. The main characteristics of the biosynthetic system forming ethylene in plant tissues have been reviewed. The dependence of synthesis on a liberal supply of oxygen is clearly indicated by the fact that atmospheres containing 3–5% oxygen prevent the synthesis in fruits. There is no close connexion between respiratory activity and synthesis. Ripening of fruits and the changes associated with it may be initiated by ethylene; under such conditions the progress of formation of the hydrocarbon is autocatalytic.2. Synthesis appears to be dependent on some degree of cell organization, since it responds acutely to changes in toxcity, tissue wounding and tissue destruction. Homogenates of many plant tissues do not produce ethylenein vitro, and the inability to use such extracts has imposed serious restrictions on biochemical studies which have in the past been mainly concerned with tracer studies and the use of tissue slices.3. The chief difficulty associated with tracer studies aimed at determining the nature of the precursor stems from the fact that the synthesis of ethylene is only a minor pathway on the general metabolism of the cell. Thus the ratio of CO2to ethylene production is of the order of 164 in the case of the apple and as high as 18,000 in the case of less vigorous producers of ethylene. The incorporation of label from labelled substrates which enter the general metabolism of the cell is thus usually very low, and this makes it difficult to determine whether the incorporation observed has any real physiological significance. In fact only where incorporation into ethylene relative to that into CO2is high, as is the case with methionine, can one conclude that the substance can be considered to be an immediate precursor.4. Because of the difficulty of obtaining clear‐cut results with tracer techniques, attention has been devoted to the production of ethylene by model systems from substances of physiological interest. The studies have revealed that many substances found in plant tissue can be decomposed to yield ethylene in model systems functioning under physiological conditions. Two such substances, which have received most attention, are methionine and linolenic acid, and conditions under which ethylene is formed from them have been described.5. Such developments have stimulated research to obtain evidence for or against the operation of such model systemsin vivo.Using tissue‐slice techniques, methionine and linolenic acid have both been found to stimulate ethylene formation in tissue slices.6. The first demonstration of the synthesis of ethylenein vitroby enzymes isolated from the florets of the cauliflower has now been reported. The system involves the intermediate formation of methional from methionine by enzymes contained in the mitochondria, and the subsequent enzymic decomposition of methional into ethylene by non‐particulate enzymes. These latter consist of a glucose oxidase and a peroxidase. The glucose oxidase in the presence of its substrate generates hydrogen peroxide, and peroxidase, in the presence of two co‐factors, ^‐coumaric acid esters and methane sulphinic acid, utilizes the peroxide to produce ethylene from methional. Although all components of this system have been isolated from extracts of floret tissue, proof that this is the actual or only processin vivofor this or other plant tissue has not as yet been achieved. The more recent demonstration of the possible involvement of linolenic acid underlines the necessity for further work.7. Whilst much work still remains to be done to establish the mechanism of synthesis, which may not be identical in different plants, the related question of the nature of the events which stimulate the tissue to produce ethylene remains to be answered. Recent work has suggested that these events, induced by ageing of the tissue, are associated with the synthesis of new enzyme proteins, which are themselves the cause of the rapid onset of synthesis of ethylene, observed in most fruits, at the climacteric.8. Much more information on the nature of events leading to and changes associated with the ripening syndrome in fruits and onset of senescence in vegetable tissues is needed before authoritative answers can be given to any of the questions raised in

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1969.tb00824.x

出版商:Blackwell Publishing Ltd    

年代:1969

数据来源: WILEY

摘要:

Summary1. The rapid increase in the knowledge of the early geological history of bryophytes which has taken place in recent years is emphasized.2. An explanation for this unexpected development is sought in a consideration of the conditions necessary for the preservation of bryophytes as fossils.3. It is concluded that the chances of preservation depend not so much on the conditions suitable for the growth of bryophytes or the possession of resistant structures (although both can be important contributory factors) but on the occurrence of the right kind of sedimentation in the right place at the right time.4. The fossil history of the main Orders of the Bryophyta is then systematically reviewed, with special reference to first records in the Palaeozoic and Mesozoic.5. A number of problematic bryophyte‐like fossils of Palaeozoic age are also reviewed. Four(ProtosalvtniaDawson,SporogonitesHalle,TetrapteritesSullivan and Hibbert, and aDicranum‐likeplant from South Africa) are accepted as probably bryophytic; three(MusciphytonGreguss,HepaticaephytonGreguss, and an alleged bryalean sporogonium from the Rhynie Chert) are rejected on the grounds of insufficient evidence.6. The bearing of the fossil evidence on bryophyte evolution is briefly considered.7. It is shown that the principal groups of both liverworts and mosses had already been differentiated before the end of the Palaeozoic.8. A polyphyletic origin of the Bryophyta is therefore highly probable.9. Beyond this, the early fossil evidence as yet gives no unequivocal answers and more detailed phylogenetic speculation based on the present state of knowledge of fossil bryophytes has little value.10. Further knowledge of the early fossil history of bryophytes is needed and it is suggested that this is most likely to be obtained by patient systematic search in finegrained freshwater sediments by bulk maceration techniq

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1969.tb00825.x

出版商:Blackwell Publishing Ltd    

年代:1969

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1969.tb00826.x

出版商:Blackwell Publishing Ltd    

年代:1969

数据来源: WILEY

摘要:

Summary1. It is vital that there should be no decrease in the rate of improvement of crop plants. In drafting the background to the inheritance of quantitative characters it is suggested that greater understanding of genetic mechanisms is needed rather than further increase in biometrical complexity. Selection is a very powerful force; it could be used to conserve genetic variation whilst effecting useful change.2. Various breeding techniques, examined in a framework of directional, stabilizing and disruptive selection, are found to be tied to certain crops. Breeders of self‐fertilizing plants handle quantitative variation differently from those working with outbreeders.3. Physiological analysis and partitioning of characters have not in themselves made large contributions to solving problems caused by complex traits, but both have been informative in conjunction with biometrical studies. In some circumstances more use could profitably be made of selection indices.4. Heterozygote advantage and genetic balance play major parts in stabilizing selection. Modal selection can sometimes lead to increased fitness, conforming with the concept of genetic homeostasis.5. Disruptive selection has apparently operated during the evolution of crop plants. To be beneficial to breeders it should be multidirectional; intercrossing of peripheral lines might then form ‘springboard’ populations allowing further advance from selection plateaux.6. As the boundaries of cultivation are extended so the need will increase for plant populations with inherent stability of performance. Few crops are yet amenable to the production ofF1hybrid seed, so polygenes should be manipulated to form balanced, heterozygous adaptive complexes. Systems of selection must be refined to deal effectively with the heterogeneous populations from which such complexes are likely to arise.7. Studies on composite crosses and related projects tie in well with other work on selective forces in adaptation. New allopolyploids offer hope in some plant groups, but induced mutation is of restricted value.8. Cryptic variation and selection can be important in many breeding methods, and should be considered when dealing with quantitative aspects.9. Changes in locality can reveal startling amounts of unsuspected variability: such ‘environmental segregation’ could be harnessed, for example, by alternation of selection centres when breeding for general adaptability.10. Persistence of variation, which may occur through selection favouring hetero‐zygotes despite strict inbreeding, is also considered in terms of heterozygous gene complexes. Tightness of linkage is the key to the durability and behaviour of super‐genes, and analysis of the simpler ones could lead to better understanding of the vastly complicated systems controlling fitness and

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1969.tb00827.x

出版商:Blackwell Publishing Ltd    

年代:1969

数据来源: WILEY

摘要:

Summary1. The term ‘meiobenthos’ (or ‘meiofauna’) has been used in the literature in a broad sense to designate collectively small individuals, mainly metazoans, which could be separated from the larger animals by fine sieves of about i mm. or 0.5 mm. mesh. It has been variously defined in terms of the sampling or sorting technique employed. Thus used it indicates a purely statistical category including temporary members which are juvenile stages of the macrofauna, as well as permanent members –species of small adult size.2. The term has also been used in a narrower sense, referring mainly to the permanent members and restricted to particular animal groups such as Nematoda, Harpacticoida, Gastrotricha, Kinorhyncha, Tardigrada, etc. This usage designates a more natural grouping of small organisms with certain biological characteristics as well as sampling requirements in common, which distinguish them from both larger and smaller organisms.3. Special collecting and sorting methods are required for meiofauna. Small core tubes are most convenient for collecting, while sorting, often with stained samples, is usually done by elutriation, decanting or repeated sieving. Other techniques are discussed.4. In the intertidal zone total population numbers range from IIXIO3to more than 16 × 106per m.2On the majority of grounds nematodes are the most numerous group, with harpacticoids second. The highest densities are usually on the softer deposits in sheltered areas.5. Subtidally the numbers on the shelf range from 4 × 103to 3.2 × 106per m.2and again nematodes and copepods are the main animals, with soft deposits richest. Density declines towards deep water but even in the abyssal zone numbers range from 1.6 to 17.0 × 104per m.2.6. Intertidal distribution of meiofauna is determined by temperature and salinity and also by the grain size of the deposit which affects the interstitial space, water content, and availability of food and oxygen.7. On intertidal muddy deposits the fauna is confined mainly to the upper few centimetres. The main controlling factor on some sediments seems to be reduced oxygen due to poor drainage, and on others the close packing of the particles leading to reduced interstitial space. This restriction to surface layers exposes the fauna to extremes of environmental conditions which in turn limit the intertidal distribution.8. On intertidal sand, with interstitial space extensive and drainage better, life can be found deeper than 1 m. below the surface and the fauna can migrate vertically to aggregate in areas of optimum conditions. Oxygen is again of prime importance, but migrations are also caused by salinity changes and by interaction between species.9. On subtidal deposits the fauna is much restricted to the superficial layers, but data on sandy grounds are sparse.10. On beaches, marked seasonal fluctuations have been observed in the meiofauna, usually correlated with temperature and reproduction. In subtidal areas these changes are much less obvious.11. The main predators on meiofauna are small fish and certain meiofauna groups such as Hydrozoa, Turbellaria and Nematoda. The meiofauna, apart from these predatory groups, feeds largely on algae and organic debris, with bacteria playing an important part in its nutrition.12. Many meiofauna species have generation times, from egg to egg production of about 1 month, and at least 2–4 generations are produced annually.13. In subtidal grounds the meiofauna is from 30–190 times greater numerically than the macrofauna but intertidally the ratios are much more varied. Interaction between the two size categories may partly explain the ratios, but the apparently greater ability of meiofauna to flourish in areas of environmental stress is perhaps also relevant.14. Certain species may be associated with particular types of deposit, but because of their small size, the distribution of meiofauna individuals should perhaps bethought of in terms of microhabitats rather than communities in the macrofauna sense. However, it may be possible to increase the precision of benthos community descriptions by including selected meiofauna species.15. A large proportion of meiofauna individuals appears to be free from predation by animals of higher trophic levels. The meiofauna thus seems to be at the top of a food chain. It will play an important part in recycling nutrients, and in some areas may compete with larger

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1969.tb00828.x

出版商:Blackwell Publishing Ltd    

年代:1969

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1969.tb00829.x

出版商:Blackwell Publishing Ltd    

年代:1969

数据来源: WILEY