摘要:

Summary1. Among animals which can be transported by air currents three main types have been distinguished:(a)large forms, from a length of roughly one decimetre down to a few millimetres;(b)small forms varying between some millimetres and about 1/10 mm.;(c)animals of a microsize, smaller than 1/10; mm.2. Large organisms are usually distributed by wind in typhoon and hurricane areas alone. During the Glacial Period, however, cyclonic disturbances proceeded along other routes, in this way creating distributional paths which are now no longer in use.3. Small organisms have considerable possibilities of distribution by convection air currents and winds at moderate altitudes. Examples are given of such distribution over great distances. But as the animals are often strictly specialized ecologically (herbivores, parasites, etc.) they have particular difficulties to overcome in their new surroundings.4. Numbers of micro‐organisms are constantly being driven up into the air to return again to earth in rain showers or downward air currents.5. Micro‐organisms are very resistant to unfavourable factors met with in the air‐sea. Some may be distributed through the air in an anabiotic stage. Being often hermaphrodite or parthenogenetic, many of them can give rise to progeny from a single individual which happens to arrive in suitable surroundings. Their resistance to low temperature, low barometric pressure and drought is superior to that of all other organisms. Nevertheless, in comparison to larger forms, they are very sensitive to radiations, especially ultra‐violet, which seem to check their distribution more than that of larger forms.6. The explanation of the fact that the microforms do not flourish everywhere is found in their very strict biotopical specialization. But wherever a biotope suitable for a certain micro‐organism exists, that organism will appear there as soon as sufficient time has elapsed to allow it to be transported through the air and to settle in the locality. The numbers which arrive are a factor in the survival and establishment of the invading species.7. No geographical borders or barriers exist for microforms. They are often cosmopolitan, or else regionally distributed around the whole globe in certain climatic belts.8. Under favourable conditions, especially in humid air, the harmful influence of radiation is diminished, and microforms may be transported alive by winds over greater distances than in clear and dr

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1948.tb00459.x

出版商:Blackwell Publishing Ltd    

年代:1948

数据来源: WILEY

摘要:

Summary1. Micro‐organisms are not evenly distributed throughout the soil, and different local soil variations and horizons are inhabited by populations differing in activity and number. The root regions of green plants constitute an important group of these microhabitats, and the population of the root regions of some plants have been examined experimentally. In the root region a population differing from the general soil microflora is found, and here increased rates of activity of certain soil processes, changing the availability of essential plant foods, have been described. Micro‐organisms of many types are subject to this rhizosphere effect, and roots differing in age and in genetic origin exert different effects on various types of micro‐organisms.2. The phenomena examined in mycorrhizal studies can be grouped naturally with general rhizosphere and root region phenomena. They differ essentially only in the relative dominance of one or few particular members of the root flora, but in typical cases of ectotrophic mycorrhiza the presence of the dominant fungus is associated with morphological changes in the root. Part of the confusion in the presentation of experimental results, and in the theoretical discussion of these results, is due to lack of appreciation of the general and widespread nature of rhizosphere effects and their dependence upon the nature and physiological state of the roots.3. Similar hypotheses, based on experimental results, have been put forward to explain both the general case of association of roots and micro‐organisms and the special cases of ectotrophic mycorrhiza. The excretion of substances causing stimulation of microorganisms, such as amino‐acids and vitamins, of food materials such as sugar, and of inhibiting substances, have all been suggested. Root excretions and extracts have been shown to affect the growth both of members of the flora of the root region and of mycorrhiza fungi.4. No explanation is at present available as to why there is intercellular penetration, morphological change and the formation of composite organs of root and fungus, in typical cases of ectotrophic mycorrhiza; nor is there again any explanation of the variability of such associations with soil conditions.5. The effect of the rhizosphere flora on the growth of the host plant has been examined. There may be a stimulation of growth by non‐mycorrhizal rhizosphere fungi and bacteria. Similar stimulation of growth by mycorrhizal fungi has frequently been claimed. A great majority of observers agree that under some conditions the growth of tree seedlings bearing mycorrhizal roots greatly surpasses that of those lacking these roots. Very few experiments on this point are, however, quite satisfactory, owing to the difficulty of providing adequate controls. These few, together with the large body of imperfect experiments, are sufficient to enforce the acceptance of the fact that mycorrhizal fungi frequently stimulate growth.6. The completeness of the living fungal sheath, and its intimate connexion with the root cortex in fully developed mycorrhizas, must be a fundamental consideration in the elaboration of hypotheses to explain the effects of infection on the host. Many of the existing theories do not take this sufficiently into account, because variations of the morphology of mycorrhizas between experimental treatments have frequently not been sufficiently described. The existing hypotheses are not all mutually exclusive, but cannot be further co‐ordinated or extended except by means of experiments more perfectly controlled than those yet described. Moreover, the activities or organisms associated with the true mycorrhiza‐former require examination. Recent reports of the growth of excised mycorrhiza roots cannot be fully accepted until the results have

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1948.tb00460.x

出版商:Blackwell Publishing Ltd    

年代:1948

数据来源: WILEY

摘要:

Summary1. The muscle fibre is regarded as a bundle of myofibrils, embedded in a viscous matrix, the sarcoplasm, the whole being surrounded by a delicate sheath, the sarcolemma.2. The sarcolemma is a thin, apparently structureless membrane. Preliminary study with the electron microscope suggests that it may be less than o‐i /u. thick. No fibrils can be seen with the electron microscope, but a large number of small nodules lie scattered over the membrane. The sarcolemma is probably not collagenous in nature.3. The sarcoplasm is a viscous, protein‐containing gel which surrounds the myofibrils. Mitochondria, glycogen and fat droplets are scattered throughout the sarcoplasm. Several enzymes have been isolated from what is believed to be the sarcoplasm. This suggests that the latter may be the site of important metabolic activity. The question of sarcoplasmic contractility is not yet settled. There is good evidence that it may occur in certain cells (myoblasts, striated fibres of tadpoles and insect larvae).4. The evidence in favour of the existence of myofibrils as contractile units is discussed. The fact that individual myofibrils or groups of myofibrils can be made to contract independently, and sometimes asynchronously, is strong support for this view.5. The electron microscope shows that the myofibrils consist of relatively straight protein chains. Several different striations(A, I, Z, M, N, H)can be recognized. There is no gross difference in orientation between the protein chains in the various bands, nor is there any evidence of a spiral structure.6. Several aspects of Szent‐Gyorgyi's work are discussed. While this work has led to a better understanding of the muscle proteins, the theories of muscle structure arising from it have found no general acceptance.7. Other theories attempting to account for the existence of the cross‐striations are reviewed. It is suggested that there is a periodic distribution of different chemical substances in the different bands. This may affect the physical, mechanical, and chemical properties of the myosin in those bands, and may account for the differences in birefringence between theAandIbands.8. Evidence is presented against the existence of aZ ‘membrane’ or other transverse membranes uniting the myofibrils. No structural connexion is believed to exist between the sarcolemma and the underlying sarcoplasm and myofibrils. The relationship may be a purely viscous or frictional one.9. Reasons are suggested for the perfect alinement of the striations in neighbouring myofibrils. A full explanation may only be possible as a result of a better understanding of intermolecular and collo

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1948.tb00461.x

出版商:Blackwell Publishing Ltd    

年代:1948

数据来源: WILEY

摘要:

Résumé1. Le contrdle hormonal de la différentiation du sexe admis par Bouin&Ancel en 1903 a trouvé une belle confirmation dans 1'observation et ľinterpretation des free‐martins (Lillie, 1916; Keller&Tandler, 1916).2. Les greffes orthotopiques effectuées par Humphrey sur ľAmbystome et les greffes intracoelomiques de Wolff sur ľembryon de Poulet établissent la production de sécrétions morphogènes par les gonades embryonnaires; Jost a également obtenu un cas positif chez ľembryon de Lapin.3. La parabiose entre larves indifférenciées de Batraciens montre aussi que les gonades des deux sexes produisent durant leur différentiation sexuelle des substances capables ďagir sur les gonades du sexe opposé (Burns, Humphrey, Witschi).4. La castration effectuée avant le début ou pendant la différentiation sexuelle somatique montre que celle‐ci obéit à un contrôle humoral, chez le Triton (de Beaumont) et chez le Lapin (Jost). Les résultats négatifs de Moore sur ľOpossum sont discutés; ils s'expli‐quent peut‐être par la date trop tardive de la castration; chez le foetus de Lapin, pour supprimer tout développement prostatique, par exemple, la castration doit en effet inter‐venir avant ľapparition des premiers bourgeons prostatiques.5. ľextraction chimique ďhormones des gonades fœtales a pu étre réalisée chez le Veau, mais n'apporte encore que des données fragmentaires.6. On peut voir un argument parlant en faveur de la production ďhormones par la gonade fœtale dans Taction des hormones sexuelles exogènes sur les stades embryonnaires.7. ďaprès les expériences réalisées jusqu'a present, ni ľhypophyse, ni les surrénales, ni les thyroïdes ne semblent jouer de rôle sensible dans la différentiation sexuelle.8. La différentiation sexuelle des gonades commence par celle de leurs constituants somatiques qui libèrent des inducteurs corticaux et médullaires. C'est la prédominance de ľun de ces inducteurs qui oriente 1'evolution de la gonade dans le sens femelle ou mâle (Witschi).9. La nature des substances inductrices de la gonade fait ľobjet de points de vue opposés. Pour Witschi elles seraient entièrement différentes des hormones sexuelles de ľadulte. Pour ďautres auteurs elles seraient soit identiques (Dantchakoff) soit de même nature (Wolff) que les hormones sexuelles. Les faits expérimentaux ne permettent pas encore ďapporter une conclusion certaine.10. ďaprès les expériences de castration de Jost le testicule foetal joue un rô1e essentiel dans la différentiation sexuelle somatique des Mammifères. Le testicule provoque le développement des glandes prostatiques, des organes génitaux externes masculins et des canaux de Wolff; il inhibe les canaux de Müller. Dans les castrats des deux sexes, et dans les femelles normales, les canaux de Wolff régressent et les canaux de Müller persistent. Ces résultats, très voisins de ceux que postulait Wiesner, ne prouvent cependant pas que ľovaire foetal ne produit aucune sécrétion.11. II n'est pas encore possible de dire si les sécrétions génitales qui dirigent la différentiation sexuelle

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1948.tb00462.x

出版商:Blackwell Publishing Ltd    

年代:1948

数据来源: WILEY