摘要:

Summary1. This article deals principally with recent experimental work on crustacean nerve‐muscle systems. A number of observations have been made which are likely to affect our views of synaptic function in vertebrates. Neuro‐muscular transmission in Crustacea, and possibly other invertebrate animals, involves three factors which in vertebrates are regarded as characteristic properties of reflex centres: convergence of excitatory impulses from different nerve fibres on to one effector cell, prolonged facilitation of motor impulses, and interplay of excitatory and inhibitory impulses. It can now be regarded as established that different nerve fibres supplying the same muscle fibres are responsible for inhibition of contraction, and for fast and slow types of facilitation (Wiersma, 1941).2. Many invertebrate motor systems are distinguished by the extremely small number of nerve axons which supply large and powerful muscles. The question arises how the speed and strength of such muscles is regulated. In vertebrates regulation is brought about by a play of numerous motor units, involving within each unit a ‘non‐stop’ transmission from the motor nerve cell to the ends of the muscle fibres. This mechanism would not permit fine gradation in muscles which are only provided with one or two motor axons. Two types of invertebrate motor systems have been described: (a) Certain specialized muscles, e.g. the ‘jet‐propelling’ mantle muscle of cephalopods, which act as units, and, like the vertebrate heart or the electric organ, invariably give a maximum response. (b) Many other muscles, e.g. in all crustacean limbs, which are capable of extremely fine gradation, despite their sparse nerve supply. In these latter muscles, the motor unit response has been broken down into much smaller ‘quanta’ by means of a barrier system and of facilitation at the nerve‐muscle junctions.3. The nature of neuro‐muscular facilitation in arthropods is discussed. There are certain analogies with partly curarized vertebrate muscle, in the existence of junctional barriers in both cases, which can be overcome by the summated action of several successive nerve impulses. There are, however, some important differences. In curarized vertebrate muscle, facilitation involves the recruitment of an increasing number of muscle fibres, each of which contributes a propagated maximum response. In normal crustacean muscle, facilitation can occur in the absence of any propagated muscle impulses and is then due to a progressive growth of local electrical and mechanical responses in the vicinity of the nerve endings. Electric recording reveals the existence of ‘end‐plate potentials’ (e.p.p.'s) which increase in size with each successive motor nerve impulse, and, at high rates of stimulation, summate to a plateau several times higher than their initial amplitude. These non‐propagated action potentials are accompanied by a local contraction whose rate and strength can be controlled continuously by the number and frequency of the motor impulses.4. In addition to these graded local responses, crustacean muscle fibres can be thrown into propagated activity, both by direct and by nerve stimulation. If the frequency of the motor nerve impulses is raised, e.p.p.'s summate and at a certain level propagated spike potentials are initiated, associated with vigorous twitches of the whole muscle fibres. Thus, the motor response in Crustacea is either local or propagated, depending upon the rate of the nerve impulses. Both kinds of response have been observed in the excised limb as well asin situ.5. There are enormous differences in the rate and power of facilitation in different muscles. Fast and slow systems have been distinguished, according as facilitation of e.p.p.'s takes several milliseconds or about one second to complete. Many muscle fibres receive branches of two motor axons, one providing a fast, the other a slow facilitation system. Thus, the rate and intensity of a muscle fibre response can be regulated, not only by the number and frequency of nerve impukes, but also by a ‘switching’ of axons.6. If the inhibitory nerve fibre is stimulated in the absence of motor activity, no electrical or mechanical change can be detected in the muscle. The inhibitory impulse is capable of breaking the transmission of motor activity at two separate stages:(a)between the motor nerve impulse and the production of the e.p.p. (a‐action), and(V)between the e.p.p. and the local contraction of the muscle (j3‐action). The two effects can be dissociated by varying the time interval between motor and inhibitory impulses. If the inhibitory impulse precedes the motor by a few milliseconds, it interferes with the production of the e.p.p. The inhibitory impulse can be made to arrive too late to affect the e.p.p. and yet in time to prevent local contraction. In both cases, the inhibitory influence is restricted to the vicinity of the motor nerve endings. Once a propagated impulse has been initiated in the muscle fibre, it cannot be stopped by an inhibitory impulse, and a twitch invariably occurs.7. Certain gaps and controversial matters in our present evidence are discussed.(a)The relative functional importance of local and propagated responses in invertebrate muscle remains to be cleared up.(b)The relation between structure and function of the various types of nerve‐muscle contacts in Crustacea, and their distribution within individual muscle fibres, require much further investigation,(c)In Crustacea as well as in many other invertebrate animals there is as yet no information about the nature of the processes, or chemical agents, involved in the facilitation and production of the ‘end‐plate

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1949.tb00568.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

摘要:

SummaryWork on the early appearance of the shoot apex in the embryo has been carried out along two lines. First, by formal descriptions of early segmentation, and, secondly, by the recognition of zones within the embryo which foreshadow the tissue systems of the seedling. The latter is the more fruitful approach when problems of wide morphological interest are being investigated. A study of this kind has suggested, for example, that the shoot is homologous with the stele of the root, the outer tissues of the root having no counterpart in the shoot.The tunica‐corpus concept can now be accepted only with considerable reservations. The depth of the tunica may fluctuate during development, and the tissues derived from an apical layer may vary quite capriciously. Nevertheless, stratification of the apex is considered a characteristic feature of angiosperm shoot apices. Complementary to this stratification is a zonation within the apical meristem corresponding to that of gymno‐sperms. That is to say, the central cells of the apical meristem are larger with less dense protoplasm than the surrounding peripheral cells. The term initiating cell is proposed for this stage in cellular development.The initiation of leaves and the delimitation of provascular meristem below the apex are briefly described, and the bearing of these facts on the relationship between stem and leaf is discussed. The shoot is regarded as a unit, and it is concluded that the angiosperm leaf may be regarded more properly as an outgrowth of the stem than as a modified branch system.Some recent work on phyllotaxis is discussed, it being shown that an experiment designed to demonstrate that the position of leaf primordia is independent of the pattern of vascular tissue below them is inadequate for this purpose. An interesting theory of phyllotaxis recently propounded by Plantefol fits many facts of apical development.Little is known of developmental changes during vegetative growth. Those which precede flowering are better known, and seem to be explicable in terms of a redistribution of growth. These changes in the position, rate and direction of cell divisions bring about changes in the organization of the apical meristem in reproductive apices. With the cessation of growth in length, the central zone is replaced by cells of the same nature as those of the peripheral zone, which then extends uniformly over the apex. Gregoire's view of the irreducibility of the reproductive and vegetative apices is examined. Most of his evidence has proved ill‐founded, but a difference in organization remains. This change in organization is regarded as one of a series which occur in the development of the shoot; only with the accomplishment of all stages in its development is a shoot com

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1949.tb00569.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

摘要:

Summary1. The article deals mainly with the last decade, although earlier work is briefly surveyed.2. In general an effect of auxins in increasing cell wall extensibility has been verified, although there is no consistent correlation of growth rate with either elastic or plastic extensibility. There is a general tendency to regard the older methods of measuring extensibilities as too drastic and therefore misleading.3. Two phases in cell elongation have been recognized, namely, an initial phase of pure wall stretching and a subsequent phase of wall growth by intussusception. Auxins may stimulate the first phase by increasing the swelling capacity of the intermicellar colloids (pectins) through the intermediary of the living protoplasm. The phase of intussusception may also be affected. Most recent theories suggest that these actions are effected by a disturbance of the intramolecular forces concerned in the formation and maintenance of the molecular lattice of the young cell wall (Diehlet al.,Burström).4. The maintenance of cellular osmotic pressures during active elongation has suggested an active solute uptake mediated by an auxin‐augmented respiratory process as the cause of cell elongation (Commoner). The results do not justify these conclusions.5. The early claims of J. Bonner of a stimulation of respiration by auxin inAvenacoleoptiles have been verified under certain experimental conditions. It seems possible that the degree of this stimulation may be influenced by the presence of various components of the Gyorgyi C4dicarboxylic acid respiration cycle, which occurs inAvenacoleoptiles. The use of monoiodoacetic acid as a differential inhibitor is misleading and results so far afford no proof of the action of auxins as a coenzyme in the C4‐acid cycle.6. Auxin has no effect on thein vitroactivity of a wide range of dehydrogenases extracted fromAvenacoleoptiles or on catalase and some plant oxidases. The heightened dehydrogenase activity of auxin‐treated coleoptiles may be due to anin vivoactivation (Berger&Avery) or to an active enzyme synthesis.7. The production of simple sugars from carbohydrate reserves after auxin application is not due to an activation of diastase or to its ‘elution’ by auxin from a protective colloid. This shift in the starch → sugar equilibrium is probably secondary to the main action of auxin.8. The original postulates of Koepfli, Thimann&Went concerning the minimum molecular requirements for auxin activity have been verified by all recent work. Most evidence supports the view that auxin is effective only in the undissociated state and not as an anion. It seems unlikely from polarographic investigations that auxins act as respiratory coenzymes by virtue of a reversible oxido‐reduction of the essential double bond in the ring. Physiological activity is probably associated with an essential high (lipophilic) surface activity of the ring and its double bond(s), the polar ‐COOH group being most effective when orientated perpendicular to the plane of this ring.9. An auxin‐induced lowering of the protoplasmic structural viscosity has suggested a dissociating action on the complex plasma proteins (Northen). The resulting heightening of cell activity could originate the many diverse phenomena evoked by auxins. Such an effect on the ‘cytoskeleton’ is supported by the antagonistic action of certain quinonoid compounds (Jones). The acceleration of protoplasmic streaming by auxins and the interaction of malate still needs verification. More direct experiments are necessary to check the suggestion (Veldstra) that auxins regulate permeability, and therefore growth, after adsorption at a lipoid interface in the cell membrane.10. Studies on the interaction of the auxins have thrown doubt on the ‘hemiauxin’ hypothesis of Went and suggest their action as coenzymes (Skoog). Further experimental data are required. Interactions with other cell metabolites are relatively unexplored, but offer profitable lines for research.11. The general conclusion drawn is that most recent theories tend to be too restricted and superficial, and, unless auxin exerts a number of discrete effects in different cell systems, we must look deeper for a more fundamental action (e.g. the theories of Northen) to explain the whole r

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1949.tb00570.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

摘要:

SummaryA comparative study of the Phaeophyceae and of the Chaetophorales (Chlorophyceae), which exhibit a development parallel to that of the simpler members of the former group, shows that, both as regards increasing vegetative specialization and elaboration of the life cycle, definite trends can be recognized in the evolution of the larger multicellular Brown Algae. The heterotrichous state, which is manifested in its simplest forms in the Chaetophorales and in the Ectocarpaceae, is met with in the juvenile stages of all the numerous remaining Ectocarpales. In these the erect filaments either become aggregated in various ways to form pseudo‐parenchymatous thalli, in part of some size, or undergo septation in diverse planes to give simple parenchymatous forms. Anatomical specialization, for example in such a series asPhloeospora‐Stictyosiphon‐Scytosiphon,arises among these parenchymatous forms by the restriction of cell division to the peripheral layer of the thallus and the marked elongation of the inner cells. The sporophytes ofChorda,formerly included in Ectocarpales, differ from those of the more specialized parenchymatous members of the latter essentially only in the disappearance of a prostrate system in the juvenile stages and in the presence of ‘trumpet‐hyphae’.Chordathus constitutes a connecting link between them and the series of large and elaborate forms characterized by marked surface development comprised in the remaining Laminariales. It is postulated that the ancestors of the Laminariales were simpleStictyosiphon‐liketypes which developed in one direction intoChordaand in another into a form likeLaminaria saccharinafrom which all other Laminariales can be derived.The Fucales represent a different and more highly specialized evolutionary line in which external elaboration has taken place by branching, whereas in the Laminariales it results from splitting of a primarily entire thallus. The anatomical resemblances between the two orders are possibly due to evolution from a commonStictyosiphon‐likeancestor. There is some evidence that the apical growth of Fucales may have been secondarily derived from the intercalary growth usual among Ectocarpales. In the possession of tetrahedral apical cells, whose segments contribute largely to the building up of the younger parts, and in the differentiation of compact tissues, with little mucilage‐formation and with hyphae developing only in the oldest parts, the bulk of the Fucales (Cystoseiraceae, Sargassaceae) show an anatomical organization approximating more to that of simple archegoniate plants than to that of the Laminariales. The Fucaceae, usually regarded as typical of the whole order, diverge in their four‐sided apical cells, their prevalent dichoto‐mous branching, and the copious mucilage‐development, the last feature being possibly related to the littoral habitat.The change from a haploid to an isomorphic life cycle, previously established in various Chlorophyceae, is now known to occur also in the heterotrichous Chaetophorales. A similar change no doubt occurred among Phaeophyceae, but no living haploid forms are known in this class. The ancestral haploid types probably bore only plurilocular sporangia producing either zoospores or gametes. When an isomorphic phase‐alternation became established, these two structures will have persisted on the respective phases, while on the diploid plant a new organ, the unilocular sporangium which became the seat of meiosis, was differentiated, perhaps arising by modification of a plurilocular sporangium. Plurilocular and unilocular sporangia remain associated on the diploid phases of most Ectocarpales and some Sphacelariales, but in other Brown Algae the former have disappeared.While the comparatively simple Ectocarpaceae have an isomorphic life cycle, the majority of the Ectocarpales have a heteromorphic one, in which the sporophytes have alone been elaborated, while the gametophytes appear as diminutive filamentous growths. The same condition is met with in Desmarestiales and Laminariales where, however, accompanying the greater vegetative specialization the isogamy of Ectocarpales is replaced by an oogamous sexual process. The Sphacelariales, whose simpler members are related to the Ectocarpaceae and in which both phases have undergone equal elaboration, show a similar progression from isogamy to oogamy in the more highly differentiated types. It is thus possible to trace an advance from Ectocarpales to Laminariales in respect of(a)increasing vegetative specialization including suppression of the prostrate system in early stages,(b)adoption of a heteromorphic life cycle, and(c)evolution of an oogamous sexual process.The Fucales stand apart from other Brown Algae, not only in vegetative organization, but also in the occurrence of sex organs on the diploid phase, their development within conceptacles, the presence of more than one ovum in the oogonium, and the elimination of gametophytes. The Fucales appear in nearly every respect to be more advanced than other Phaeophyceae and, although they no doubt originated fromEctocarpus‐likttypes, it is more difficult to trace their mode of origin than in the case of the Laminariales. There is some reason to believe that their distinctive life cycle may have originated from the tendency of the normally asexual swarmers of the unilocular sporangia to behave as gametes which has been established in a number of the less specialized Brown Algae.The progressive elaboration, which can thus be traced among Brown Algae, probably shows considerable parallelism with the changes which occurred during the evolution of archegoniate plants, although the former exhibit many peculiarities of their own (e.g. the meristematic surface layer, hyphae) which appear already

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1949.tb00571.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1949.tb00572.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY