摘要:

SummaryInsects have a haemocoele, in which the only tissue fluid, the blood, circulates. The blood consists of haematocytes and plasma.Most of the haematocytes are phagocytic leucocytes. These alter considerably in appearance at different stages of deuelopment, assuming many different forms. Respiratory corpuscles are never present.The phagocytes assist in breaking down obsolescent tissues. They are most active during the pupal period in endopterygote ikects, but also exhibit considerable activity at each moult in both endopterygote and exopterygote forms. Haematocytes take some part in producing internal membranes.The phagocytes serve to protect the insect's body against invasion, particularly by bacteria. Immunity against bacterial diseases can be developed, either naturally or by processes analogous to vaccination, and in immune insects the phagocytes attack the bacteria more rapidly and more successfully than otherwise. Metman parasites may be overcome by the haematocytes, which then surround the invader with a capsule. Parasites which are adapted to living inside specific insect hosts suffer no inconvenience from the activities of the blood cells. The capsules produced may even be necessary for the well‐being of these parasites.The plasma is a viscous fluid which may be coloured or colourless. Except for haemoglobin in solution in the plasma of chironomid larvae, no respiratory pigments are found. The fluid consists mainly of water, but the percentage of dry matter, the proportion of many of the various constituents and the reaction of the liquid shows considerable variations even in the same insect at different stages of development.The water in the plasma serves as a useful reserve, and allows the insects to withstand considerable desiccation. Under such circumstances the blood becomes more concentrated and more viscous.Food substances are transported and stored in the blood, which also carries hormones about the body.Respiration in insects is primarily the function of the tracheal system, but the blood has certain subsidiary functions. The tracheoles do not enter all tissues, and some cells receive their oxygen from solution in the blood, which serves as an intermediary between the tissues and the tracheae. Various structures have been described as being especially adapted to oxygenate the blood, but these have mostly been found to be unimportant. The osmotic pressure of the blood may govern the extent to which air extends into the tracheoles. Muscular activity liberates metabolites which increase the blood's osmotic pressure, and this removes some fluid from the tracheoles and draws the air up among the active tissues, thus increasing their oxygen supply.The blood sometimes contains poisonous substances; “reflex bleeding” may then protect the insect from attack.The plasma as well as the haematocytes may be concerned in producing immunity to bacterial infection.The blood has important mechanical functions. It is the means by which pressure is transferred from one part of the body to another, and thus assists in hatching and moulting. Desiccation, by reducing the blood volume, may interfere with these processes. The volume of blood also serves to maintain the body size, and if the volume is decreased during development an undersized adult may r

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1939.tb00933.x

出版商:Blackwell Publishing Ltd    

年代:1939

数据来源: WILEY

摘要:

Summary1. InDrosophilathe great majority of mutations are reversible in direction, and very commonly the “reverse mutation” appears to reconstitute precisely the original gene. The mutation from the normal to the abnormal type has been found usually to be a change from a more active to a less active condition of the gene, and is hence to be regarded as constituting, itself, a reversal of the original direction of evolution. The so‐called reverse mutation, in such a case, is really a mutation in the direction of past evolution. As the latter changes usually occur less readily than the former (except in the rare cases of highly mutable mutant genes), it is to be inferred that evolution proceeded contrary to the prevailing mutation pressure, and hence only by the aid of selection. Thus, with selection relaxed, a certain reverse evolution would tend to occur, so far as individual loci were concerned.2. The reversibility of most mutations is significant in the theory of the gene and of evolution in showing, first, that most mutations pre not mere losses of genes. Secondly, as Timoféëff‐Ressovsky, Zimmer and Delbrück (1935) have pointed out, the fact of their fairly high reversibility indicates that most mutations involve canalized reactions in a unified gene structure. For, in the case of radiation mutations at any rate, it can be shown that activation of any one of a large number of atoms, and of gene parts, results in sensibly the same mutation; that this principle applies even to reverse mutations indicates that precisely the same gene‐part as was struck in making the original mutation need not be struck again to make the reversal. This adds to the already existing evidence (Muller, 1932) that the gene mutation process involves a chain of reactions of which the primary one may even have lain outside the gene. It is at present uncertain whether or not this process involves a breakage and linear rearrangement of the chromonema similar to that occurring in obvious gene rearrangements but much more minute. This conception, which might require a revision of older notions of distinctly delimited genes, seems however to meet with some difficulties in explaining the comparative readiness with which apparently exact reversions may be produced.3. Considerations of mutation frequency show that a mere removal of selection for given genes would be attended by reversal of evolution, in the sense of loss of organs and of traits (e.g. pigmentation) dependent on organized reaction systems, in, geologically, a comparatively short time. In practice, there are difficulties in the way of such a stoppage of selection for given genes (or even for given alleles of them) inasmuch as there is a tendency for genes (and gene differences) to become increasingly pleiotropic in the course of evolution, through mutational transfer of functions. Stopping selection in respect to the major function of a gene, then, can only slowly, with a speed dependent on the recency with which the gene has acquired this function, lead to a genetic reversal, involving loss of this function. For such a process is now contingent upon the establishment of mutations in other genes, that accidentally happen to have the effect of taking over the secondary functions of the gene in question. Eventually, however, loss of any function must follow stoppage of selection for it, since an ever greater number of mutations must become established (both through selection for other functions, and through “drift”) that happen to disturb the organized reaction system whereby the given function is carried out.4. There can be apparent reversal of evolution with respect to given characters brought about by selection of mutations as well as by the genetic disintegration attendant upon mere removal of selection. But in neither case will the final product be genically identical or even very similar to the archetype. For the mutations of many different genes have equivalent end‐effects, especially in the case of the “small mutations”, which are more numerous and less harmful, and hence more apt to furnish evolutionary material than the large ones. For this reason the determination of the exact mutational path of evolution involves a large element of accident and, considered from a genic point of view, this path can never really be retraced, nor paralleled, in a second evolutionary sequence, nor can the same complex genic system be twice arrived at. The probability of the phenotypic similarity being thorough‐going will depend, among other things, on the length and complexity of the path to be retraced (or paralleled), and on the extent to which the reverse (or parallel) selection applies to all features at once (as a departure in one respect will tend to influence the conditions for other features).5. In the case of a longer, more complex path, there is an increasing role played by the complicating circumstance (mentioned in (3)) that some of the evolutionary steps have later acquired accessory functions that can no longer be dispensed with readily. At the same time their own genetic basis has spread so as to depend on an increasing number of genes, by a kind of genetic diffusion. These circumstances will often prevent even the appearance of retracement, so that an equivalent end‐result (e.g. adoption of fish‐like form by mammals) will obviously embody a quite different developmental mechanism or have a demonstrably different morphological or physiological basis. There will thus be a tendency for the old gene reactions of development and of physiology to persist in the basis of the life complex and only to be overlaid, as it were, by the newer acquirements, which would tend to develop later in ontogeny (recapitulation), and this principle would apply no matter whether these newer acquirements represented progressively different stages, or more or less phenotypic reversal to an earlier stage (a superficial reversion). Nevertheless, especially in the case of shorter paths (and more closely related organisms) there should often be the possibility both of parallel and reverse evolution involving a more nearly real retracement (forwards or backwards) of steps which, though genically somewhat different, embody essentially the same reaction changes, as judged from the point of view of ordinary embryology, physiology and morphology. For here the steps have not yet become so indispensable; moreover, the thousands of primary gene reactions are necessarily canalized into certain definite channels, that limit the possible effects of their change, as viewed by these methods, which still deal with characters standing relatively far from the gene itself. To some extent, then, reversal, as well as parallelism in evolution, maybe “real”, and to deny the homologies of the resultant forms is to make an arbitrary metaphysical distinction, created to suit the point to be proved. 6. The complex systems of chemical reactions upon which fertility and viability depend become changed by numerous mutations, differing in different populations, that become established in, geologically, a very short time. Some of these mutations, though first indifferent or only an asset, finally become necessary, through the later establishment of other mutations, which without them would be detrimental to fertility or viability. Thereafter, crossing between one of the populations in question and one like the original (or one likewise evolved from the latter) will result in hybrids that are sterile or inviable, owing to the action of these harmful mutant genes, inadequately balanced by the ones that had made them tolerable. Two groups of organisms which are not ordinarily allowed to cross with one another will thus automatically become increasingly immiscible, and their genic, chemical paths of evolution will diverge more and more. This will occur even in cases where their evolution is, from the phenotypic standpoint, strikingly parallel (owing to similar selective conditions and similar developmental and physiological bases for change), or where one of the groups undergoes a striking appearance of reversion towards the other, ore closely related groups, the parallelism or reversion may involve physiological and ontogenetic processes lying on a relatively deep plane of analysis. There must also be a hidden shift in the chemical, genic basis of a population which, phenotypically, remains relatively constant. But although these deeper‐lying genic changes may for a long time remain cryptic, they will eventually find more and more expression in “unnecessary” features of the life processes, discoverable by the chemist, the physiologist, the embryologist, or the morphologist, and an ever more different basis will be laid conditioning the future evolutionary possibil

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1939.tb00934.x

出版商:Blackwell Publishing Ltd    

年代:1939

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1939.tb00935.x

出版商:Blackwell Publishing Ltd    

年代:1939

数据来源: WILEY

摘要:

SummaryThe impetus which modern work on growth has given to wall studies promises to combine these into a branch of plant science of considerable importance to botanists in many fields. The present review represents an attempt to discuss, in relatively simple terms, the work which has led to present views on the structure of cellulose, the chief structural polysaccharide of the plant cell wall. To this end, the results of organic chemistry, X‐ray analysis, and of polarization optics, are mentioned, and data from other sources are summarized where necessary.Cellulose is shown to consist of long molecular chains, of which the links are β‐glucose residues bound together, in the chain, by primary valences. The linkage is universally of the 1:4 type. The chains themselves are in turn bound together by secondary valences, or van der Waals forces, into ill‐defined bundles which correspond to the classical micelle of Nageli. Incrusting substances like pectin and lignin are certainly deposited in the intermicellar spaces, as against hemicelluloses like xylan, which are equally certainly intramicellar. The evidence for the existence of micellar aggregates, both in the wall itself and in the cytoplasm, is discussed in some detail.In any one layer of the secondary walls in the majority of plant cells, the cellulose chains form a single spiral round the cell. With many fibres and tracheids this spiral retains the same sign, and approximately the same pitch, from layer to layer of any one wall. On the other hand, both the sign and th? pitch of the spiral varies widely in some cell types. This has been demonstrated most clearly in the algae (e.g.Valonia, Cladophora, Chaetomorpha), where the cellulose chains of odd layers, say, point in exactly the same direction, but at a considerable angle (some 83° inValonia) to the direction of those in the even layers.In spite of the increasing prevalence of the conception that the cellulose chains in the primary wall lie transversely, it is quite clear that in those cases critically examined they form a spiral resembling that in the secondary wall. It would seem that the development of a spiral in the secondary wall is not unconnected with its existence in the primary wall.The various theories of wall growth which are based on a transverse orientation of the cellulose chains, in the primary wall, must clearly be abandoned. Such evidence as there is points to a change, during growth, of the inclination of an original spiral.The nature, both of the cell wall and of the cytoplasm, is probably involved in wall deposition. Thus, a new layer deposited on a wall may be so influenced by the existing layers that the cellulose chains composing it lie parallel to those of the old wall. Yet the fundamental orienting mechanism must lie in the cytoplasm, for each wall originates as a new layer at cytokinesis. The evidence pointing to protoplasmic streaming as the mechanism involved is hardly convincing, and it seems not unreasonable to suggest that the configuration of the protein molecules at the cytoplasm‐wall interface may be

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1939.tb00936.x

出版商:Blackwell Publishing Ltd    

年代:1939

数据来源: WILEY

摘要:

SummaryInhibition of growth is widespread in the plant kingdom, Just as the promotion of growth is controlled by the growth hormones, comprising principally the group known as the auxins, so also inhibitions are, in many cases at least, due to the action of auxins.The application of very high concentrations of auxin inhibits the growth of shoots directly. Such concentrations retard the rate of protoplasmic streaming and are close to the range at which these substances are definitely toxic. Another effect which results from very high concentrations of auxins, and is perhaps of a more indirect nature, is the inhibition of the growth of parts morphologically above the point of the auxin application. Whether or not these phenomena of very high auxin concentration have any bearing upon normal growth inhibitions is still not clear. They may play a part in pathological inhibition.The inhibiting effect of the root tip upon the growth of the root may be readily imitated by application of very low concentrations of auxin, probably of the order of those present in the tip. Hence this inhibition, where it occurs, is due to the auxin coming from the root tip, A somewhat lower range of auxin concentrations accelerates root growth. These effects are observable on isolated roots. The response of roots to auxin can thus be represented by an optimum curve with the peak at very low concentration. The growth of roots which have been inhibited may become accelerated when the auxin is removed; this may lead to a definite acceleration of shoot growth.The inhibition of the development of lateral buds by the terminal bud of growing shoots can also be quantitatively imitated by the application of auxin in concentrations not much greater than those which are produced by the terminal bud. Hence this inhibition, the release of which comprises the basis of pruning, is due also to auxin, and in fact auxin is produced in rather large amount in young terminal buds of the majority of plants. The inhibition of buds by the leaves in whose axil they stand is similarly caused.Not only buds, but also young developing shoots may be inhibited by another shoot or by auxin in suitable (physiological) concentration. The adventitious outgrowths on fern prothallia are subject to a similar auxin inhibition. In tubers the inhibiting action of one bud upon another is, at least in part, also an auxin effect.Differences in bud inhibition between sympodially and monopodially growing trees, between normal and dwarf forms, or between related species of different growth habit, may be, and in some instances have been satisfactorily explained by, differences in the rates of auxin production or consumption. This concept unifies a scattered and diverse group of observations on plant behaviour.However, the paradox that auxin, which typically promotes growth by cell enlargement in shoots, should inhibit growth of buds and of roots, has not been satisfactorily explained. The divergent views on bud inhibition have engendered nine theories. The principal point at issue is whether the inhibition is due to the auxin itself or to some effect of auxin on the production or movement of other substances. None of these theories is entirely adequate to explain both the inhibition of buds and that of young growing shoots, though it is still possible that one of four may be established by further study. It is pointed out that the inactivation of auxin in inhibited parts may play an important role which has not yet been considered.

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1939.tb00937.x

出版商:Blackwell Publishing Ltd    

年代:1939

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1939.tb00938.x

出版商:Blackwell Publishing Ltd    

年代:1939

数据来源: WILEY