摘要:

SUMMARYThe ideal classification of a group of organisms is one which takes account of all that is known about the group. It should, therefore, include information about its evolution. There are two aspects of evolutionary theory, (1) that which concerns the past history of the group and which can only be known with certainty from the fossil record, (2) that which concerns present‐day organisms and is expressed in the differences between advanced and primitive types. As regards angiosperms, there is only faint hope that their history will ever be known in sufficient detail for it to be made the basis for a phylogenetic classification. The extent to which the second aspect of evolutionary theory can be incorporated into classification depends on the ability of botanists to assess relative advancement.The first step in deciding which organisms are primitive is to discover which characters are primitive. For this purpose, a number of doctrines have been employed, often without due consideration of their philosophical justification. The doctrines of ‘conservative regions’, of ‘recapitulation’, of ‘teratology’ and of ‘sequences’ are unreliable, since their application is entirely subjective; in the absence of evidence from some other source, it is impossible to judge whether they are appropriate to the structures concerned. The doctrine of’ association’ is unsatisfactory because of the assumptions which have to be made before it can be applied. The doctrine of ‘the basic ground plan’, if applied in the strict sense, is relatively unprofitable; if applied loosely it is dependent on subjective judgement and if applied in the ‘Gestalt’ sense leads only to figments of the imagination. The doctrine of ‘correlation’ has been applied with success to the evolution of secondary wood in dicotyledons and to other characters, both floral and vegetative. Its results are of value since the subjective element is reduced to a minimum and, by its means, it is possible to assess the relative advancement of present‐day taxa.There have been many attempts to represent in two dimensions the phylogenetic classification of the angiosperms, but most of these are at fault because of a misconception of the tree of evolution. The ideal representation is visualized as resembling a target whose concentric rings correspond with relative advancement. On this are disposed the various families in such a way that closely similar ones are close together within circles of affinity, while, at the same time, their radial position corresponds to their relative advancement. In this way the second of the two aspects of evolutionary theory is catered for, and the classification may, therefore, be described as a phylogenetic one. The history of the group, if it becomes known from the fossil record, may then be inserted as a tree of evolution in a plane (corresponding to evolutionary time) which is perpendicular to tha

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1956.tb01550.x

出版商:Blackwell Publishing Ltd    

年代:1956

数据来源: WILEY

摘要:

SUMMARYThe crown pattern of the tooth is essentially that of the surface of the dentine (dentine‐enamel junction), modified by the deposition of enamel which may be uneven in thickness. The dentine‐enamel junction preserves in the completed tooth the form of the membrana praeformativa, the basement membrane of the inner enamel epithelium of the enamel organ. Folding of this membrane creates the crown pattern.The inner enamel epithelium is subjected to pressure from both sides. On the basal side there is the rapidly growing mesenchyme of the dental papilla, and on the occlusal side there is the stellate reticulum, which swells by the accumulation of fluid. The stellate reticulum prevents distortion of the epithelium by growth of the papilla, and thus ensures that folding of the epithelium is due to its intrinsic growth pattern. This makes for more accurate control of the crown pattern, the details of which are of importance in the function of chewing.The enamel knot is a region of the inner enamel epithelium from which cells are contributed to the stellate reticulum. It represents the tip of the primary cusp. The enamel cord (‘enamel septum’) which consists of cells which are in process of transforming into stellate reticulum, has been confused with two other structures that develop later: a cleavage septum, preparatory to the formation of crown cementum, and an epithelial septum, found in marsupials and crocodilians. The epithelial nodules of monotremes are probably degenerate relics of an epithelial septum.The inner enamel epithelium is a diaphragm passing across the interior of the dental follicle, and folding to adapt its increased area to a confined space. The cement organ, which in some mammals develops from the follicle, probably plays no part in the deformation of the epithelium, but the follicle as a whole may be subject to compression by adjacent follicles.A cusp is a centre of precocious maturation of the cells of the inner enamel epithelium. Here growth ceases (perhaps after a transitory burst of mitosis) and eventually the hard tissues are deposited. The process starts at the tip of the cusp and extends basally, so that growth continues longest in the valleys, intensifying the crown relief.Dens in denteis due to retarded maturation of an area of the enamel epithelium.Throughout the development of the crown there is a marginal zona cingularis, where growth continues. The crown pattern depends upon the position and the stage of growth in which cusps are differentiated from the zona cingularis, by accelerated maturation of groups of cells. Cusps which appear late in development stand low on the crown and frequently form part of a cingulum. Changes in the timing of cusp formation play an important part in serial modifications of pattern, as well as in phylogeny.Ridges are probably produced by tensions set up in the epithelium by the relative movement of cusps, owing to unequal growth or to changes in the shape of the follicle. They form in areas where growth has slowed down but the apposition of hard tissues has not begun.The lobes of the basal outline of the tooth are produced by growth centres in the papilla, which cause evagination of the follicle. Each growth centre is supplied by a bundle of blood vessels. The roots form in relation to these blood vessels, and so reflect the organization of the papilla. Hertwig's epithelial sheath grows between the follicle and the base of the papilla, the direction of its growth being controlled by the surrounding tissues owing to the absence of stellate reticulum on the basal portion of the tooth.There is no constant relation between cusps and roots, although marginal cusps frequently arise in association with lobes of the basal outline. The crown pattern results from an interaction between the growth pattern of the inner enamel epithelium and that of the papilla, the latter controlling the shape of the margin which limits the folding epi

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1956.tb01551.x

出版商:Blackwell Publishing Ltd    

年代:1956

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1956.tb01552.x

出版商:Blackwell Publishing Ltd    

年代:1956

数据来源: WILEY

摘要:

SUMMARY1In the lysogenic bacterium the bacterial virus exists as prophage. It is composed of desoxyribosenucleic acid (DNA) and occupies a defined site on the bacterial chromosome.2Normally the prophage divides in harmony with the nucleus of the bacterium, so that lysogenesis is hereditary.3Occasionally the prophage reverts from symbiosis to virulence, and multiplies at the expense of its host, producing a brood of free phage particles. This is believed to occur when the prophage becomes detached from the chromosome.4Free phage particles may be temperate (capable of reproducing lysogenesis in a sensitive bacterium) or virulent (unable to produce lysogenesis and multiplying only at the expense of the doomed host). Temperate particles greatly outnumber virulent particles, which are in fact mutants. Reasons are given for believing that all virulent phages (such as the T series of coliphages) originate in this way.5Free phage particles have a characteristic tadpole‐like structure of varying proportions. The tail and the covering of the head are composed of protein, which has specific antigenic characters. The contents of the head consist of DNA.6The protein envelope and tail of the particle are chiefly mechanical in function. The tail adheres to the host bacterium and breaches its covering membrane. The contents of the head are then injected into the bacterium. They constitute the infective element.7A temperate phage may produce lysogenesis or may multiply in the same way as a virulent particle. Various factors facilitate or interfere with the establishment of lysogenesis. There is evidence to suggest that some temperate particles are specially endowed to produce lysogenesis, while others can undergo only productive development at the expense of the host.8Virulent particles multiply only by productive development. The infecting virus displaces the genetic apparatus of the bacterium and directs the bacterial enzymes to its own ends. The nucleic acid of the bacterium, augmented if necessary by newly synthesized material from the culture medium, is broken down to nucleotide level or lower, and resynthesized into viral DNA. Protein is synthesized separately. Ultimately both are assembled into mature particles.9Lysogenic bacteria are immune to the phage they produce, and to allied types. Occasional virulent mutants are exceptions to this rule.10In certain cases temperate phage particles transduce genetic characters from one bacterium to another.11Bacterial species (the classical example beingSalmonella typhi) can be broken down into numerous ‘phage‐types’ by tests which exploit the hereditary immunity conferred by the various symbiotic phages. Alternatively, different races of a bacterial species can be recognized by isolating and identifying the symbiotic phage which the

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1956.tb01553.x

出版商:Blackwell Publishing Ltd    

年代:1956

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1956.tb01554.x

出版商:Blackwell Publishing Ltd    

年代:1956

数据来源: WILEY