THE DYNAMIC APPROACH TO PLANT STRUCTURE AND ITS RELATION TO MODERN TAXONOMIC BOTANY

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Abstract
Summary: A truly dynamic approach to plant structure has been a matter of the past century. To Linné the species was something ‘established in the beginning’, and each specimen was made up of distinct ‘species’ of organ. Some conflict must exist between taxonomy, with its methods dating from Linnean times, and other more recently formed branches of botany. When von Mohl and others had elucidated the finer structure of plants the stage was set for the appearance of Darwin's The Origin of Species and Hofmeister's papers on morphology. Much work followed, filling in the details of a map whose outlines Darwin and Hofmeister had sketched. Such theories as that of the carpel took on a new significance. Divergence of opinion showed itself in the objection of Coulter & Chamberlain to the ‘hard categories’ of the ‘old morphologists’. With the appearance of Mendelism a new mode of thought was engendered. The last vestiges of justification for a static view of plant structure vanished. Within recent decades, with ever‐increasing specialization there is nevertheless a tendency towards interdependence between various branches of the subject. A feature of the modern outlook is a certain fluidity of concept. The present article deals with evidence of this outlook drawn from morphological and anatomical fields, and its bearing on taxonomy.Few publications on structural botany have been more controversial than those of such observers as McLean Thompson and Grégoire on the gynaeceum. McLean Thompson carries an emancipated viewpoint to its logical conclusion of almost complete agnosticism. Recent publications indicate that a less rigid attitude is being assumed towards apices, and to the tissues arising from them. In the work of Esau on phloem one is struck by the dynamic approach adopted. The formalized interpretation of the structure of the secondary body enunciated by Jeffrey was opposed by Bailey and others whose critical studies are a further illustration of a dynamic‐viewpoint. The work of Priestley, Salisbury, and Thoday may be cited as further illustrations of a modern approach. A like trend of thought is apparent in the realm of gross morphology. Arber concludes that leaf and stem are no more than convenient descriptive terms. Thomas arrives at equally revolutionary conclusions. Genetical work has influenced structural concepts where the species is concerned. The new attitude is far from universal, but is well expressed in the stress recently laid by Thoday on the significance of the natural selection of harmonious changes, and the importance of obtaining light on the laws of harmonious development.The foundations of taxonomy, the primary function of which is systematic arrangement, were laid by Linne. Within recent years systems have appeared purporting to give the ‘ true’ or natural classification. Systematists have thus become interpreters. It is doubtful how far the study of external form and gross morphology can unfold the story of organic evolution. A conflict in fact exists as regards what constitutes valid evidence. Modern views have had repercussions on the working units of the taxonomist. The writings of Guppy and Willis show evidence of mistrust of the Darwinian conception of the mechanism of evolution. Du Rietz goes far in this direction, whilst Lotsy and Hayata provide instances of extreme iconoclasm. A parallel exists between the agnosticism of Arber in morphology and that of Hayata in taxonomy. Taxonomy, as the science of arrangement, presents a formidable task in itself; its concern with relationships must be secondary. Precise structural units are essential, so that such theories as that of ‘ acarpous flowering’ spell confusion to the taxonomist. Here lies the central conflict between taxonomy and much of the modern work on structure. Taxonomy is faced with the alternative of either undergoing a revolution within itself or ceasing to claim to present more than a convenient arrangement of organisms. Elaborate genetical and other studies do not promote the primary end of taxonomy, if that end is taken to be a workable arrangement of plants. Systematic botany, relatively conservative, may become to some extent divorced from the discussion of phyletic trends. Du Rietz, in pressing for reform in the methods of taxonomy, is focusing attention on the essential conflict between the older systematic methods and the modern work on heredity and allied subjects. Two conflicting interests are concerned, on the one hand absolute truth, on the other a system convenient and workable.No treatment of the plant's anatomical organization, however dynamic, can pass beyond ultimate structural units. The conflict above noted is thus of limited extent. With increasing knowledge of the processes involved in anatomical development has come the statistical method, as employed by Bailey and others. The agnosticism of Arber and Hayata is in the last analysis unjustifiable. Different classes of evidence may be utilized towards a common end. Advances–no matter in what branch of botanical knowledge–enter the field of accepted fact in terms of structural units, and so become available, alike for phyletic discussion and for use as taxonomic criteria.