No abstract

Some of the biological problems presented by bracken, Pteridium aquilinum (L.) Kuhn, are posed. Its taxonomic position within the Pteridophyta and the delimitation of entities within the genus are discussed on the basis of morphological and cytological evidence. The geographical ranges of the various brackens world‐wide are described and mapped in outline, and emphasis placed on reviewing the natural ecological role of bracken in plant communities throughout the world. Further geographic areas where taxonomic investigation of Pteridium is most needed are indicated, and evidence of the reproductive, dispersal, establishment, colonizing ability and vegetative persistence of bracken is reviewed. Its palaeobiological spread, with associated vegetational history, and the effects on this of anthropogenic influences—better known than are comparable details for any other pteridophyte—are detailed, and the present magnitude of the resulting bracken problem in Britain (and especially in upland Britain) indicated.

Summary The use of the scanning electron microscope to survey the surface relief of Equisetum reveals with clarity the existence of hitherto little known fine surface features. These patterns of sculpturing are characteristic and distinct from species to species, and form a valuable basis on which to construct taxonomic and phylogenetic ideas independently of the features of habit and gross morphology used currently but known to be highly variable. A new taxonomic layout for the species of subgenus Equisetum is suggested on this evidence, and the interrelationships and evolution of the species concerned are considered.

Evidence is presented from new observations in morphological, anatomical and numerical fields which supports the view that in Equisetum the units of which the vegetative and fertile axes are composed are serially homologous throughout the shoot. Though greatly different in their modern morphology, these differences can be accounted for by the evolutionary specialization of each from a once common type of unit. The structure of the primitive unit is most closely approached today by that of the fertile axis of Equisetum, especially in the construction of its vascular system. A relationship also exists between the appendages borne from the units of each of the two axes—the leaves and sporangiophores. The evidence, however, fails to support the widely held notion that sporangiophore and leaf are wholly homologous. Instead, only part of the Equisetum sporangiophore (the stalk plus the upper half of its peltate head) is homologous with the modern leaf. Based on this interpretation, a new schematic sequence of evolutionary stages is simultaneously proposed for both the axes and appendages of the type seen in Equisetum. Evidence from the fossil record, as well as from the structure of living Equisetum, is cited in support of the outline of evolutionary morphology proposed.