An interpretation of the morphology and evolution of the cone and shoot of Equisetum

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.