XVI International Botanical Congess

Recent molecular phylogenetic and cladistic analyses suggest that early vascular plants diverged into two clades, microphyllous lycopods, and megaphyllous pteropsids and sphenopsids. In the evolution of vascular plants, the aerial part of their body was elaborated into shoots composed of the stems and leaves. The evolutionary trend in the shoot apical meristem may be toward the complex meristem with multiple initial cells as in seed plants. The simple meristem with a single apical cell is common in the megaphyllous pteridophytes. In the megaphyllous plants the apical meristem of the shoot is similar to that of the leaves, while there is no such similarity in the microphyllous plants. The proposed evolution of the megaphylls from primitive axial branches and the microphylls from enations or sporangia is discussed from a morphogenetic point of view.