Shoot apical meristem and initial vascular development of a late Palaeozoic spermatophyte (order Medullosales).

Medullosans exhibited a peculiar stem development not comparable to that of most extant seed plants. Only extant cycads show several specific functional analogies (e.g. a 'stem-girdling' vasculature) that might refer to arborescent growth by a pachycaulous stem. This configuration requires sophisticated hydraulic support at an early developmental stage, including vertical and horizontal transportation of water. Our study demonstrates that the stelar evolutionary path of the medullosans was complex and characterized by various adaptive modifications and transitional stages.