The two parts are very different in structure: in the main axis, as shown in the common Coal Measure form Stigmaria ficoides, the centre was occupied by the pith, which was surrounded by a zone of wood, centrifugally developed throughout.
Here f is the termination of the primary axis, and this flower expands first, while the other flowers are developed centrifugally on separate axes.
The floral axes are thus centrifugally developed.
The heads of flowers, on the other hand, taken collectively, expand centrifugally - the central one a first.
21), the heads of flowers,Mixe d taken as a whole, are developed centrifugally, the terminal head first, while the florets, or small flowers on the receptacle, open centripetally, those at the circumference first.