In a bundle examined in the basal portion of a leaf the bulk of the xylem is found to be centrifugal in position, but internally to the protoxylem there is a group of centripetal tracheids; higher up in the petiole the xylem is mainly centripetal, the centrifugal wood being represented by a small arc of tracheids external to the protoxylem and separated from it by a few parenchymatous elements.
Archaeocalamites appears to have had a similar structure, but in some specimens from the Lower Carboniferous of Burntisland, provisionally named Protocalamites pettycurensis, centripetal wood was present in the stem.
The primary wood of Lepidodendron forms a continuous cylinder, not broken up into distinct bundles; its development was clearly centripetal, the spiral elements forming more or less prominent peripheral groups.
In the latter case individual stamens may move in succession towards the pistil and discharge their contents, as in Parnassia palustris, or the outer or the inner stamens may first dehisce, following thus a centripetal or centrifugal order.
In other species, however, the centripetal primary xylem is represented.