Reconstruction of the bundle of wood vessels of a very young specimen of Psaronius by Stidd and Phillips (1968). The height of this coal ball plant was 4.5 cm.
The central strand of wood vessels starts as a little tube with a diameter of 0.5 cm. At a certain point a part of this tube separates, curls inward and becomes a new tube. This is going to be a petiole. Above the point of separation a temporary gap appears in the bundle of wood vessels. This separation repeats itself from time to time. When the gap is closing, both sides curle inward and the curles give rise to a new oblong (partial) bundle inside the original bundle. In this way the total central strand gets an increasing diameter because repeatedly new partial bundles come into existence inward. The xylem bundles of the petioles become C-shaped and after some time they leave the central strand and subsequently the trunk.
In the picture small roots are visible at three points on the left hand side.