Primitive ovulate plants and their precursors (Progymnospermopsida)

The class Progymnospermopsida contains only fossil plants. The concept of progymnospermy, a stage at which plants with conifer-like anatomy and morphology were still reproducing by spores, followed the surprising discovery that certain well-preserved trees of the Upper Devonian bore frond-like sprays of branches, some of which bore clusters of sporangia. Callixylon, the first progymnosperm to be recognized, provides a splendid example of this stage of evolution (Fig. 8.1a). The trunks reached a diameter of 1.5m (5 ft) and a length of 8m (26ft) or more. Permineralized remains reveal fine details of the anatomy. A central pith was surrounded by mesarch primary xylem. Outside this lay a considerable thickness of well-developed secondary xylem traversed by narrow rays. The pits in the radial walls of the tracheids were frequently grouped, the groups aligned horizontally and in register with tracheids in the rays. Dense wood of this kind, also characteristic of modern conifers, is termed pycnoxylic.

Although Callixylon was known for many years as the trunk of a late Devonian tree, only much later were discovered specimens in organic connection with frond-like branches. These branches were already known as Archaeopteris and had been assumed to be the fronds of ferns. Archaeopteris is known both sterile and fertile (Fig. 8.1b). The sporangia were spindle-shaped, up to 3.5mm (0.14in.) in length, and occasionally with stomata in the epidermis.