Stems The relatively slender stems (fossil genus
Callistophyton) had a
eustele with a well-developed zone of secondary wood, and unlike most (but not all) other Palaeozoic pteridosperms, showed
axillary branching. These characters strongly point to its having been a scrambling or climbing plant. A characteristic feature of the stems is the presence in the cortex of spherical secretory structures. Similar structures have also been found in associated ovules, pollen-organs and foliage, and were one of the main lines of evidence on which the reconstruction of the plant was based (compare with similar evidence used to reconstruct the
Lyginopteris-bearing fossil plant).
Ovules The small
ovules (fossil genus
Callospermarion) with the characteristic secretory structures have an
integument that was only fused to the
nucellus in the
basal part of the
ovules and so superficially resemble
medullosalean ovules. Unlike the
Medullosales, the
ovules appear to be bilaterally symmetrical, although details of the
vasculature suggest they were in fact evolved from plants with radially symmetrical
ovules. The
apical part of the
nucellus has a
lagenostome-like projection, which breaks down to form the pollen chamber. The full
ontogeny of these
ovules has been worked out in some detail and seems to be essentially similar to that seen in modern-day
gymnosperms, including the use of a pollen drop to help capture and draw the pollen into the pollen chamber and a pollen tube to deliver the generative nucleus. The ovules were borne on the underside of pinnules that did not differ significantly in form from those of the purely vegetative fronds.
Synangia The pollen-producing organs (fossil genus
Idanothekion) consisted of small, radially symmetrical synangia, with each pollen-sac having a longitudinal dehisence structure. Like the ovules, the synangia were attached to the underside of pinnules that did not differ significantly in form from those of the purely vegetative fronds, and so can give a superficially similarity to fertile fern fronds. Unlike ferns, however, these pollen-organs produced monolete, bisaccate pollen (fossil genus
Vesicaspora) bearing some similarity to the pollen of many conifers.
Foliage The foliage, which is the part of these plants most widely-found as
macrofossils, consists of fronds with a basal dichotomy of the main rachis, each branch producing pinnately divided segments, but with no pinnae attached below the dichotomy. Such fronds, when found as adpressions, are known as
Dicksonites. The pinnules tend to be tongue-shaped or somewhat lobed, and can closely resemble pinnules of
Lyginopteridales fronds such as
Mariopteris. Distinguishing the fronds of these two orders of pteridosperms can in fact be very difficult unless the pinnules are fertile, although generally the pinnule lamina of
Dicksonites fronds tend to be somewhat vaulted, whereas
Mariopteris pinnules are usually flatter. ==Distribution==