Introduction to the Zosterophylls

Evolving from a rhyniophyte-like ancestor around the beginning of the
Devonian, the Zosterophyllophyta
is an extinct taxon of small, rather unprepossessing Devonian plants.
Zosterophylls occupy a key place in the evolution of land plants; several
fossils show characters that are otherwise very typical of the
lycophytes. Though the
evolutionary relationships of the zosterophylls have not yet been
worked out completely, several taxa traditionally placed in the
Zosterophyllophyta appear to be close to the ancestry of the Lycophyta.

Superficially, zosterophylls looked very much like
rhyniophytes and like
trimerophytes, the other two
prominent plant groups of the lower Devonian. Zosterophylls lacked true leaves
and roots, and photosynthesis was probably carried out all over the stems.
Zosterophylls generally showed dichotomous branching -- each stem
dividing into two branches of equal size. Like some trimerophytes, most
zosterophylls were covered with enations, scale-like flaps of tissue
that lacked vascularization and so were not true leaves.

But zosterophylls differed from other plants of the time in two main ways:
their sporangia were round to reniform (kidney-shaped) and they were not
born on the ends of branches, but on small stalks arranged in clusters along
branches. These characters are also seen in the Lycophyta. In fact, some
Devonian fossil
plants, like Asteroxylon, have been placed both in the Lycophyta and
in the Zosterophyllophyta by different paleobotanists. Thus the
Zosterophyllophyta is probably a paraphyletic assemblage of early
plants, including the ancestors and close relatives of the Lycophyta.

Source:
Stewart, W.N. and Rothwell, G.W. 1993. Paleobotany and the Evolution of
Plants. Second edition. Cambridge University Press, Cambridge.