Once thought to be descendants of early vascular plants (the
Psilophyta of the
Devonian period),
Psilotaceae have been shown by molecular phylogenetics to be ferns (
Polypodiopsida), and a sister group of the
Ophioglossaceae. The family contains two genera,
Psilotum and
Tmesipteris. The first genus,
Psilotum, consists of small shrubby plants of the dry tropics commonly known as "whisk ferns". The other genus,
Tmesipteris, is an
epiphyte found in
Australia,
New Zealand, and
New Caledonia. All members of Psilotaceae are
vascular plants without any true roots. Rather, the plants are anchored by an underground system of
rhizomes. The small, stem-like
gametophytes of Psilotaceae are located in this rhizome system, and they aid in a plant's nutrient absorption through the soil. This is primarily achieved through
saprotrophic feeding on organic soil matter and
mycorrhizal interactions. Psilotaceae do not have leaves. Some species have leaf-like structures called
enations which have no vascular tissue except for a small bundle at the base. These are almost peg-like, stubby and are generally not considered true leaves, though they likely evolved from them. Members of
Tmesipteris may appear to have leaves, but these are really
phylloclades, or flattened stems. The
sporangia of Psilotaceae are fused together into small and distinctive yellow balls called synangia (shown in the picture of
P. nudum above). These synangia are located off the stems of the plants. They contain two sporangia each in
Tmesipteris species, and three sporangia each in
Psilotum species. A thick
tapetum nourishes the developing
spores, as is typical of
eusporangiate ferns like Psilotaceae. ==Classification==