Psilotum is a genus of fern-like . It is one of two Genus in the family Psilotaceae commonly known as whisk ferns, the other being Tmesipteris. Plants in these two genera were once thought to be descended from the earliest surviving , but more recent phylogenies place them as basal ferns, as a sister group to Ophioglossales. They lack true and Leaf are very reduced,
Description and life cycle
Whisk ferns in the genus
Psilotum lack true roots but are anchored by creeping
. The stems have many branches with paired
, which look like small leaves but have no
vascular tissue. Above these enations there are synangia formed by the fusion of three
Sporangium and which produce the
. When mature, the synangia release yellow to whitish spores which develop into a
gametophyte less than long. The gametophyte lives underground as a mycoheterotroph, tapping into mycorrhizal networks to access carbon and other nutrients. When the gametophyte is mature, it is
monoicous, producing both egg and sperm cells. The sperm cells swim using several
Flagellum and when they reach an egg cell, unite with it to form the young
sporophyte. A mature sporophyte may grow to a height of or more but has no apparent
leaves. The stem has a core of thick-walled protostele in its centre surrounded by an
endodermis which regulates the flow of water and nutrients. The surface of the stem is covered with
which allow gas exchange with the surroundings.
The gametophyte of Psilotum is unusual in that it branches dichotomously, lives underground and possesses vascular tissue.
Taxonomy and naming
The genus
Psilotum was first formally described in 1801 by
Olof Swartz and the description was published in
Journal für die Botanik (Schrader).
The name of the genus is from the
Ancient Greek word
psilos meaning "bare", "smooth" or "bald"
referring to the lack of the usual plant organs,
and the seeming lack of leaves.
Species and distribution
There are two species,
Psilotum nudum and
Psilotum complanatum, with a hybrid between them known,
Psilotum ×
intermedium W. H. Wagner.
The distribution of Psilotum is tropical and subtropical, in the New World, Asia, and the Pacific, with a few isolated populations in south-west Europe. The highest latitudes known are in South Carolina, Cádiz province in Spain,
Relation to ferns
Psilotum superficially resembles certain extinct early vascular plants, such as the
and the trimerophyte genus
Psilophyton. The unusual features of
Psilotum that suggest an affinity with early vascular plants include dichotomously branching sporophytes, aerial stems arising from horizontal rhizomes, a simple vascular cylinder, homosporous and terminal eusporangia and a lack of roots.
Unfortunately, no fossils of psilophytes are known to exist. A careful study of the morphology and anatomy suggests that whisk ferns are not closely related to rhyniophytes, and that the ancestral features present in living psilophytes represent a reduction from a more typical modern fern plant. Significant differences between
Psilotum and the rhyniophytes and trimerophytes are that the development of its vascular strand is exarch, while it is centrarch in rhyniophytes and trimerophytes.
The sporangia of
Psilotum are
locule synangia resulting from the fusion of three adjacent sporangia,
[ and these are borne laterally on the axes. In the rhyniophytes and trimerophytes the sporangia were single and in a terminal position on branches.]
Molecular evidence strongly confirms that Psilotum is a fern (in the broad sense that includes horsetails) and that psilophytes are sister to ophioglossoid ferns.[Qiu, Y-L and Palmer, J (1999) "Phylogeny of early land plants: insights from genes and genomes." Trends in Plant Science 4(1), 26-30]
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