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Psilotum
(pronounced "zi-lo-tum") is a
low-growing plant devoid of any roots or leaves.
Instead it has branched rhizomes under the soil
surface, and forked green photosynthetic stems above
which bear small leaf-like or scale-like appendages.
Laced terminally on very small branches are
thick-walled homosporous sporangia which release the
plant's spores.
Macro
detail of a Psilotum nudum sporangium.
This
is a very primitive plant indeed; its closely
related ancestors predate leaves, seeds, flowers,
and even wood. It only has half the vascular system
modern plants have; a xylem for transporting water,
but no phloem for the movement of sugars. In order
to carry out the vital process of photosynthesis
(the reaction which converts light energy to
chemical energy for the plant) the stems of this
plant are green all over and contain chlorophyll,
the pigment that allows photosynthesis to
occur.
To
maximise exposure to light, Psilotum sends up
branches from its rhizomes which turn upwards and
become aerial shoots. Like the photosynthetic stems,
these shoots also have small appendages which have
no vascular tissue.
Terminal
growth of Psilotum nudum, showing the forked
green photosynthetic stems and yellow-coloured
sporangia.
To
control water loss, P. nudum has stomata
(like all modern plants). Each stoma consists of two
guard cells, which open and close depending on how
much water is in the plant. If there is plenty of
water (for instance after rain), then the stomata
will open and allow water within the plant to be
released in a process called transpiration. (The
releasing of water allows more water to be 'sucked
in' through the rhizomes). If it is dry, then the
stomata are closed and water loss is reduced.
Because
P. nudum's primitive system of absorbing
nutrients and water through rhizomes is terribly
inefficient, the plant forms a relationship with
mycorrhiza (a type of fungus). The fungus has access
to the plants rhizoidal hairs - structures
which act as absorption points for nutrients and
water on the rhizomes. The fungus is better at
nutrient and water uptake than the Psilotum,
and so its relationship allows both organisms to
exist in arid environments.
Close
detail of Psilotum nudum.
The
gametophyte of P. nudum resembles a piece of
sporophyte rhizome, as was the case of Rhynia
species that lived 400 million years ago. It
contains branched axes lacking chlorophyll with sex
organs borne together, these producing the mobile
gametes.
Psilotum
nudum is important because it so closely
resembles the early Rhynia- and Cooksonia-type
plants that lived in the Silurian era. The fact that
representatives of such basic and primitive plants
still exist is remarkable, and perhaps highlights
the importance of conservation attempts.
Psilotum
nudum is found growing in the Australian states
of Western
Australia, New
South Wales, Victoria and Queensland. The
species is also indigenous to parts of South-East
Asia and North America. At present, the species is
not threatened in Australia. The
genus Psilotum belongs to the Psilotaceae
family and contains three species; P. nudum; P.
complanatum and the hybrid P. ×intermedium.
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