Plant Science Portal 
Psilotum nudum

Meet Psilotum nudum from Australia. 400 million years has passed since the first terrestrial vascular plants evolved, yet this modern-day wonder appears to show little difference from these first vascular plants.
Is it the plant that evolution forgot?

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.
Psilotum nudum
Psilotum nudum.
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.

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.
Psilotum nudum
Psilotum nudum macro detail
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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