PLANT OF THE WEEK

Dr. T. Ombrello - UCC Biology Department

 

PSILOTUM

 

Common names:      Whisk Fern  

 

Scientific name:    Psilotum nudum

 

Explanation of scientific name:

Psilotum - from the Greek word “psilos” meaning bare.

nudum - Latin for naked, nude.

The scientific names reflect this species’ lack of many of the organs typically present on the common plants around us.

While not the most attractive of plants, Psilotum nudum has the distinction of representing the most primitive vascular plants on Earth today.  The vascular (circulatory) system of Psilotum nudum consists of a series of tube-shaped cells that conduct water, minerals, and organic materials throughout the plant.  This vascular system is essentially the same as those found in the trees, shrubs, flowers, and grasses we are familiar with.  The similarity to these plants, however, ends with the vascular system.

Psilotum nudum lacks most typical plant organs.  The plant has no roots, leaves, flowers, fruits, or seeds.  It consists primarily of stems.  Besides reaching about 1 foot above the soil surface, the stems extend beneath the soil, branching to form a network of smaller stems to hold the plant erect and absorb water and minerals for nutrition.  These underground stems, called rhizomes, do not have the same elaborate and efficient means of gathering water as do “true” roots, but they grow in intimate contact with special fungi that aid their absorptive capabilities.  The above ground stems of Psilotum nudum have small scale-like appendages called enations and clustered yellow spherical structures called synangia.  The synangia produce microscopic, single-celled reproductive units called spores.  When mature, the synangia open, releasing the spores to be dispersed by the wind.  The dust-like spores can be carried for miles to produce the next generation.

Psilotum nudum is native to the Untied States.  Ranging from Arizona and Texas to Louisiana and Florida, it occasionally is found as far north as South Carolina.  The species grows on moist, rich soil or as an epiphyte (living upon other plants, such as in the humus accumulated in the crotches of trees). 

Psilotum nudum belongs to the taxonomic division of plant life called Psilophyta.  Today this division has only 10 living species representing it, but in the past members of Psilophyta made up a significant portion of the vegetation in North America, Europe, and Asia.  Fossil evidence, first uncovered in Scotland in the 1800s, indicates that Psilophytes were thriving over 400 million years ago, appearing much the same as Psilotum nudum does today.

Although the Psilophytes are the oldest and simplest vascular plants of which we have any record, their origin is unknown.  Whether they arose independently or evolved from more primitive plants such as the green algae is still debated by scientists.  Another question that remains unresolved is whether the Psilophytes were the predecessors to the modern flora of the world, or evolutionary “dead ends” that have become practically extinct as more advanced plant forms appeared.  Recent evidence has led some botanists to classify the living Psilophytes as primitive ferns, and not as an independent division of plants.  Unless more complete fossil evidence is found, these questions about the origin and role of the Psilophytes in the history and evolution of Earth’s plant life may never be resolved.

Besides serving as a botanical curiosity to frustrate scientists, Psilotum nudum has little importance to mankind.  Its common name, Whisk Fern, alludes to its use in the past as a small broom, made by tying a handful of its branches together.  Psilotum nudum was once prized as an ornamental in Japan, and about 100 varieties were known.

A close-up view of a Psilotum plant growing in one of Union County College's greenhouses.  Note the yellow, spore producing synangia.

A Psilotum plant growing in one of Union County College's greenhouses.