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Psilotum - November 14, 2007

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Whisk Fern or Psilotum nudum

Florida is home to the most primitive living vascular plant on earth.  (Vascular plants are those that have specialized tissue for the conduction of food and water, phloem and xylem.)   The common name is whisk fern and you can perhaps see why - it’s similar to the old whisk brooms used to clean clothes and cars before the advent of vacuums.  However, it is not a true fern.  Instead, it is classified as a close relative, or fern ally.  The scientific name is Psilotum nudum.  It does look pretty nude.  It has primitive dichotomous (forked) branching and no true roots or leaves.  Instead of leaves, it has very tiny, often degenerate prophylls which are nonphotosynthetic.  The stem is the only photosynthetic part of the plant, so the stem serves as one giant leaf.  It’s anchored in the soil by an underground stem called a rhizome.  Attached to the rhizome are root-like structures called rhizoids - but they are not true roots.

The stem is unusual for more than its ability to photosynthesize.  The stem has stomata with guard cells ñ typically found in true leaves, not stems and used in gas exchange.  The rhizome has a fungus which lives in association with the cells and the fungal filaments actually penetrate the rhizome cells making it an endomycorrhizal relationship - a relationship between a fungus and another cell.  The whisk fern cannot live without this relationship.

I remember the first time I saw this in the wild.  I was fishing in the Everglades at Holiday Park and got out to explore the sides of one of the canals.  Growing in a dry area of the canal was a bed of Psilotum.  Psilotum is native to Florida, southern Louisiana and southern Texas.  It can also be found in other parts of the world but only these locations in the United States.  Apparently Mississippi and Alabama are too far north in latitude to allow growth in those states

Whisk fern reproduces by spores.  Located in the axils of some of the prophylls, sporangia develop and produce sexual spores.  These spores germinate to produce a very tiny gametophyte (haploid plant body) which develops archegonia and antheridia.  The antheridia produce sperm which then swim to another gametophyte and fertilize the eggs in the archegonia.  The fertilized egg gives rise to the plant (sporophyte or diploid plant body) you see here. 

I found this specimen growing in my flower pot in the back yard.  It’s not uncommon for trees to be cut in the Everglades and used for mulch.  Probably, some of the spores came from that.  Whisk fern grows on South Campus, often in the mulch of flower beds.  This particular plant did not have sporangia on them when the photo was taken, however, they typically do produce them sometimes during its life.  A cross section of the sporangium (taken from a slide in biology) shows the spores inside the sporangium.   

© Fred Searcy 2017