Whisk Brush, Anyone?

Whisk fern (Psilotum nudum) is on infrequently found growing at the Oslo Riverfront Conservation Area (ORCA), usually as an epiphyte. Karen Schuster (Class of 2008) spotted it growing on the adventitious roots on the trunk of a cabbage palm tree (along with a bit of poison ivy) on the hammock loop trail on a walk there on Saturday, January 2, 2021.

Once upon a time, whisk fern was thought to be a “living fossil”, a very primitive plant that persisted from days gone bye. Current research suggests instead that whisk fern is a much-reduced modern plant that evolved from a fern in the adder’s tongue family, Ophioglossaceae.

Whisk fern is usually called a fern ally since somehow evolution has done away with its leaves and roots. It has a wide geographic distribution that includes tropical Africa, tropical Asia, southern Japan, Hawaii, Australia, central America, and subtropical & tropical America. In Florida it often grows on the trunks of cabbage palms (Sabal palmetto) as an epiphyte in moist places, but you also will find it “volunteering” in your potted plants and is a common “visitor” in greenhouses.

It looks like a bunch of branched stems, and the settlers were said to have tied together a handful of branches to make a whisk brush. Its genus name, Psilotum, means bare in Greek, and its species name, nudum, is rather redundant.

Its epidermal layer conducts photosynthesis and has tiny holes (stomata) to regulate gas exchange. A very close look reveals some tiny “scales” called enations that look like miniature leaves. They are not really leaves because they lack internal vascular tissue.

You might think that the yellow spheres are fruits, but they are not. Whisk fern does not flower and reproduces via spores.

Mature sphorophytes are yellow and inside are the sporangia (spore cases) that house that dust-like spores that are spread by wind.

Whisk ferns attach to to a tree trunk or the ground via hair-like projections called rhizoids and initially are “fed” by symbiotic mycorrhizal fungi. When the “baby” whisk fern (gametophyte) begins to perform photosynthesis, the co-worker fungi finally begin to reap their rewards.

Whisk ferns that grow in the soil or in your plant pot easily can be overlooked because at a quick glance they resemble a grass especially when the yellow sporophytes are not yet present. They are listed as a threatened plant by the state of Florida. Enjoy seeing this fanciful fern ally whenever you can!