Throwback Thelypteris Thursday: Hottentot or Willdenow’s Fern

Fern, Oslo Riverfront Conservation Area

Pictured above is hottentot fern (Thelypteris interrupta), a native fern that grows along the mesic (moist) hammock loop trail at the Oslo Riverfront Conservation Area (ORCA). Its fronds are usually 3 – 4′ long and grow from an underground rhizome (underground plant stem).

The genus name, Thelypteris, means female fern in Greek. The species name, interrupta, is said to refer to the spacing of the frond along the dark subterranean rhizome from which they grow.

The common names, Hottentot fern and Willdenow’s fern, have an interesting derivation. The Hottentots are people who live in arid places in Africa, and this fern has a wide distribution that includes Africa, tropical Asia, India, and the American tropics. Carl Ludwig Willdenow was a German botanist (1765 – 1812), who is considered to be the founder of phytogeography, the study of the geographical distribution of plants.

Bob Stolze, Curator of Ferns at the Chicago Natural History Museum, provided this description …

Hottentot fern – Thelypteris interrupta
Many species of the genus look very much alike – distinguished only by subtle characters. The frond of this Thelypteris is glossier, thicker and less hairy than those of its two cousins along the trail, and is obviously widest at its base. Also, the lobes of the leaflets (pinnae) are shorted and broader, whereas the lobes of the other Thelypteris here are narrower and cut more deeply to the base. The basal veins of each segment merge with the opposing nodes, and from their juncture springs another, which then leads to the edge of the segment. The leaf stalks are very widely spaced on the black, long-creeping underground stem (rhizome).
Habitat: Rather common in wet hammocks and cypress swamps in Florida. Pantropical.

Above is a photo taken along the hammock loop trail in 2014 by Diane LaRue (Class of 2012), knowing that the distinctive zig-zag pattern of the “spores” would assist with ID. Technically, we are looking at the sori, clusters of sporangia, the structures that produce and contain the spores. Also notice, per the description provided by Bob Stolze, the lack of hairs, the glossiness of the top of the leaflets (pinnae), and the shallowness of each lobe of the leaflets (other species have  much “deeper” lobes).

The way that its “spores” are held is really quite distinctive. Below, for comparison, are the sori from last Throwback Thursday’s fern, the naturalized downy shield fern (Thelypteris dentata) …

If you are lucky enough to find a fertile frond, then the ID is evident. Hottentot fern has distinctive zig-zag “spores”.