Excerpts from Jim Conrad's
from the August 18, 2013 Newsletter issued from the Frio Canyon Nature Education
Center in the valley of the Dry Frio River in northern Uvalde County, southwestern Texas, on the southern border of the Edwards Plateau, USA
Most but certainly not all fern species occupy moist habitats; our edge-of-desert location is better for finding cacti than ferns. Therefore, it's a happy time when a new-for-me fern species turn up at this location. The species encountered this week was a robust, knee-high one with numerous fronds emerging from a moist crevice in the Dry Frio River's limestone bed, as shown above.
Notice how the fronds' subdivisions, or pinnae, are toothed along their margins, and how each frond's lowest pair of pinnae is smaller than the pinnae above it, and that the lower pinnae point backwards. These features are just like those of the most common fern on bayou walls back in Mississippi, the Southern Shield-Fern, which you can see at http://www.backyardnature.net/n/h/shield-f.htm.
However, that fern needs a moister climate, its westernmost point of distribution being humid eastern Texas. Southern Shield-Ferns don't make this far west.
Southern Shield-Ferns are members of the genus Thelypteris, so I figured that this week's fern also was a Thelypteris. Thelypteris ferns are beautiful and interesting but they can be hard to identify to species level. About 875 species are recognized, occurring nearly worldwide, with 21 found naturally in North America. I had problems distinguishing Thelypteris species until I learned the secret of "earmark veins." Therefore, this week as soon as I thought I might have a new Thelypteris, I looked for those earmark veins. You can see them below:
In that picture the tiny, spherical, dark, spore-bearing bags called sporangia also show up, scattered in loose groups on the pinnae undersides.
The "earmark veins" are the two veins forking off the bottom of each pinna's midvein. Notice that the very lowest vein terminates a little above the bottom of a neighboring sinus, while the second-lowest vein terminates at the very bottom of the other sinus. These are the earmark veins. In many Thelypteris species the earmark veins merge below the sinus to form a new vein that shoots upward to the sinus base. In other species, both earmark veins connect with the sinus well above the sinus's base. Sometimes two Thelypteris species will be almost identical, but their earmark veins can behave very differently.
Our Thelypteris turns out to be THELYPTERIS OVATA, often referred to as the Ovate Maiden Fern. The species is widely distributed from Belize and Guatemala north through Mexico and the Caribbean area, and extends into the US in Florida and nearby Coastal Plain zones from South Carolina to Alabama, and also here in southwestern Texas. The US's Florida populations are recognized as the variety ovata, while our Texas ones are variety lindheimeri; sometimes ours are called Lindheimer's Maiden Fern. The Lindheimer's habitat is described as "riverbanks and moist canyons," which is close enough to where ours was found.
So, this is a good find, not only because it's a very pretty species in an area with few ferns, but also because it's always a pleasure to find subtle "variations on a theme." The Lindheimer's Maiden is very similar looking to the US Southeast's Southern Shield-Fern, yet the two species have different habitat preferences, and their earmark veins are ever so slightly differently configured.