I was ready to return with my fern book and finally start learning their identities, yet I didn’t realize how little I know about ferns.
Maidenhair Fern, April 23, 2010.
I figured that the distinctive silhouettes of the fern fronds would be enough to arrive at a positive ID, but no. In many cases, you have to turn the fern and study the pattern of bumps, called sori, on the underside of the leaf.
A sorus (pl. sori) is a cluster of sporangia.
In ferns, these form a yellowish or brownish mass on the edge or underside of a fertile frond. In some species, they are protected during development by a scale or film of tissue called the indusium, which forms an umbrella-like cover.
Dicksonia antarctica. Picture taken by DanielCD on 17 May 2005. Picture is of the underside of a fern frond. It shows a fertile frond which is covered with sori (sing. sorus)
Sori occur on the sporophyte generation, the sporangia within producing haploid meiospores. As the sporongia mature, the indusium shrivels so that spore release is unimpeded. The sporangia then burst and release the spores.
The shape, arrangement, and location of the sori are often valuable clues in the identification of fern taxa. Sori may be circular or linear. They may be arranged in rows, either parallel or oblique to the costa, or randomly. Their location may be marginal or set away from the margin on the frond lamina. The presence or absence of indusium is also used to identify fern taxa.
The best time to identify ferns is when the sori are fully developed. Here’s one more description of what to look for:
Many ferns bear their spore cases (also known as sporangia, sori, or fruit-dots) on the undersides of some of the leaflets—turn over the leaves and look for small dots, often brown. Other species have separate stems devoted to holding spore cases. These structures have fertile leaves that usually look like miniature versions of the larger plant but later turn brown and curly.
Identification of many of the twice-compound species requires examining placement of spore cases; comparison of sizes, shapes, veining patterns, and numbers of leaflets; and other meticulous evaluations, which obsessive botanists usually enjoy.
Read more at Suite101: How to Identify Ferns: Primitive and Beautiful Plants of Woods and Meadows http://botany.suite101.com/article.cfm/how_to_identify_ferns#ixzz0rvFoo332
Sori (containing spores) on the underside of a curling Polypodium fern.
Catskill Mountains, New York, USA
I had already been contemplating the fractal* quality of ferns, even before stumbling onto the bit about spore cases with leaves that resemble the larger plant.
[*Fractals being processes or images that exhibit something called self-similarity, something made up of a reduced version of itself.]
Trees and ferns are fractal in nature and can be modeled on a computer by using a recursive algorithm. This recursive nature is obvious in these examples—a branch from a tree or a frond from a fern is a miniature replica of the whole: not identical, but similar in nature.
The connection between fractals and leaves are currently being used to determine how much carbon is contained in trees.
Barnsley's fern computed using an iterated function system
I haven’t been back to the ferns, but I’ll find them in due time. I still don’t know much, but more than I did before.
Two lessons, for now.
One, when you look at a fern, you’re looking at mathematics in action.
And two, always look on the underside of the leaf!
The Great Smoky Mountains All Taxa Biodiversity Inventory reports 53 species of ferns representing the Pteridophyta division of plants (within the national park).
Finally, as if that’s not enough reason to go out and learn a fern, there’s this:
Finnish tradition holds that one who finds the "seed" of a fern in bloom on Midsummer night will, by possession of it, be guided and be able to travel invisibly to the locations where eternally blazing Will o' the wisps called aarnivalkea mark the spot of hidden treasure. These spots are protected by a spell that prevents anyone but the fern-seed holder from ever knowing their locations.
Links on ferns and fractals: