Shady, moist, and relatively cool, the canyons near Austin have long provided a respite from the summer heat for people and wildlife alike. The Colorado River and major streams, such as Barton Creek and Bull Creek, have carved many of these canyons, especially along the Balcones Escarpment that arcs along Austin’s western boundary. But smaller streams dissecting the Edwards Plateau have also left deep ravines in the limestone—in places such as Wild Basin, Hamilton Pool, and Westcave Preserve.
Part of the allure of these oases is the lush plant growth, for many plant species also find refuge here from the desiccating sun and wind. Ferns, in particular, can be abundant in these microhabitats. Verdant drapes of Southern Maidenhair Fern (Adiantum capillus-veneris) seem to cloak every dripping limestone wall. But other fern species are common here. One, Mexican Fern (Anemia mexicana), fairly common in Barton Creek canyons, reaches the northern limit of its range in Travis County.
We often think of ferns in association with moist places, but why is that?
Ferns and their relatives are very different from most other plants. For most plants, reproduction is a matter of transporting pollen grains—and the sperm encased within them—from the staminate (male) flowers of one individual to the pistillate (female) flowers of another. The agents of transport are usually wind or animals. Once a pollen grain lands on a receptive pistil, the sperm will tunnel through the flower tissue to fertilize the ovary at the base of the pistil. Ferns, though, developed before that elegant (and effective) system evolved.
Among the oldest of all vascular plants (vascular equals having a system of water-conducting tissue), ferns have no flowers and produce no seeds. For that matter, the plants we consider ferns are actually only one generation of a complex life cycle—and a generation that is not directly involved in sexual reproduction at all. The sexually-reproductive stage is a separate growth called a prothallus. The prothallus is a tiny, inconspicuous, disc-shaped plant only about the size of a fingernail.
Specialized structures form on the surface of a prothallus that are capable of producing eggs; other structures produce sperm. Both structures may be present on the same prothallus, or only one or the other. But for genetic crossing to occur— allowing for a healthy population and the ability to evolve to meet changing conditions—the challenge is getting the sperm of one prothallus to the egg of another. Unlike in flowering plants, the sperm are free-swimming, not contained in a protective pollen grain. So for genetic crossing to occur, you need: water.
Not a lot of water is necessary, only enough to allow the sperm to swim to another plant and fertilize an egg on it. But a constant, or at least frequent, film or drip of water is ideal to allow fertilization to take place through much of the growing season. That makes the seeping limestone walls of Edwards Plateau canyons superb niches for ferns.
The problem with the life cycle so far, though, is that it does not allow for wide dispersal of ferns. How do new colonies develop in suitable but isolated moist spots in an otherwise hostile Texas landscape? That’s the role of this second life stage that is sprouting from the fertilized egg, the one that makes the lush curtains around Hamilton Pool. Once the egg is fertilized, the next generation of the life cycle begins to grow. Sometimes called the sporophyte (because it produces spores), this is the lacy-leaved fern that we recognize so readily.
If you turn over some of those fern fronds and look at the undersides, you may see them lined with fuzzy brown spots, or the margins may be crowded with tiny brown or black beads. Those are not insect eggs or fungal infections, but rather structures that contain the spores. Depending upon the species of fern, the spores may be contained simply in sporangia (spore cases), or the sporangia can be wrapped in a modified flap of leaf tissue called an indusium for further protection. Sometimes, as with Southern Maidenhair Ferns, the margins of the frond will fold over the spore cases in what is called a “false indusium.” How the spores are arranged and contained is a primary way to distinguish among different groups of ferns.
When you see the sporangia or the indusia under the fern frond, you are not seeing the spores themselves. The spores are extremely small—so small that 100 of them may fit on the period at the end of this sentence. Since they are so small, they are easily transportable by wind, water, and possibly also by animals. When a spore lands in a congenial location, it will sprout into a prothallus, and the life cycle starts again.
One difference between spores and seeds is that spores are not the product of sexual reproduction. Most contain only half the number of chromosomes of the “parent” (sporophyte) plant. Once the spore sprouts into the prothallus, and produces sperm or eggs, cross-fertilization will restore the proper number of chromosomes.
Some ferns, however, have adapted strategies for exploiting particularly isolated microhabitats in arid regions. In the desert of Big Bend, the chances are pretty low of the wind carrying two spores of the same species to the same distant spot, and close enough together that the sperm of one can swim to the egg of the other. So, many arid-land species have developed spores with a full complement of chromosomes. These spores grow into fully-developed ferns, skipping the sperm-and-egg stage altogether.
Because there has been no cross-fertilization, these plants are essentially clones of the parent. This strategy, then, allows for colonization of isolated habitat patches, but not for adapting to changing conditions through genetic variation. It may also help to explain how 80 species of ferns and their relatives have been identified in the arid trans-Pecos region of Texas!
Ferns have developed other strategies to deal with hot, dry conditions. Many have scales or long hairs that shield the surface of the frond from desiccating sun and wind, or leathery outer surfaces to minimize water loss. Arid-land ferns often have very small, finely-cut fronds, which allow for rapid heat dispersal. The Powdery Cloak Fern (Argyrochosma dealbata), which grows on rocks in Balcones canyons, has awhitish, mealy-looking, waxy substance (called farina) on the underside of the frond. In dry times, the leaf curls up to expose this light- and heat-reflecting surface, which also inhibits water loss.
Some ferns can lose up to 90% of the water from their tissues and still survive; some species can be dormant for several years. The fronds turn brown and brittle, but, upon wetting, “resurrect.” Such is the case with the Resurrection Fern (Pleopeltis polypodioides) common in east Texas, and at the extreme edge of its range here. The fronds of this species, too, curl upward when dry. But instead of farina, on the undersurface of the frond are specialized scales that can rapidly funnel water to the leaf tissue. The plant can resurrect in as little as 15 minutes!
The primary reference for this article was The Ferns and Lycophytes of Texas, by George M. Diggs, Jr. and Barney L. Lipscomb.