Ferns and Lycophytes of Texas


The beauty, variety, and fascinating biology of ferns and similar plants catch our imaginations and almost demand our attention. More than a half-century ago they captured the interest of the Texas botanist Donovan Stewart Correll. In 1956 he published the landmark Ferns and Fern Allies of Texas, his first major work on Texas botany. No book since that time has focused on these unique plants throughout Texas despite the fact that a great deal has changed. Since the mid-1950s, the number of ferns and similar plants known for Texas has increased by nearly 29%, including some new to science. Also, the scientific naming (nomenclature) of these plants has changed dramatically, and perhaps most importantly, our understanding of their biology and evolutionary relationships has greatly improved. Indeed, some changes have been so dramatic that Haufler (2007) recently noted, “a revolution began in evolutionary studies” of ferns and similar plants “about 25 years ago.” As a result, now seems to be an appropriate time for a new volume that follows in Correll’s tradition and focuses on these often rare but always interesting plants.

“Record-Holders” and “Prize-Winners”

One of the reasons we are attracted to ferns and similar plants is that they are endlessly fascinating and full of surprises. Before beginning to discuss their biology, it is worthwhile to look at a few of the unofficial record-holders and prize-winners among the species that occur in Texas. Table 1 gives an overview of a number of our favorites; more information about these plants can be found in the species treatments. Although some of our choices are subjective, reading about these “champion” species will give you an intuitive sense of how special some of the ferns and similar plants that occur in Texas really are.

Table 1

The Plants Included: “Pteridophytes”

As indicated by Correll’s title, his book dealt with more than ferns—in addition, it included the “fern allies,” those plants that in some ways resemble ferns and that are alluded to in such phrases as “ferns and similar plants.” Indeed, almost all fern books cover these similar and equally interesting plants and this book is no exception. Botanists have traditionally referred to ferns and fern allies collectively as “pteridophytes.” The word is from the Greek pteron, feather or wing, due to the form of the leaves, and -phyte from the Greek phuton, plant. Scientists who study pteridophytes are referred to as pteridologists and their subject as pteridology. The pteridophytes all share some things in common:

  1. they have vascular or conducting tissue;
  2. their life cycles include two very different alternating phases or generations (explained in the section titled “The Sexual Life Cycle” on page 16);
  3. their fertilization involves swimming sperm;
  4. they are the surviving descendants of evolutionary lineages reaching back hundreds of millions
    of years to shortly after plants colonized land; and
  5. perhaps most significantly, they are “seed-free”—unlike gymnosperms and flowering plants, they lack seeds and instead reproduce by microscopic, single-celled structures called spores. (In fact, in Shakespeare’s time they were considered mystical plants because it was not understood how they could reproduce without flowers, fruits, or seeds [Moran 2004].)

Seedless reproduction is the ancestral condition of the earliest land plants, and lacking these structures can be a serious drawback. Seeds, basically an embryo (baby plant) packaged with stored food and surrounded by a protective waterproof coating, have several important advantages. They have freed seed plants from the necessity of a moist environment for reproduction, they have a supply of food that allows the embryo to grow rapidly, they are capable of remaining dormant for long periods of time, and they can be modified to allow effective dispersal by animals. Indeed, such advantages have allowed seed plants to be incredibly successful—more than 95% of all plants alive today reproduce by seeds.

On the other hand, microscopic spores do have some advantages. They are so small that ferns can make tremendous numbers of them, and they are so lightweight that they can easily be blown or otherwise carried for hundreds or even thousands of miles and reach distant locales. In fact, atmospheric sampling has “… found fern spores at high altitudes, including in the jet stream” (Moran 2008). Long-distance dispersal by wind is thought to be “… far more common in pteridophytes than in seed plants” (Wagner & Smith 1993). The high percentage of ferns on remote oceanic islands in comparison with mainland areas demonstrates this advantage (Smith 1972, 1993; Kramer 1993; Moran 2004, 2008). Likewise, the isolated occurrences of a number of tropical ferns far removed from others of their kind, in tiny pockets of appropriate habitat in Texas, is testament to the dispersal ability of spores. Thus the fern life cycle itself can help these plants to colonize remote places. However, although some ferns are remarkably well-suited for long-distance dispersal, it should be noted that probably less than 10% of ferns can accomplish self-fertilization (most ferns have mechanisms to prevent this and thus ensure cross-fertilization). We now know there is great variation in the probability that different fern species will successfully colonize over long distances, with some being excellent candidates for long-distance dispersal and others not so (Peck et al. 1990; Wild & Gagnon 2005; Ranker & Geiger 2008). Details of the fern life cycle will be discussed in a later section.

These pteridophytes, seedless vascular plants reproducing by spores, include more than 10,000 species, with some authorities suggesting as many as 13,000 or more. This is a sizable number, but these relatively unusual plants still represent less than five percent of all known living vascular plants. Such modern pteridophytes are representatives of evolutionary lineages that have astonishingly long histories. From fossil evidence we know that pteridophytes were once extremely abundant and dominated many of the planet’s ecosystems, some becoming tree-like in size. For example, lycophytes, horsetail relatives, and other ferns were among the dominant plants in the extensive swamps of the Carboniferous Period (359–299 million years ago). Over geologic time, the compressed ancient remains of these swamp plants became coal (Hoshizaki & Moran 2001); in fact, the word “carboniferous,” means “coal-bearing.” In this form, the stored energy of sunlight captured in swamp forests hundreds of millions of years ago provides the modern world with much of its energy.

Because of their similarities, earlier taxonomists often formally grouped ferns and similar plants together as Division Pteridophyta (e.g., Correll 1956). Later, they were often divided into four distinct groups: Division Lycopodiophyta (the lycophytes, including club-mosses, spikemosses, and quillworts), Division Polypodiophyta (the true ferns), Division Psilophyta (the whisk ferns or psilophytes), and Division Equisetophyta (the horsetails). Even though they have only a few species each, the visually distinctive whisk ferns and horsetails have often been segregated into their own groups because they were thought to represent separate evolutionary lineages.

Recent Discoveries

In the past several decades, molecular studies (including DNA sequence data) have revolutionized our understanding of plant evolutionary relationships. No longer do we have to rely solely on comparing the form and structure of living organisms or interpreting the fossil remains of long dead plants, as DNA now provides us a very different and much more detailed way to learn about the evolutionary history of organisms. In a real sense it gives us an exciting new window into the evolutionary history of the species with which we share our planet.

Based on all this new information, it is now clear that ferns and similar plants actually represent only two evolutionarily distinct lineages, which are recognized in this book. These are the lycophytes (club-mosses, spike-mosses, and quillworts), technically referred to as the Division Lycopodiophyta, and the ferns, known as the Polypodiophyta (including not only “typical” ferns, but also both the horsetails, a group of 15 species, and the psilophytes, comprised of 4 to 8 species). Genetic and fossil evidence now shows that about 400 million years ago a major evolutionary split occurred. This split gave rise on one hand to the lycophytes, and on the other hand the remaining vascular plants. The latter lineage—the remaining vascular plants, subsequently split into a fern branch and a branch including all seed plants: the gymnosperms (conifers and their relatives) and angiosperms (flowering plants). So, although lycophytes and ferns do share a number of characteristics (e.g., reproduction by spores), it can be noted from Figure 1 that ferns are actually more closely related to seed plants (i.e., they share a more recent common ancestor) than they are to lycophytes—therefore, lycophytes and ferns need to be treated as separate taxonomic groups.