Pteridophytes

Ferns and their allies belong to the Pteridophytes, a group of four phyla comprising nearly forty families that include some 9,000 to 12,000 species worldwide. Of these, 97 percent are true ferns, the Polypodiophyta (or Pterophyta, or Filicinophyta). They can vary in size from tree ferns with 12-foot leaves to mosquito ferns just 1/16 inch long. The other phyla are Psilophyta, made up of two genera in the whisk fern family (Psilotaceae); Lycopodio-phyta, made up of the club moss or ground pine family (Lycopodiaceae), the spike moss family (Selaginellaceae), and the quillwort family (Isoetaceae); and the Phylum Equiseto-

phyta, containing only the single genus Equi-setum in the horse tail or scouring rush family (Equisetaceae). Pteridophytes are the earliest plants to appear on land, approximately 400 million years ago, during the Devonian Era, before the rise of flower- and seed-bearing plants. Pteridophytes are second only to flowering plants in terms of their diversity among living land plants. The pteridophytes dominated the flora of the Carboniferous Era, and their fossil remains yield most of the world's coal and oil. The three types of pteridophytes other than true ferns are considered "living fossils," which have changed little since the Mesozoic. Today in North America north of Mexico, ninety-three species of pteridophytes are known, and of these twenty-six species are listed as endangered.

The K/T Boundary Fern Spike

Looking deep into the geological past, we find that the end of the Cretaceous Era and the start of the Tertiary Era is marked by the presence of a vast number of fern fossils and little other evidence of life. This phenomenon is known as the K/T boundary fern spike. At that time, the devastation caused by the impact of a meteor nearly 10 km in diameter in Yucatan, Mexico, wiped out most living species on earth. Scientists believe that some fern species survived the fires and dense cloud cover around the world because ferns are very tolerant of shade, and their underground rhizomes can survive fire, sending up new shoots that will grow in soil covered with ash. The death of most other plants and animals gave these ancient pteridophytes a competition-free environment in which they flourished briefly before a succession of new life appeared in the Tertiary.

Structure and Life Cycle of Ferns and Their Allies

Morphology. Most ferns have thin, wiry roots that are shallow and grow from the bottom and sides of the rootstock, called the stock, trunk, or rhizome. The roots hold the plant in place and absorb water and minerals. The rhizome may be short and thick, or more often longer, narrow, and growing horizontally at ground level, either partially or shallowly buried or above ground, but in all cases sprouting stems and leaves from the upper surface. The structure of rhizomes is comparable to that of the stem of a flowering plant. It contains the conducting tissues xylem and phloem, and the supporting tissues called sclerenchyma fibers. The rhizome is perennial and commonly covered with scales or hairs. A fern leaf is called a frond or blade and may be evergreen or annual, according to the species and ecological conditions. The stalk, also called the stipe, stem, or petiole, supports the leaf. The stalk is usually flat or concave in front and rounded in back and often covered with hairs or scales.

Immature fronds start life as tightly coiled shoots that look rather like the curled top of a violin; hence they are called fiddleheads. The way they uncoil as they grow is called cir-cinate vernation. Fronds come in a great array of shapes and sizes, varying from a simple single leaf to extremely compound laciness. The divisions of a compound leaf are known as leaflets or pinnae, and the entire leaf is called once-cut or pinnate. The part of the stem that carries the leaflets is the axis or rachis. A frond is called twice-cut or bipinnate when the leaflets are decompound, or cut into subleaflets or pinnules. If the subleaflets have divisions too, they are named lobes or pinnulets, making the frond thrice-cut, lacy-cut, or tripinnate.

Reproduction. The reproductive structure of ferns and their allies consists of the sporangia, containing the dustlike spores that serve instead of seeds to generate a plant with male and female sex organs that in turn gives rise to the next generation of spore-bearing fern. Sporangia tend to be arranged in a group, called a sorus, of usually sixty-four or sometimes thirty-two or sixteen spores, depending on the species. Sori occur on the underside of fronds that grow after the first, purely vegetative, growth of the season. Young sporangia are pale and darken as the spores mature. In some genera, the entire sori may be protected by a membrane called an indusium, or by a cuplike structure, while in other ferns sori with no protection are called naked.

In most ferns, the sporangia are formed of stalked capsules only one cell thick. Some of the cells that grow in a row are thicker, making them function like a spring as the sporangium matures and dries out. Finally, increased tension bursts open the sporangium, at which spores may fall nearby or float on air or water to great distances. When conditions such as moisture, temperature, light, and soil composition are suitable, a spore will germinate into a tiny green, heart-shaped plant called the gametophyte or prothallus. Sex organs develop on the underside: the antheridia, which produces numerous sperms or antherozoids, and a number of archegonia, each containing a single female cell. Fern sperm require a damp environment, since they can reach archegonia only by swimming through free water to their destination. Hybridization, or the fertilization of one species of fern by a different one, is not uncommon, because antheridia form before archegonia mature, increasing the chance that a gametophyte may fertilize another one, rather than itself. Under ideal circumstances, the sporeling fern, which looks simpler in structure than its mature form, grows from the fertilized gametophyte to produce the sporophyte, or spore-bearing fern.

Reproduction of allied species. These more primitive relatives of ferns also reproduce through a separate gametophyte stage, but there are some differences. Club mosses and quillworts always have only one sporangium per leaf, located at the base. Although some club mosses have bisexual gametophytes like ferns, quillworts and other club mosses have two kinds of sporangia, a larger one that becomes a female gametophyte and a smaller one that develops into a male. Horsetails produce their sporangia on structures called spo-rangiophores, which are grouped into a cone-shaped point usually on top of the main shoot or on the end of branch shoots in some species. Each spore has four armlike elators that begin to flail in response to moisture in the air, helping to disperse the spores. Horsetails also reproduce vegetatively quite readily, since they grow in sections that, if broken, can grow roots and generate a full plant. Many true ferns also reproduce vegetatively by extending new rhizomes that sprout, and often a hillside or grove of ferns will prove to be a single genetic plant.

Human Uses of Ferns and Their Allies Ferns have played a role in human economies since earliest times. Although the bracken can sometimes be mildly toxic, the young, still-curled frond tips called fiddleheads, or croziers, of brackens and other species are often eaten, either cooked or raw. The bracken fern root was once the food staple of the Maori people of New Zealand, and many people today consider fiddleheads a delicacy. Several species can also serve as teas that in some cultures are considered to have medicinal properties, as does the allied Equisetum. Before the invention of modern pesticides, dry bracken fronds were used as a mattress stuffing to repel insects such as bedbugs, and their antimold and insect-repellent properties made fresh brackens an effective packing material for the transport and preservation of fish, fruits, and vegetables. As early as 800 C.E. in Europe, ferns as well as fern ashes were used both as a dye and a fixative for dyes, giving a soft, earthy color as well as helping to preserve fabric.

Ferns such as Lycopodium clavatum produce huge amounts of tiny, powdery spores, which have been used like talcum powder for the skin as well as to coat condoms. "Lycopodium powder," as the spores were known, is also explosively inflammable and was once a popular feature of magic shows, theatricals, and firework displays. Dry fern fronds make excellent fuel and have even been used as money in barter. Early U.S. pioneers called the fern ally common horsetail (Equisetum arvense) the "scouring rush," because the high concentration of silica in its leaves gives it a scratchy texture that works like fine sandpaper or steel wool, suitable for polishing pewter or brass and scouring cookware. In the Middle Ages, horsetails were used to polish armor. At that time, people also thought that fern seeds could make you invisible by sympathetic magic. The logic was that only flowers produce seeds, and therefore fern flowers must be invisible. The myth said that these flowers bloomed on midsummer night, and whoever was touched by the seeds then would remain invisible until daybreak.

Ferns have been decorative items ever since the so-called Victorian fern craze, or pterido-mania, that was made possible in the 1830s when the London resident Dr. Nathaniel Bagshaw Ward discovered that he could grow ferns, which often died in England's industrially polluted gardens, in an airtight glass box, or terrarium, he invented and marketed as the Wardian Case. By the mid-1800s ferns dominated Victorian society, both as design elements in everything from garden furniture to building decoration, and as houseplants that required little light in typically ill-lit rooms. The modern horticulture industry sells a great variety of ferns for use in floral arrangements, as houseplants, and as outdoor ornamentals. They are particularly useful as shade-

loving perennials that flourish where many flowers cannot.

Environmental Aspects of Ferns

Because it's less expensive to collect than grow them, there has been a commercial demand for wild ferns since the Victorian era, which contributes to the present loss of fern species. Fern diversity has also suffered from environmental degradation and loss of habitat, such as clear-cutting forests; human-induced forest fires; replacing ecologically mixed environments with monocultures or human habitations; habitat invasion by alien species; land pollution through agriculture, industry, improper waste disposal, and inadvertent toxic spillage; and redirection, pollution, and siphoning off of waterways and their surrounding landscapes. On the plus side, horsetails have been used in gold prospecting, because of their great capacity to concentrate metals in their leaves, while recent studies indicate that bracken ferns have the potential to remove arsenic from polluted soil by drawing great quantities of it into their leaves.

As conservation agents, ferns play a role in a great variety of environments. In eastern North America the Christmas fern (Polystichum acrostichoides) helps to control erosion, because its fronds rise nearly vertically to pierce through the spring leaf litter on the forest floor, then gradually droop until frost lays them low, trapping the fall leaf accumulation. Species such as the rock-cap ferns (Polypodium vulgare, P. vir-ginianum, and P. appalachianum), which grow on rock ledges, and those that grow on rocky slopes, such as the fragile fern (Cystopteris protrusa) and the maidenhair fern or the narrow glade fern (Adiantum pedatum and Athyrium pycnocarpon), also serve to hold organic matter that falls on the densely growing mat of fronds and roots and help convert rocky litter to soil. On sand banks and river edges, the ostrich fern (Matteuccia struthiopteris), which propagates vegetatively by sending out a network of underground roots to form colonies, stabilizes land against the current's pull, while the cinnamon fern (Osmunda cinnamomea), the royal fern (Osmunda regalis), and the chain fern (Woodwardia virginica), which grow in swampy areas, form little hummocks or islands that aid in making marshy land more solid.

—Mick Wycoff

See also: Botany; Bryophytes; Cretaceous-Tertiary Extinction; Draining of Wetlands; Ecosystems; Evolutionary Biodiversity; Interior Wetlands Bibliography

Cleal, Christopher J., and Barry A. Thomas. 1999. Plant Fossils: The History of Land Vegetation. Vol. 3: Fossils Illustrated. New York: Boydell and Brewer; Cobb, Boughton, and Laura Louise Foster. 1999. A Field Guide to Ferns and Their Related Families: Northeastern and Central North America. Boston: Houghton Mifflin; Fernald, Merritt Lyndon, and Asa Gray. 1987. Portland, OR: Timber; Flora of North America Editorial Committee. 1993. Flora of North America (North of Mexico). Vol. 2: Pteridophytes and Gymnosperms. Oxford: Oxford University Press; Jones, David L. Encyclopedia of Ferns. 1987. Portland, OR: Timber; Kunkel, Guenther. 1984. Plants for Human Consumption: An Annotated Checklist of the Edible Phanerogams and Ferns. Koenigstein: Koeltz Scientific Books; Lellinger, David B., and A. Murray Evans. 1985. A Field Manual of the Ferns and Fern-Allies of the United States and Canada. Washington, DC: Smithsonian Institution Press; Margulis, Lynn, and Karlene Schwartz. 1998. Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth, 3d ed. New York: W. H. Freeman; Raghavan, Vajayamghat. 1989. Developmental Biology of Fern Gametophytes. Cambridge: Cambridge University Press; Vajda, Vivi, J. Ian Raine, and Christopher J. Hollis. 2001. "Indication of Global Deforestation at the Cretaceous-Tertiary Boundary by New Zealand Fern Spike." Science 294:1700-1702.

Punctuated Equilibria

Punctuated equilibria is a term coined in 1972 by paleontologists Niles Eldredge and Stephen Jay Gould for a mode of evolution they con sidered common—and not broadly understood or appreciated in the world of evolutionary biology up to the 1970s. They contrasted it with the notion of phyletic gradualism—that is, the supposition that evolution is largely a matter of the slow, steady ("gradual") transformation of entire species over time, under the general guidance of natural selection. Eldredge and Gould (1972) attributed phyletic gradualism to Darwin and claimed that it had been the general view in paleontology and evolutionary biology ever since 1859.

The core notion of punctuated equilibria is based on the empirical observation of stasis: contrary to Darwinian expectations, most species seen in the fossil record tend to remain very stable—recognizably the same, often for millions of years, from the moment of their first appearance to their very last. This great stability is the "equilibria" part of the term. The observation had been made by a number of paleontologists in Darwin's day, but it had largely been forgotten—most likely inasmuch as stasis was not what was expected to be observed in the new science of evolutionary biology of the late nineteenth century.

Eldredge and Gould claimed that stasis is nearly universal among all sexually reproducing species, especially those that have left a fossil record over the past 540 million years. Although once considered in itself a controversial claim, the reality, frequency of occurrence, and importance of stasis has since come to be acknowledged by the large majority of paleontologists and, increasingly, evolutionary biologists, such as experimental, population, and ecological geneticists.

Thus evolutionary change is concentrated in relatively rapid bursts, estimated to last only 5,000 to 50,000 years—rapid in geological terms, but not especially fast given what is known to be possible in terms of genetic change in modern populations of organisms. This, the "punctuated" part of the term, was considered especially controversial when it was introduced: the phenomenon was often confused with outmoded and discredited ideas of "saltationism," which postulated evolution by sudden jumps through macromutations or other such undocumented genetic processes. In contrast, however, the original term was always firmly associated with the notion of allopatric speciation (see Speciation), as developed especially by the geneticist Theodosius Dobzhansky and the avian systematist Ernst Mayr in New York in the late 1930s and early 1940s. True speciation—as opposed to the model of "phyletic gradualism"—had never been fully explored, let alone accepted, in paleontology.

Stasis recognizes the considerable geographic variation that is developed by many species: the term does not imply that there is no variation within a species. It simply means that the variation seen within a species at any one time is not likely to be transformed into substantial directional change over geological time. Stasis is thought to come about, not because organisms lack sufficient genetic variation to evolve, or because natural selection is weak, but rather because, as environments change, species tend to change their locations—occupying familiar habitat whenever they can get to it. Once there, species tend to remain unchanged, a phenomenon known as "habitat tracking."

The other, and perhaps most important, cause of stasis comes from the fact that most species are fairly widespread, and they are broken up into many localized populations that play roles in their local ecosystems. Circumstances of life are bound to be different in all of these different ecosystems, so that there would be no way for natural selection to change the phenotypic and genotypic properties of all the organisms within a single species in one direction for any great length of time (see Species for more discussion of the geographic structure of species populations).

There are a number of ideas closely associated with punctuated equilibria. One is the notion of "species selection," which grew out of the paradox that, if natural selection is not constantly changing the appearance of organisms within a species over geological time, how then to explain the many examples of long-term, apparently directional, evolutionary trends? For example, one would predict that the fossil record of human evolution would involve a progressive enlargement in the size of the hominid brain—based on the realization that chimpanzees have brain sizes in the range of 400 to 450 ml, while the average brain size of a modern human being is 1,350 ml. And, sure enough, that is what the fossil record shows: the earliest hominids (genus Australopithecus) had brains roughly in the chimpanzee range; earliest members of the genus Homo evolved from these ancestors about 2.5 million years ago and had larger brains, around 750 ml. Later, Homo ergaster and Homo erectus had still larger brains (approximately 1,000 ml).

These data were traditionally interpreted in straightforward phyletic gradualism fashion: gradual evolution over 4 million years took the brains from 450 ml to 1,350 ml, as natural selection would be expected to favor a gradual increase in brain size (hence in presumed intelligence) over time. In contrast, though, fossil species such as Homo erectus show great stability (that is, stasis) in brain size over time, and it is now accepted by the majority of paleoanthropologists that brain size increase in human evolution occurred in a stepwise fashion associated with true speciation. In other words, human evolution accords very well with the notion of punctuated equilibria.

How, then, to explain the increase in brain size? Eldredge and Gould (ibid.) suggested that such examples of directional evolutionary trends could be reconciled with punctuated equilibria simply by recognizing that closely related species in a lineage may well compete for space and resources; in the case of hominids, those with bigger brains might be expected to win out over their smaller-brained relatives, thus driving their ancestors to extinction in a form of "species selection." Indeed, it was Charles Darwin who originally thought that most extinction came about in precisely that way, with newly evolved, superior species actively driving their less endowed, earlier-evolved kin species to extinction simply by out-competing them. The modern view of extinction, however, holds that most extinction events occur through physical stress to ecosystems, rather than through competition between species within an ecosystem—the exception being the current wave of extinction engulfing the planet's species now, an event caused by the presence of ourselves, Homo sapiens.

Paleontologist Elisabeth S. Vrba, however, pointed out that true species selection should have reference to properties of entire species. Brain size, however, is a property of organisms, even though we can calculate an average brain size for entire species and see that the averages differ. She suggested instead that many evolutionary trends could arise simply because some lineages speciate (and thus evolve) and go extinct more quickly than others, and the reasons for this might have more to do with the biological properties of the component organisms themselves—a phenomenon she called the "effect hypothesis." In other words, it would be incorrect to consider the trend in brain size increase in human evo lution "species selection," as the survival of the progressively bigger-brained species was due, not to selection of entire species, but simply to the competitive success of the individuals within the bigger-brained species.

—Niles Eldredge

See also: Evolution; Habitat Tracking; Human Evolution; Natural Selection; Paleontology; Specia-tion; Species

Bibliography

Eldredge, Niles. 1989. Time Frames: The Evolution of Punctuated Equilibria. Princeton: Princeton University Press; Eldredge, Niles. 1999. The Pattern of Evolution. New York: W. H. Freeman; Eldredge, Niles, and Stephen J. Gould. 1972. "Punctuated Equilibria: An Alternative to Phyletic Gradualism." In Models in Paleobiology, edited by T. J. M. Schopf, pp. 82-115. San Francisco: Freeman, Cooper; Futuyma, Douglas J. 1997. Evolutionary Biology. Sun-derland, MA: Sinauer.

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