The Surviving Nonseed Tracheophytes

The nonseed tracheophytes have a large, independent sporo-phyte and a small gametophyte that is independent of the sporophyte. The gametophytes of the surviving nonseed tra-cheophytes are rarely more than 1 or 2 centimeters long and are short-lived, whereas their sporophytes are often highly visible; the sporophyte of a tree fern, for example, may be 15 or 20 meters tall and may live for many years.

The most prominent resting stage in the life cycle of a non-seed tracheophyte is the single-celled spore. This feature makes their life cycle similar to those of the fungi, the green algae, and the nontracheophytes, but not, as we will see in the next chapter, to that of the seed plants. Nonseed tracheo-phytes must have an aqueous environment for at least one stage of their life cycle because fertilization is accomplished by a motile, flagellated sperm.

The ferns are the most abundant and diverse group of nonseed tracheophytes today, but the club mosses and horsetails were once dominant elements of Earth's vegetation. A fourth group, the whisk ferns, contains only two genera. In this section we'll look at the characteristics of these four groups and at some of the evolutionary advances that appeared in them.

The club mosses are sister to the other tracheophytes

The club mosses and their relatives (together called lycopods, phylum Lycophyta) diverged earlier than all other living tra-cheophytes—that is, the remaining tracheophytes share an ancestor that was not ancestral to the Lycophyta. There are relatively few surviving species of club mosses.

The lycopods have roots that branch dichoto-mously. The arrangement of vascular tissue in their stems is simpler than in the other tracheophytes. They bear only mi-crophylls, and these simple leaves are arranged spirally on the stem. Growth in club mosses comes entirely from apical cell division, and branching is dichotomous, by a division of the apical cluster of dividing cells.

The sporangia in many club mosses are contained within conelike structures called strobili (singular, strobilus; Figure 29.15). A strobilus is a cluster of spore-bearing leaves inserted on an axis tucked into the upper angle between a specialized leaf and the stem. (Such an angle is called an axil.) Other club mosses lack strobili and bear their sporangia in the axil between a photosynthetic leaf and the stem. This placement contrasts with the apical sporangia of the rhyniophytes. There are both homosporous species and heterosporous

Microsporangium

Microsporangium

Microsporangium

Phylum Lycophyta Club Mosses

(a) Lycopodium obscurum

29.15 Club Mosses (a) Strobili are visible at the tips of this club moss. Club mosses have microphylls arranged spirally on their stems. (b) A thin section through a strobilus of a club moss, showing microsporangia.

(a) Lycopodium obscurum

29.15 Club Mosses (a) Strobili are visible at the tips of this club moss. Club mosses have microphylls arranged spirally on their stems. (b) A thin section through a strobilus of a club moss, showing microsporangia.

species of club mosses. Although only a minor element of present-day vegetation, the Lycophyta are one of two phyla that appear to have been the dominant vegetation during the Carboniferous period. One type of coal (cannel coal) is formed almost entirely from fossilized spores of the tree ly-copod Lepidodendron—which gives us an idea of the abundance of this genus in the forests of that time (see Figure 29.11). The other major elements of Carboniferous vegetation were horsetails and ferns.

Horsetails, whisk ferns, and ferns constitute a clade

Once treated as distinct phyla, the horsetails, whisk ferns, and ferns form a clade, the phylum Pteridophyta (pteridophytes, or "ferns and fern allies"). Within that clade, the whisk ferns and the horsetails are both monophyletic; the ferns are not. However, about 97 percent of all fern species, including those with which you are most likely to be familiar, do belong to a single clade, the leptosporangiate ferns. In the pteridophytes— and in all seed plants—there is differentiation (overtopping) between the main axis and side branches.

horsetails grow at the bases of stem segments. Like the club mosses, the horsetails are represented by only a few

present-day species. All are in a single genus, Equisetum. These plants are sometimes called "scouring rushes" because silica deposits found in their cell walls made them useful for cleaning. They have true roots that branch irregularly. Their sporangia curve back toward the stem on the ends of short stalks called sporangiophores (Figure 29.16a). Horsetails have a large sporophyte and a small gametophyte, both independent.

The small leaves of horsetails are reduced megaphylls and form in distinct whorls (circles) around the stem (Figure 29.16b). Growth in horsetails originates to a large extent from discs of dividing cells just above each whorl of leaves, so each segment of the stem grows from its base. Such basal growth is uncommon in plants, although it is found in the grasses, a major group of flowering plants.

Sporangium

Sporangium

Microphyll Equisetum Arvense

(a) Equisetum arvense

(b) Equisetum palustre

29.16 Horsetails (a) Sporangia and sporangiophores of a horsetail. (b) Vegetative and fertile shoots of the marsh horsetail. Reduced megaphylls can be seen in whorls on the stem of the vegetative shoot on the right; the fertile shoot on the left is ready to disperse its spores.

(a) Equisetum arvense

(b) Equisetum palustre present-day whisk ferns resemble the most ancient tracheophytes. There once was some disagreement about whether rhyniophytes are entirely extinct. The confusion arose because of the existence today of two genera of rootless, spore-bearing plants, Psilotum and Tmesipteris, collectively called the whisk ferns. Psilotum nudum (Figure 29.17) has only minute scales instead of true leaves, but plants of the genus Tmesipteris have flattened photosynthetic organs—reduced mega-phylls—with well-developed vascular tissue. Are these two genera the living relics of the rhyniophytes, or do they have more recent origins?

Psilotum and Tmesipteris once were thought to be evolu-tionarily ancient descendants of anatomically simple ancestors. That hypothesis was weakened by an enormous hole in the geological record between the rhyniophytes, which apparently became extinct more than 300 million years ago, and Psilotum and Tmesipteris, which are modern plants. DNA sequence data finally settled the question in favor of a more modern origin of the whisk ferns from fernlike ancestors. These two genera are a clade of highly specialized plants that evolved fairly recently from anatomically more complex ancestors by loss of complex leaves and true roots. Whisk fern gametophytes live below the surface of the ground and lack chlorophyll. They depend upon fungal partners for their nutrition.

Ferns evolved large, complex leaves

The sporophytes of the ferns, like those of the seed plants, have true roots, stems, and leaves. Their leaves are typically large and have branching vascular strands. Some species have small leaves as a result of evolutionary reduction, but

29.16 Horsetails (a) Sporangia and sporangiophores of a horsetail. (b) Vegetative and fertile shoots of the marsh horsetail. Reduced megaphylls can be seen in whorls on the stem of the vegetative shoot on the right; the fertile shoot on the left is ready to disperse its spores.

even these small leaves have more than one vascular strand, and are thus megaphylls.

The ferns constitute a group that first appeared during the Devonian period and today consists of about 12,000 species. The ferns are not a monophyletic group, although, as already mentioned, 97 percent of the species—the leptosporangiate ferns—do constitute a monophyletic group. The leptospo-rangiate ferns differ from the other ferns in having sporangia with walls only one cell thick, borne on a stalk.

Tracheophytes Psilophyta Whisk Fern

Psilotum nudum

29.17 A Whisk Fern Psilotum nudum was once considered by some to be a surviving rhyniophyte and by others to be a fern. It is now included in the phylum Pteridophyta, and it is widespread in the Tropics and Subtropics.

Psilotum nudum

29.17 A Whisk Fern Psilotum nudum was once considered by some to be a surviving rhyniophyte and by others to be a fern. It is now included in the phylum Pteridophyta, and it is widespread in the Tropics and Subtropics.

Lycophyta Pteridophyta

(a) Adiantum pedatum

29.18 Fern Fronds Take Many Forms (a) The fronds of Northern maidenhair fern form a pattern in this photograph. (b) The "fiddle-head" (developing frond) of a common forest fern; this structure will unfurl and expand to give rise to a complex adult frond such as those in (a). (c) The tiny fronds of a water fern.

(c) Marsilea mutica

(a) Adiantum pedatum

29.18 Fern Fronds Take Many Forms (a) The fronds of Northern maidenhair fern form a pattern in this photograph. (b) The "fiddle-head" (developing frond) of a common forest fern; this structure will unfurl and expand to give rise to a complex adult frond such as those in (a). (c) The tiny fronds of a water fern.

Ferns are characterized by fronds (large leaves with complex vasculature; Figure 29.18a). During its development, the fern frond unfurls from a tightly coiled "fiddlehead" (Figure 29.18fr). Some fern leaves become climbing organs and may grow to be as much as 30 meters long.

Because they require water for the transport of the male gametes to the female gametes, most ferns inhabit shaded, moist woodlands and swamps. Tree ferns can reach heights of 20 meters. Tree ferns are not as rigid as woody plants, and they have poorly developed root systems. Thus they do not grow in sites exposed directly to strong winds, but rather in

Pictures Spore Bearing Tracheophyte

Dryopteris intermedia

29.19 Fern Sori Are Clusters of Sporangia Sori,each containing many spore-producing sporangia, have formed on the underside of this frond of the Midwestern fancy fern.

Dryopteris intermedia

29.19 Fern Sori Are Clusters of Sporangia Sori,each containing many spore-producing sporangia, have formed on the underside of this frond of the Midwestern fancy fern.

(c) Marsilea mutica ravines or beneath trees in forests. The sporangia of ferns are found on the undersurfaces of the fronds, sometimes covering the whole undersurface and sometimes only at the edges. In most species the sporangia are found in clusters called sori (singular, sorus) (Figure 29.19).

The sporophyte generation dominates the fern life cycle

Inside the sporangia, fern spore mother cells undergo meio-sis to form haploid spores. Once shed, the spores travel great distances and eventually germinate to form independent ga-metophytes. Old World climbing fern, Lygodium microphyllum, is currently spreading disastrously through the Florida Everglades, choking off the growth of other plants. This rapid spread is testimony to the effectiveness of windborne spores.

Fern gametophytes have the potential to produce both an-theridia and archegonia, although not necessarily at the same time or on the same gametophyte. Sperm swim through water to archegonia—often to those on other gametophytes— where they unite with an egg. The resulting zygote develops into a new sporophyte embryo. The young sporophyte sprouts a root and can thus grow independently of the gametophyte. In the alternating generations of a fern, the ga-metophyte is small, delicate, and short-lived, but the sporo-phyte can be very large and can sometimes survive for hundreds of years (Figure 29.20).

Most ferns are homosporous. However, two groups of aquatic ferns, the Marsileaceae and Salviniaceae, are derived from a common ancestor that evolved heterospory. The megaspores and microspores of these plants (which germinate to produce female and male gametophytes, respectively) are produced in different sporangia (megasporangia and mi-crosporangia), and the microspores are always much smaller and greater in number than the megaspores.

29.20 The Life Cycle of a Fern The most conspicuous stage in the fern life cycle is the mature, diploid sporophyte

Meiosis

A few genera of ferns produce a tuberous, fleshy gameto-phyte instead of the characteristic flattened, photosynthetic structure produced by most ferns. Like the gametophytes of whisk ferns, these tuberous gametophytes depend on a mutualistic fungus for nutrition; in some genera, even the sporophyte embryo must become associated with the fungus before extensive development can proceed. In Chapter 31 we will see that there are many other important plant-fungus mutualisms.

All the tracheophytes we have discussed thus far disperse themselves by spores. In the next chapter we will discuss the plants that dominate most of Earth's vegetation today, the seed plants, whose seeds afford new sporophytes protection unavailable to those of the nonseed tracheophytes.

29.20 The Life Cycle of a Fern The most conspicuous stage in the fern life cycle is the mature, diploid sporophyte

Virus Aquatic Fern

Meiosis

HAPLOID (n) DIPLOID (2n)

Fertilization

Mature sporophyte (typically 0.3-1 m tall)

HAPLOID (n) DIPLOID (2n)

Fertilization

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Responses

  • mike
    Are whiskfern a tracheophyte?
    8 years ago
  • eileen
    What is a "spore bearing tracheophyte"?
    8 years ago
  • pantaleone
    How do whisk ferns disperse spores?
    8 years ago
  • Teuvo
    What is an example of a plant that is a nonseed tracheophyte?
    8 years ago
  • kalevi
    What phylum does adiantum pedatum belong to?
    8 years ago
  • D
    Do horsetails and club mosses have life cycles independent of aqueous enviornments?
    7 years ago
  • MENEGILDA
    How do ancient members of Lycophyta phylum differ from present day?
    7 years ago
  • wallace
    Are ferns, whisk ferns and horsetails monophyletic?
    7 years ago
  • lisa
    How do clubmosses disperse spores?
    6 years ago

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