Introducing the Tracheophytes

Although they are an extraordinarily large and diverse group, the tracheophytes can be said to have been launched by a single evolutionary event. Sometime during the Paleozoic era, probably well before the Silurian period (440 mya), the sporophyte generation of a now long-extinct plant produced a new cell type, the tracheid (Figure 29.10). The tra-cheid is the principal water-conducting element of the xylem in all tracheophytes except the angiosperms, and even in the angiosperms, tracheids persist alongside a more specialized and efficient system of vessels and fibers derived from them.

The evolution of a tissue composed of tracheids had two important consequences. First, it provided a pathway for long-distance transport of water and mineral nutrients from a source of supply to regions of need. Second, its stiff cell walls provided something almost completely lacking—and unnecessary—in the largely aquatic green algae: rigid structural support. Support is important in a terrestrial environment because plants tend to grow upward as they compete for sunlight to power photosynthesis. Thus the tracheid set the stage for the complete and permanent invasion of land by plants.

The tracheophytes feature another evolutionary novelty: a branching, independent sporophyte. A branching sporophyte can produce more spores than an unbranched body, and it can develop in complex ways. The sporophyte of a tra-cheophyte is nutritionally independent of the gametophyte at maturity. Among the tracheophytes, the sporophyte is the large and obvious plant that one normally notices in nature. This pattern is in contrast to the sporophyte of nontracheo-phytes such as mosses, which is attached to, dependent on, and usually much smaller than the gametophyte.

The present-day evolutionary descendants of the early tra-cheophytes belong to seven distinct phyla (see Figure 29.10). The tracheophytes have two types of life cycles, one that involves seeds and another that does not. The nonseed tra-cheophytes (the two basal phyla) include the club mosses and the ferns and their relatives: horsetails and whisk ferns. We will describe these phyla in detail after taking a closer

Common ancestor

Tracheids; branching, independent sporophyte

Common ancestor

Tracheids; branching, independent sporophyte

True Fern Early Stage Sporophyte

29.10 The Evolution of Today's Plants The nine phyla of extant tracheophytes are divided between those that produce seeds and those that do not.

29.10 The Evolution of Today's Plants The nine phyla of extant tracheophytes are divided between those that produce seeds and those that do not.

look at tracheophyte evolution. The five phyla of seed plants will be described in the following chapter.

Tracheophytes have been evolving for almost half a billion years

The evolution of an effective cuticle and of protective layers for the gametangia (archegonia and antheridia) helped make the first tracheophytes successful, as did the initial absence of herbivores (plant-eating animals) on land. By the late Silurian period, tracheophytes were being preserved as fossils that we can study today. Two groups of nonseed tracheophytes that still exist made their first appearances during the Devonian period (409-354 mya): the lycopods (club mosses) and the pteridophytes (including horsetails and ferns). Their proliferation made the terrestrial environment more hospitable to animals. Amphibians and insects arrived soon after the plants became established.

Trees of various kinds appeared in the Devonian period and dominated the landscape of the Carboniferous. Mighty forests of lycopods up to 40 meters tall, horsetails, and tree ferns flourished in the tropical swamps of what would become North America and Europe (Figure 29.11). The remnants of those forests are with us today as huge deposits of coal.

In the subsequent Permian period, the continents came together to form a single gigantic land mass, called Pangaea. The continental interior became warmer and drier, but late in the period glaciation was extensive. The 200-million-year reign of the lycopod-fern forests came to an end as they were replaced by forests of seed plants (gymnosperms), which dominated until other seed plants (angiosperms) became dominant less than 80 million years ago.

The earliest tracheophytes lacked roots and leaves

The earliest known tracheophytes belonged to the now-extinct phylum Rhyniophyta. The rhyniophytes were among the only tracheophytes in the Silurian period. The landscape at that time probably consisted of bare ground, with stands of rhyniophytes in low-lying moist areas. Early versions of the structural features of all the other tracheophyte phyla appeared in the rhyniophytes of that time. These shared features strengthen the case for the origin of all tracheophytes from a common nontracheophyte ancestor.

Tree Ferns From Carboniferous

29.11 An Ancient Forest

This reconstruction is of a Carboniferous forest that once thrived in what is now Michigan.The dominant "trees" are lycopods of the genus Lepidodendron; ferns are also abundant.

29.11 An Ancient Forest

This reconstruction is of a Carboniferous forest that once thrived in what is now Michigan.The dominant "trees" are lycopods of the genus Lepidodendron; ferns are also abundant.

In 1917, the British paleobotanists Robert Kidston and William H. Lang reported their finding of well-preserved fossils of tracheophytes embedded in Devonian rocks near Rhynie, Scotland. The preservation of these plants was remarkable, considering that the rocks were more than 395 million years old. These fossil plants had a simple vascular system of phloem and xylem. Some of the plants had flattened scales on the stems, which lacked vascular tissue and thus were not comparable to the true leaves of any other tracheophytes.

These plants also lacked roots. They were apparently anchored in the soil by horizontal portions of stem, called rhizomes, that bore water-absorbing rhizoids. These rhizomes also bore aerial branches, and sporangia—homologous with the nontracheophyte capsule—were found at the tips of these branches. Their branching pattern was dichotomous; that is, the shoot apex divided to produce two equivalent new branches, each pair diverging at approximately the same angle from the original stem (Figure 29.12). Scattered fragments of such plants had been found earlier, but never in such profusion or so well preserved as those discovered by Kidston and Lang.

Sporangia

Dichotomous branching

Sporangia

Dichotomous branching

Rhyniophyta

Rhizome

29.12 An Ancient Tracheophyte Relative This extinct plant, Aglaophyton major (phylum Rhyniophyta), lacked roots and leaves. It had a central column of xylem running through its stems, but true tracheids were lacking. The rhizome is a horizontal underground stem, not a root. The aerial stems were less than 50 cm tall, and some were topped by sporangia. Other very similar rhyniophytes such as Rhynia did have tracheids.

Rhizome

The presence of xylem indicated that these plants were tra-cheophytes. But were they sporophytes or gametophytes? Close inspection of thin sections of fossil sporangia revealed that the spores were in groups of four. In almost all living nonseed tracheophytes (with no evidence to the contrary from fossil forms), the four products of meiosis and cytokinesis remain attached to one another during their development into spores. The spores separate only when they are mature, and even after separation their walls reveal the exact geometry of how they were attached. Therefore, a group of four closely packed spores is found only immediately after meiosis, and a plant that produces such a group must be a diploid sporophyte—and so the Rhynie fossils must have been sporophytes. Gametophytes of the Rhyniophyta were also found; they, too, were branched, and depressions at the apices of the branches contained archegonia and antheridia.

Although they were apparently ancestral to the other tra-cheophyte phyla, the rhyniophytes themselves are long gone. None of their fossils appear anywhere after the Devonian period.

Early tracheophytes added new features

A new phylum of tracheophytes—the Lycophyta (club mosses)—also appeared in the Silurian period. Another—the Pteridophyta (ferns and fern allies)—appeared during the Devonian period. These two groups arose from rhyniophyte-like ancestors. These new groups featured specializations not found in the rhyniophytes, including one or more of the following: true roots, true leaves, and a differentiation between two types of spores.

the origin of roots. The rhyniophytes had only rhizoids arising from a rhizome with which to gather water and minerals. How, then, did subsequent groups of tracheo-phytes come to have the complex roots we see today?

It is probable that roots had their evolutionary origins as a branch, either of a rhizome or of the aboveground portion of a stem. That branch presumably penetrated the soil and

Vascular tissue

Sporangia

Microphyll
Sporangium Microphyll

branched further. The underground portion could anchor the plant firmly, and even in this primitive condition it could absorb water and minerals. The discovery of fossil plants from the Devonian period, all having horizontal stems (rhizomes) with both underground and aerial branches, supported this hypothesis.

Underground and aboveground branches, growing in sharply different environments, were subjected to very different selection pressures during the succeeding millions of years. Thus the two parts of the plant axis—the aboveground shoot system and the underground root system—diverged in structure and evolved distinct internal and external anatomies. In spite of these differences, scientists believe that the root and shoot systems of tracheophytes are homolo-gous—that they were once part of the same organ.

the origin of true leaves. Thus far we have used the term "leaf" rather loosely. We spoke of "leafy" mosses and commented on the absence of "true leaves" in rhyniophytes. In the strictest sense, a leaf is a flattened photosynthetic structure emerging laterally from a main axis or stem and possessing true vascular tissue. Using this precise definition as we take a closer look at true leaves in the tracheophytes, we see that there are two different types of leaves, very likely of different evolutionary origins.

The first leaf type, the microphyll, is usually small and only rarely has more than a single vascular strand, at least in plants alive today. Plants in the phylum Lycophyta (club mosses), of which only a few genera survive, have such simple leaves. The evolutionary origin of microphylls is thought by some biologists to be sterile sporangia (Figure 29.13a). The principal characteristic of this type of leaf is that its vascular

29.13 The Evolution of Leaves (a) Microphylls are thought to have evolved from sterile sporangia. (b) The megaphylls of pterido-phytes and seed plants may have arisen as photosynthetic tissue developed between branch pairs that were "left behind" as dominant branches overtopped them.

29.13 The Evolution of Leaves (a) Microphylls are thought to have evolved from sterile sporangia. (b) The megaphylls of pterido-phytes and seed plants may have arisen as photosynthetic tissue developed between branch pairs that were "left behind" as dominant branches overtopped them.

Evolution Leaves
The end branches evolved into the veins of leaves.

strand departs from the vascular system of the stem in such a way that the structure of the stem's vascular system is scarcely disturbed. This was true even in the fossil lycopod trees of the Carboniferous period, many of which had leaves many centimeters long.

The other leaf type is found in ferns and seed plants. This larger, more complex leaf is called a megaphyll. The megaphyll is thought to have arisen from the flattening of a dichotomously branching stem system and the development of overtopping (a pattern in which one branch differentiates from and grows beyond the others). This change was followed by the development of pho-tosynthetic tissue between the members of overtopped groups of branches (Figure 29.13b). Mega-phylls may have evolved more than once, in different phyla of tracheophytes showing overtopping of branches.

homospory and heterospory. In the most ancient of the present-day tracheophytes, both the gametophyte and the sporophyte are independent and usually photosynthetic. Spores produced by the sporophytes are of a single type, and they develop into a single type of gameto-phyte that bears both female and male reproductive organs. The female organ is a multicellular archegonium, typically containing a single egg. The male organ is an antheridium, containing many sperm. Such plants, which bear a single type of spore, are said to be homosporous (Figure 29.14a).

A different system, with two distinct types of spores, evolved somewhat later. Plants of this type are said to be heterosporous (Figure 29.14b). One type of spore, the megaspore, develops into a larger, specifically female gametophyte (a megagametophyte) that produces only eggs. The other type, the microspore, develops into a smaller, male gametophyte (a microgametophyte) that produces only sperm. The sporophyte produces megaspores in small numbers in megaspo-rangia on the sporophyte, and microspores in large numbers in microsporangia.

The most ancient tracheophytes were all homo-sporous, but heterospory evidently evolved independently several times in the early descendants of the rhyniophytes. The fact that heterospory evolved repeatedly suggests that it affords selective advantages. Subsequent evolution in the plant kingdom featured ever greater specialization of the heterosporous condition.

Capsule Showing Spore Mother Cells

Spore mother cell (2n)

Zygote (2n)

Spore mother cell (2n)

Zygote (2n)

Sporangium (2n)

Sporophyte (2n)

Embryo (2n)

(b) Heterospory

Sporophyte (2n)

(b) Heterospory

Fig Mature Gametophyte Fern
Spore mother cell (2n)

Zygote (2n)

Spore mother cell (2n)

er S

Spore mother cell (2n)

Zygote (2n)

Megasporangium (2n)

Microsporangium (2n)

Sporophyte (2n)

Embryo (2n)

29.14 Homospory and Heterospory (a) Homosporous plants bear a single type of spore. Each gametophyte has two types of sex organs, antheridia (male) and archegonia (female). (b) Heterosporous plants, which bear two types of spores that develop into distinctly male and female gametophytes, evolved later.

Some tracheophyte clades arose and became extinct in the course of evolution. The earliest clades to arise and survive to this day belong to the nonseed tracheophytes.

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Responses

  • charli alexander
    What was first lycopod or whisk fern?
    8 years ago
  • bisrat
    What is tracheophytes?
    8 years ago
  • BRENDA
    Where are the tracheophytes belong?
    8 years ago
  • arnor
    What came first branched sporophyte or tracheids?
    8 years ago
  • madoc
    How are tracheophytes divided?
    8 years ago
  • pirkko
    What is a mature gametophyte?
    8 years ago
  • jennifer
    How did the tracheophytes appear on land?
    8 years ago
  • arcangelo
    Which evolved first branced sporophyte or traechids?
    7 years ago
  • ASMAIT
    Which evolved first: a branched sporophyte or tracheids?
    7 years ago
  • bellina
    How did microphyll arose?
    7 years ago

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