The Nontracheophytes Liverworts Hornworts and Mosses

Most liverworts, hornworts, and mosses grow in dense mats, usually in moist habitats. The largest of these plants are only about 1 meter tall, and most are only a few centimeters tall or long. Why have the nontracheophytes not evolved to be taller? The probable answer is that they lack an efficient system for conducting water and minerals from the soil to distant parts of the plant body. To limit water loss, layers of maternal tissue protect the embryos of all nontracheophytes. All nontracheophyte clades also have a cuticle, although it is often very thin (or even absent in some species) and thus not highly effective in retarding water loss. Nontracheophytes lack the leaves, stems, and roots that characterize tracheo-phytes, although they have structures analogous to each.

Most nontracheophytes live on the soil or on other plants, but some grow on bare rock, dead and fallen tree trunks, and even on buildings. Nontracheophytes are widely distributed over six continents and exist very locally on the coast of the

Nontracheophyte

Gametophytes (n)

Photosynthetic filament

Gametophytes (n)

Photosynthetic filament

Rhizoid

Nonvascular Plants Rhizoid Stomata Foot

DIPLOID (2n) Sporophyte generation

Archegonium (n)

Rhizoid

DIPLOID (2n) Sporophyte generation

Sporophyte (2n)

Archegonium (n)

Hornwort Archegonium

Fertilization in nontracheophytes requires water so that sperm can swim to eggs.

fiffi 29.5 A Nontracheophyte Life I 'J "JN Cycle The life cycle of nontra-i / cheophytes, illustrated here by a 1 moss, is dependent on an external source of liquid water. The visible green structure of nontracheophytes is the gametophyte.

Fertilization in nontracheophytes requires water so that sperm can swim to eggs.

Sporophyte (2n)

Gametophyte Sporophyte Attached

Embryo (2n)

The sporophyte is attached to and nutritionally dependent on the gametophyte.

Gametophyte (n)

Embryo (2n)

The sporophyte is attached to and nutritionally dependent on the gametophyte.

Gametophyte (n)

seventh (Antarctica). They are successful plants, well adapted to their environments. Most are terrestrial. Some live in wetlands. Although a few nontracheophyte species live in fresh water, these aquatic forms are descended from terrestrial ones. There are no marine nontracheophytes.

Nontracheophyte sporophytes are dependent on gametophytes

In nontracheophytes, the conspicuous green structure visible to the naked eye is the gametophyte (Figure 29.5). In contrast, the familiar forms of tracheophytes, such as ferns and seed plants, are sporophytes. The gametophyte of nontracheo-phytes is photosynthetic and therefore nutritionally independent, whereas the sporophyte may or may not be photo-synthetic, but is always nutritionally dependent on the gametophyte and remains permanently attached to it.

A nontracheophyte sporophyte produces unicellular, hap-loid spores as products of meiosis within a sporangium, or capsule. A spore germinates, giving rise to a multicellular, haploid gametophyte whose cells contain chloroplasts and are fiffi 29.5 A Nontracheophyte Life I 'J "JN Cycle The life cycle of nontra-i / cheophytes, illustrated here by a 1 moss, is dependent on an external source of liquid water. The visible green structure of nontracheophytes is the gametophyte.

thus photosynthetic. Eventually gametes form within specialized sex organs, the gametangia. The archegonium is a multicellular, flask-shaped female sex organ with a long neck and a swollen base, which produces a single egg (Figure 29.6a). The antheridium is a male sex organ in which sperm, each bearing two flagella, are produced in large numbers (Figure 29.6b).

Once released, the sperm must swim or be splashed by raindrops to a nearby archegonium on the same or a neighboring plant. The sperm are aided in this task by chemical attractants released by the egg or the archego-nium. Before sperm can enter the archegonium, certain cells in the neck of the archegonium must break down, leaving a water-filled canal through which the sperm swim to complete their journey. Note that all of these events require liquid water.

On arrival at the egg, the nucleus of a sperm fuses with the egg nucleus to form a zygote. Mitotic divisions of the zygote produce a multicellular, diploid sporophyte embryo. The base of the archegonium grows to protect the embryo during its early development. Eventually, the developing sporophyte elongates sufficiently to break out of the archegonium, but it remains connected to the gametophyte by a "foot" that is embedded in the parent tissue and absorbs water and nutrients from it. The sporophyte remains attached to the gametophyte throughout its life. The sporo-phyte produces a capsule, within which meiotic divisions produce spores and thus the next gametophyte generation.

The structure and pattern of elongation of the sporophyte differ among the three nontracheophyte phyla—the liverworts (Hepatophyta), hornworts (Anthocerophyta), and mosses (Bryophyta). The probable evolutionary relationships of these three phyla and the tracheophytes can be seen in Figure 29.4.

29.6 Sex Organs in Plants

(a) Archegonia and (b) anthe-ridia of the moss Mnium (phylum Bryophyta).The gametophytes of all plants have archegonia and antheridia, but they are much reduced in seed plants.

Archegonia develop at the tip of a gametophyte. In the archegonium, the egg will be fertilized and begin development into a sporophyte.

Antheridia are also located at the tip of a gametophyte.

Archegonia develop at the tip of a gametophyte. In the archegonium, the egg will be fertilized and begin development into a sporophyte.

Antheridia are also located at the tip of a gametophyte.

29.6 Sex Organs in Plants

(a) Archegonia and (b) anthe-ridia of the moss Mnium (phylum Bryophyta).The gametophytes of all plants have archegonia and antheridia, but they are much reduced in seed plants.

Archegonium

The large egg cell is in the center of the archegonium.

These antheridia contain a large number of sperm. When released, the sperm can be carried by water to an archegonium and then swim down its neck to the egg.

The large egg cell is in the center of the archegonium.

These antheridia contain a large number of sperm. When released, the sperm can be carried by water to an archegonium and then swim down its neck to the egg.

Liverworts may be the most ancient surviving plant clade

The gametophytes of some liverworts (phylum Hepato-phyta) are green, leaflike layers that lie flat on the ground (Figure 29.7a). The simplest liverwort gametophytes, however, are flat plates of cells, a centimeter or so long, that produce antheridia or archegonia on their upper surfaces and anchoring and water-absorbing filaments called rhizoids on their lower surfaces. Liverwort sporophytes are shorter than those of mosses and hornworts, rarely exceeding a few millimeters.

The liverwort sporophyte has a stalk that connects capsule and foot. In most species, the stalk elongates and thus raises the capsule above ground level, favoring dispersal of spores when they are released. The capsules of liverworts are simple: a globular capsule wall surrounding a mass of spores. In some species of liverworts, spores are not released by the sporophyte until the surrounding capsule wall rots. In other liverworts, however, the spores are thrown from the capsule by structures

29.7 Liverwort Structures Members of the phylum Hepatophyta display various characteristic structures. (a) Gametophytes. (b) Structures bearing antheridia and archegonia. (c) Gemmae cups.

that shorten and compress a "spring" as they dry out. When the stress becomes sufficient, the compressed spring snaps back to its resting position, throwing spores in all directions.

Among the most familiar liverworts are species of the genus Marchantía (Figure 29.7a). Marchantía is easily recognized by the characteristic structures on which its male and female gametophytes bear their antheridia and archegonia (Figure 29.7b). Like most liverworts, Marchantía also reproduces asexually by simple fragmentation of the gametophyte. Marchantía and some other liverworts and mosses also reproduce asexually by means of gemmae (singular, gemma), which are lens-shaped clumps of cells. In a few liverworts, the gemmae are loosely held in structures called gemmae cups, which promote dispersal of the gemmae by raindrops (Figure 29.7c).

Hornworts evolved stomata as an adaptation to terrestrial life

The phylum Anthocerophyta comprises the hornworts, so named because their sporophytes look like little horns (Fig-

29.7 Liverwort Structures Members of the phylum Hepatophyta display various characteristic structures. (a) Gametophytes. (b) Structures bearing antheridia and archegonia. (c) Gemmae cups.

Hepatophyta Antheridia Anthoceros Gametophyte

Anthoceros sp.

29.8 A Hornwort The sporophytes of hornworts can resemble little horns.

Anthoceros sp.

29.8 A Hornwort The sporophytes of hornworts can resemble little horns.

sible interpretation of the current data. The exact evolutionary status of the hornworts is still unclear, and in some phyloge-netic analyses they are placed as the most ancient plant clade.

Water and sugar transport mechanisms emerged in the mosses

The most familiar nontracheophytes are the mosses (phylum Bryophyta). There are more species of mosses than of liverworts and hornworts combined, and these hardy little plants are found in almost every terrestrial environment. They are often found on damp, cool ground, where they form thick mats (Figure 29.9a). The mosses are probably sister to the tra-cheophytes (see Figure 29.4).

Many mosses contain a type of cell called a hydroid, which dies and leaves a tiny channel through which water can ure 29.8). Hornworts appear at first glance to be liverworts with very simple gametophytes. These gametophytes consist of flat plates of cells a few cells thick.

However, the hornworts, along with the mosses and tra-cheophytes, share an advance over the liverwort clade in their adaptation to life on land. They have stomata—pores that, when open, allow the uptake of CO2 for photosynthesis and the release of O2. Stomata may be a shared derived trait (synapomorphy) of hornworts and all other plants except liverworts, although hornwort stomata do not close and may have evolved independently.

Hornworts have two characteristics that distinguish them from both liverworts and mosses. First, the cells of hornworts each contain a single large, platelike chloroplast, whereas the cells of other nontracheophytes contain numerous small, lens-shaped chloroplasts. Second, of all the nontracheophyte sporophytes, those of the hornworts come closest to being capable of indeterminate growth (growth without a set limit). Liverwort and moss sporophytes have a stalk that stops growing as the capsule matures, so elongation of the sporo-phyte is strictly limited. The hornwort sporophyte, however, has no stalk. Instead, a basal region of the capsule remains capable of indefinite cell division, continuously producing new spore-bearing tissue above. The sporophytes of some hornworts growing in mild and continuously moist conditions can become as tall as 20 centimeters. Eventually the sporophyte's growth is limited by the lack of a transport system.

To support their metabolism, the hornworts need access to nitrogen. Hornworts have internal cavities filled with mucilage; these cavities are often populated by cyanobacteria that convert atmospheric nitrogen gas into a form usable by the host plant.

We have presented the hornworts as sister to the clade consisting of mosses and tracheophytes, but this is only one pos-

Mucilage Hornworts
29.9 The Mosses (a) Dense moss forms hummocks in a valley on New Zealand's South Island. (b) The moss capsule, from which spores are dispersed, grows at the tip of the plant.

travel. The hydroid may be the progenitor of the tracheid, the characteristic water-conducting cell of the tracheophytes, but it lacks lignin (a waterproofing substance that also lends structural support) and the cell wall structure found in tra-cheids. The possession of hydroids and of a limited system for transport of sucrose by some mosses (via cells called lep-toids) shows that the old term "nonvascular plant" is somewhat misleading when applied to mosses.

In contrast to liverworts and hornworts, the sporophytes of mosses and tracheophytes grow by apical cell division, in which a region at the growing tip provides an organized pattern of cell division, elongation, and differentiation. This growth pattern allows extensive and sturdy vertical growth of sporophytes. Apical cell division is a shared derived trait of mosses and tracheophytes.

The moss gametophyte that develops following spore germination is a branched, filamentous structure called a pro-tonema (see Figure 29.5). Although the protonema looks a bit like a filamentous green alga, it is unique to the mosses. Some of the filaments contain chloroplasts and are photosynthetic; others, called rhizoids, are nonphotosynthetic and anchor the protonema to the substratum. After a period of linear growth, cells close to the tips of the photosynthetic filaments divide rapidly in three dimensions to form buds. The buds eventually differentiate a distinct tip, or apex, and produce the familiar leafy moss shoot with leaflike structures arranged spirally. These leafy shoots produce antheridia or archegonia (see Figure 29.6). The antheridia release sperm that travel through liquid water to the archegonia, where they fertilize the eggs.

Sporophyte development in most mosses follows a precise pattern, resulting ultimately in the formation of an absorptive foot anchored to the gametophyte, a stalk, and, at the tip, a swollen capsule, the sporangium. In contrast to hornworts, whose sporophytes grow from the base, the moss sporophyte stalk grows at its apical end, as tracheophytes do. Cells at the tip of the stalk divide, supporting elongation of the structure and giving rise to the capsule. For a while, the archegonial tissue grows rapidly as the stalk elongates, but eventually the archegonium is outgrown and is torn apart by the expanding sporophyte.

The lid of the capsule is shed after the completion of meio-sis and spore development. In most mosses, groups of cells just below the lid form a series of toothlike structures surrounding the opening. Highly responsive to humidity, these structures dig into the mass of spores when the atmosphere is dry; then, when the atmosphere becomes moist, they fling out, scooping out the spores as they go (Figure 29.9b). The spores are thus dispersed when the surrounding air is moist—that is, when conditions favor their subsequent germination.

Mosses of the genus Sphagnum often grow in swampy places, where the plants begin to decompose in the water af ter they die. Rapidly growing upper layers compress the deeper-lying, decomposing layers. Partially decomposed plant matter is called peat. In some parts of the world, people derive the majority of their fuel from peat bogs. Sphagnum-dominated peatlands cover an area approximately half as large as the United States—more than 1 percent of Earth's surface. Long ago, continued compression of peat composed primarily of other nonseed plants gave rise to coal.

With their simple system of internal transport, the mosses are, in a sense, vascular plants. However, they are not tra-cheophytes because they lack true xylem and phloem.

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  • arlene
    What does nontracheophytes mean?
    8 years ago
  • birgit
    Why are nontracheophytes small?
    8 years ago
  • rudigar
    What does a hornwort sporophyte look like?
    8 years ago
  • Fearne Sinclair
    Where are nontracheophytes found?
    8 years ago
  • Lucio Padovesi
    Which clade would be considered the out group: hornworts, liverworts, or mosses?
    8 years ago
  • hessu koskela
    How to hornwort release o2?
    8 years ago
  • adeline
    Where is the archegonia located?
    8 years ago
  • Patryk Macleod
    Why arent liverworts hornworts and mosses a clade?
    8 years ago
  • senja
    Are the sporophyte of a nontracheophyte dependent or independent?
    8 years ago
  • melanie koertig
    What are some conditions that hornworts need.?
    8 years ago
  • ilse
    Why mosses liverworts and hornworts all live close to the ground?
    8 years ago
  • george woolum
    Which structures can be seen with naked eye on a hepatophyta?
    7 years ago
  • STEPHAN
    What are the differentiation of mosses; liverworts, and hornworts?
    7 years ago
  • crispus smallburrow
    Is the gametophyte, sporophyte, or both nutritionally independent in Phylum Hepatophyta?
    7 years ago
  • Hagos
    Why don't liverwort mosses and hornworts form a clade?
    7 years ago
  • caradoc sandyman
    Are marchantia nontracheophytes?
    7 years ago
  • seija
    Are spores of nontracheophytes multicellular?
    7 years ago
  • Abraham
    What advantage might there be to the liverworts being flat and close to the ground?
    7 years ago
  • sarah
    Why anthoceros called hornworts?
    7 years ago
  • holly
    Why are mosses liverworts and hornworts so much shorter than other plants?
    6 years ago
  • Semere Girmay
    How do nontracheophytes transport?
    6 years ago
  • Dahlak
    How are hornworts and moss sporophytes different?
    4 years ago
  • Bella
    What is the egg bearing structures in the nontracheophyte?
    4 years ago
  • kyllikki
    How to see the archegonia in moss?
    4 years ago
  • isengrim
    Is the gametophyte sporophyte or both nutritionally independent in phylum hepatipcphya?
    3 years ago
  • rowan
    Which plant has a Foot and Rhizoid?
    2 years ago
  • klaus
    How can diferentiate mosses liverworts and hornwort?
    2 years ago

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