BSC 1011C
General Biology II
Dr. Graeme Lindbeck
glindbeck@valenciacollege.edu


Plant Diversity I: How Plants Colonized Land

Outline

A. An Overview of Land Plant Evolution

  1. Evolutionary adaptations to terrestrial living characterize the four main groups of land plants
  2. Charophyceans are the green algae most closely related to land plants
  3. Several terrestrial adaptations distinguish land plants from charophycean algae

B. The Origin of Land Plants

  1. Land plants evolved from charophycean algae over 500 million years ago
  2. Alternation of generations in plants may have originated by delayed meiosis
  3. Adaptations to shallow water preadapted plants for living on land
  4. Plant taxonomists are reevaluating the boundaries of the plant kingdom
  5. The plant kingdom is monophyletic

C. Bryophytes

  1. The three phyla of bryophytes are mosses, liverworts, and hornworts
  2. The gametophyte is the dominant generation in the life cycles of bryophytes
  3. Bryophyte sporophytes disperse enormous numbers of spores
  4. Brophytes provide many ecological and economic benefits

D. The Origin of Vascular Plants

  1. Additional terrestrial adaptations evolved as vascular plants descended from mosslike ancestors
  2. A diversity of vascular plants evolved over 400 million years ago

E. Pteridophytes: Seedless Vascular Plants

  1. Pteridophytes provide clues to the evolution of roots and leaves
  2. A sporophyte-dominant life cycle evolved in seedless vascular plants
  3. Lycophyta and Pterophyta are the two phyla of modern seedless vascular plants
  4. Seedless vascular plants formed vast "coal forests" during the Carboniferous period

Introduction

More than 280,000 species of plants inhabit Earth today.

Most plants live in terrestrial environments, including deserts, grasslands, and forests.

Land plants (including the sea grasses) evolved from a certain green algae, called charophyceans.

1. Evolutionary adaptations to terrestrial living characterize the four main groups of land plants

There are four main groups of land plants: bryophytes, pteridophytes, gymnosperms, and angiosperms.

Mosses and other bryophytes have evolved several adaptations, especially reproductive adaptations, for life on land.

The other major groups of land plants evolved vascular tissue and are known as the vascular plants.

Ferns and other pteridiophytes are sometimes called seedless plants because there is no seed stage in their life cycles.

The evolution of the seed in an ancestor common to gymnosperms and angiosperms facilitated reproduction on land.

The early seed plants gave rise to the diversity of present-day gymnosperms, including conifers.

The great majority of modern-day plant species are flowering plants, or angiosperms.

Bryophytes, pteridiophytes, gymnosperms, ands angiosperms demonstrate four great episodes in the evolution of land plants:

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2. Charophyceans are the green algae most closely related to land plants

What features distinguish land plants from other organisms?

Plants are multicellular, eukaryotic, photosynthetic autrotrophs.

Land plants have cells walls made of cellulose and chlorophyll a and b in chloroplasts.

Land plants share two key ultrastructural features with their closet relatives, the algal group called charophyceans.

The plasma membranes of land plants and charophyceans possess rosette cellulose-synthesizing complexes that synthesize the cellulose microfibrils of the cell wall.

A second ultrastructural feature that unites charophyceans and land plants is the presence of peroxisomes.

In those land plants that have flagellated sperm cells, the structure of the sperm resembles the sperm of charophyceans.

Finally, certain details of cell division are common only to land plants and the most complex charophycean algae

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3. Several terrestrial adaptations distinguish land plants from charophycean algae

Several characteristics separate the four land plant groups from their closest algal relatives, including:

In terrestrial habitats, the resources that a photosynthetic organism requires are found in two different places.

Therefore, plants show varying degrees of structural specialization for subterranean and aerial organs - roots and shoots in most plants.

The elongation and branching of the shoots and roots maximize their exposure to environmental resources.

This growth is sustained by apical meristems, localized regions of cell division at the tips of shoots and roots.

Multicellular plant embryos develop from zygotes that are retained within tissues of the female parent.

This distinction is the basis for a term for all land plants, embryophytes.

The parent provides nutrients, such as sugars and amino acids, to the embryo.

All land plants show alternation of generations in which two multicellular body forms alternate.

One of the multicellular bodies is called the gametophyte with haploid cells.

Mitotic division of the diploid zygote produces the other multicellular body, the sporophyte.

Mitotic division of a plant spore produces a new multicellular gametophyte.

Unlike the life cycles of other sexually producing organisms, alternation of generations in land plants (and some algae) results in both haploid and diploid stages that exist as multicellular bodies.

While the gametophyte and sporophyte stages of some algae appear identical macroscopically in some algae, these two stages are very different in their morphology in other algal groups and all land plants.

The relative size and complexity of the sporophyte and gametophyte depend on the plant group.

Plant spores are haploid reproductive cells that grow into a gametophyte by mitosis.

Multicellular organs, called sporangia, are found on the sporophyte and produce these spores.

Within a sporangia, diploid spore mother cells undergo meiosis and generate haploid spores.

The outer tissues of the sporangium protect the developing spores until they are ready to be released into the air.

The gametophytes of bryophytes, pteridophytes, and gymnosperms produce their gametes within multicellular organs, called gametangia.

A female gametangium, called an archegonium, produces a single egg cell in a vase-shaped organ.

Most land plants have additional terrestrial adaptations including:

Male gametangia, called antheridia, produce many sperm cells that are released to the environment.

A sperm fuses with an egg within an archegonium and the zygote then begins development into an embryo.

In most land plants, the epidermis of leaves and other aerial parts is coated with a cuticle of polyesters and waxes.

Pores, called stomata, in the epidermis of leaves and other photosynthetic organs allow the exchange of carbon dioxide and oxygen between the outside air and the leaf interior.

Except for bryophytes, land plants have true roots, stems, and leaves, which are defined by the presence of vascular tissues.

Tube-shaped cells, called xylem, carry water and minerals up from roots.

Phloem is a living tissue in which nutrient-conducting cells arranged into tubes distribute sugars, amino acids, and other organic products.

Land plants produce many unique molecules called secondary compounds.

Examples of secondary compounds in plants include alkaloids, terpenes, tannins, and phenolics such as Flavonoids.

Humans have found many applications, including medicinal applications, for secondary compounds extracted from plants.

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B. The Origin of Land Plants

1. Land plants evolved from charophycean algae over 500 million years ago

Several lines of evidence support the phylogenetic connection between land plants and green algae, especially the charophyceans, including:

Homologous chloroplasts - The chloroplasts of land plants are most similar to the plastids of green algae and of eulgenoids which acquired green algae as secondary endosymbionts.

Homologous cellulose walls - In both land plants and charophycean algae, cellulose comprises 20-26% of the cell wall.

Homologous peroxisomes - Both land plants and charophycean algae package enzymes that minimize the costs of photorespiration in peroxisomes.

Phagmoplasts - These plate-like structures occur during cell division only in land plants and charopyceans.

Many plants have flagellated sperm, which match charophycean sperm closely in ultrastructure.

Molecular systematics - In addition to similarities derived from comparisons of chloroplast genes, analyses of several nuclear genes also provide evidence of a charophycean ancestry of plants.

All available evidence upholds the hypothesis that modern charophyceans and land plants evolved from a common ancestor.

The oldest known traces of land plants are found in mid-Cambrian rocks from about 550 million years ago.

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2. Alternation of generations in plants may have originated by delayed meiosis

The advanced charophyceans Chara and Coleochaeta are haploid organisms.

The zygote of a charophyceans undergoes meiosis to produce haploid spores, while the zygote of a land plants undergoes mitosis to produce a multicellular sporophyte.

A reasonable hypotheses for the origin of sporophytes is a mutation that delayed meiosis until one or more mitotic divisions of the zygote had occurred.

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3. Adaptations to shallow water preadapted plants for living on land

Many charophycean algae inhabit shallow waters at the edges of ponds and lakes where they experience occasional drying.

The evolutionary novelties of the first land plants opened an expanse of terrestrial habitat previously occupied by only films of bacteria.

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4. Plant taxonomists are reevaluating the boundaries of the plant kingdom

The taxonomy of plants is experiencing the same turmoil as other organisms as phylogenetic analyses revolutionize plant relationships.

Even "deeper" down the phylogenetic tree of plants is the branching of the whole land plant clade from its algal relatives.

The traditional scheme includes only the bryophytes, pteridophytes, gymnosperms, and angiosperms in the kingdom Plantae.

Others expand the boundaries to include charophyceans and some relatives in the kingdom Streptophyta.

Still others include all chlorophytes in the kingdom Viridiplantae.

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5. The plant kingdom is monophyletic

The diversity of modern plants demonstrates the problems and opportunities facing organisms that began living on land.

Because the plant kingdom is monophyletic, the differences in life cycles among land plants can be interpreted as special reproductive adaptations as the various plant phyla diversified from the first plants.

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C. Bryophytes

1. The three phyla of bryophytes are mosses, liverworts, and hornworts

Bryophytes are represented by three phyla:

Note, the name Bryophyta refers only to one phylum, but the informal term bryophyte refers to all nonvascular plants.

The diverse bryophytes are not a monophyletic group.

Liverworts and hornworts may be the most reasonable models of what early plants were like.

Mosses are the bryophytes most closely related to vascular plants.

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2. The gametophyte is the dominant generation in the life cycles of bryophytes

In bryophytes, gametophytes are the most conspicuous, dominant phase of the life cycle.

Bryophyte spores germinate in favorable habitats and grow into gametophytes by mitosis.

The gametophyte is a mass of green, branched, one-cell-thick filaments, called a protonema.

When sufficient resources are available, a protonema produces meristems.

These meristems generate gamete-producing structures, the gametophores.

Bryophytes are anchored by tubular cells or filaments of cells, called rhizoids.

Bryophyte gametophytes are generally only one or a few cells thick, placing all cells close to water and dissolved minerals.

Most bryophytes lack conducting tissues to distribute water and organic compounds within the gametophyte.

Lacking support tissues, most bryophytes are only a few centimeters tall.

They are anchored by tubular cells or filaments of cells, called rhizoids.

The gametophytes of hornworts and some liverworts are flattened and grow close to the ground.

The gametophytes of mosses and some liverworts are more "leafy" because they have stemlike structures that bear leaflike appendages.

The "leaves" of most mosses lack a cuticle and are only once cell thick, features that enhance water and mineral absorption from the moist environment.

Some mosses have more complex "leaves" with ridges to enhance absorption of sunlight.

Some mosses have conducting tissues in their stems and can grow as tall as 2m.

The mature gametophores of bryophytes produce gametes in gametangia.

When plants are coated with a thin film of water, sperm swim toward the archegonia, drawn by chemical attractants.

The zygotes and young sporophytes are retained and nourished by the parent gametophyte.

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3. Bryophyte sporophytes disperse enormous numbers of spores

While the bryophyte sporophyte does have photosynthetic plastids, they cannot live apart from the maternal gametophyte.

A bryophyte sporophyte remains attached to its parental gametophyte throughout the sporophyte's lifetime.

Bryophytes have the smallest and simplest sporophytes of all modern plant groups.

Liverworts have the simplest sporophytes among the bryophytes.

Hornwort and moss sporophytes are larger and more complex.

Moss sporophytes consist of a foot, an elongated stalk (the seta), and a sporangium (the capsule).

The foot gathers nutrients and water from the parent gametophyte via transfer cells.

The stalk conducts these materials to the capsule.

In most mosses, the seta becomes elongated, elevating the capsule and enhancing spore dispersal.

The moss capsule (sporangium) is the site of meiosis and spore production.

When immature, it is covered by a protective cap of gametophyte tissue, the calyptra.

The upper part of the capsule, the peristome, is often specialized for gradual spore release.

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4. Bryophytes provide many ecological and economic benefits

Wind dispersal of lightweight spores has distributed bryophytes around the world.

They are common and diverse in moist forests and wetlands.

Some even inhabit extreme environments like mountaintops, tundra, and deserts.

Sphagnum, a wetland moss, is especially abundant and widespread.

Peatlands, extensive high-latitude boreal wetland occupied by Sphagnum, play an important role as carbon reservoirs, stabilizing atmospheric carbon dioxide levels.

Sphagnum has been used in the past as diapers and a natural antiseptic material for wounds.

Today, it is harvested for use as a soil conditioner and for packing plants roots because of the water storage capacity of its large, dead cells.

Bryophytes were probably Earth's only plants for the first 100 million years that terrestrial communities existed.

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D. The Origin of Vascular Plants

Modern vascular plants (pteridophytes, gymnosperms, and angiosperms) have food transport tissues (phloem) and water conducting tissues (xylem) with lignified cells.

In vascular plants the branched sporophyte is dominant and is independent of the parent gametophyte.

The first vascular plants, pteridophytes, were seedless.

1. Additional terrestrial adaptations evolved as vascular plants descended from mosslike ancestors

Vascular plants built on the tissue-producing meristems, gametangia, embryos and sporophytes, stomata, cuticles, and sproropollenin-walled spores that they inherited from mosslike ancestors.

The protracheophyte polysporangiophytes demonstrate the first steps in the evolution of sporophytes.

Like bryophytes, they lacked lignified vascular tissues, but the branched sporophytes were independent of the gametophyte.

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2. A diversity of vascular plants evolved over 400 million years ago

Cooksonia, an extinct plant over 400 million years old, is the earliest known vascular plant.

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E. Pteridophytes: Seedless Vascular Plants

The seedless vascular plants, the pteridophytes consists of two modern phyla:

These phyla probably evolved from different ancestors among the early vascular plants.

1. Pteridophytes provide clues to the evolution of roots and leaves

Most pteridophytes have true roots with lignified vascular tissue.

These roots appear to have evolved from the lowermost, subterranean portions of stems of ancient vascular plants.

Lycophytes have small leaves with only a single unbranched vein.

In contrast, the leaves of other vascular plants, megaphylls, are much larger and have highly-branched vascular system.

The fossil evidence suggests that megaphylls evolved from a series of branches lying close together on a stem.

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2. A sporophyte-dominant life cycle evolved in seedless vascular plants

From the early vascular plants to the modern vascular plants, the sporophyte generation is the larger and more complex plant.

Ferns also demonstrate a key variation among vascular plants: the distinction between homosporous and heterosporous plants.

A homosporous sporophyte produces a single type of spore.

A heterosporous sporophyte produces two kinds of spores.

Regardless of origin, the flagellated sperm cells of ferns, other seedless vascular plants, and even some seed plants must swim in a film of water to reach eggs.

Because of this, seedless vascular plants are most common in relatively damp habitats.

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3. Lycophyta and Pterophyta are the two phyla of modern seedless vascular plants

Phylum Lycophyta - Modern lycophytes are relicts of a far more eminent past.

Modern lycophytes include tropical species that grow on trees as epiphytes, using the trees as substrates, not as hosts.

Others grow on the forest floor in temperate regions.

The lycophyte sporophytes are characterized by upright stems with many microphylls and horizontal stems along the ground surface.

Roots extend down from the horizontal stems.

Specialized leaves (sporophylls) bear sporangia clustered to form club-shaped cones.

Spores are released in clouds from the sporophylls.

They develop into tiny, inconspicuous haploid gametophytes.

The phylum Pterophyta consists of ferns and their relatives.

Psilophytes, the whisk ferns, used to be considered a "living fossil".

Their dichotomous branching and lack of true leaves and roots seemed similar to early vascular plants.

However, comparisons of DNA sequences and ultrastructural details, indicate that the lack of true roots and leaves evolved secondarily.

Sphenophytes are commonly called horsetails because of their often brushy appearance.

During the Carboniferous, sphenophytes grew to 15m, but today they survive as about 15 species in a single wide-spread genus, Equisetum.

Horsetails are often found in marshy habitats and along streams and sandy roadways.

Roots develop from horizontal rhizomes that extend along the ground.

Upright green stems, the major site of photosynthesis, also produce tiny leaves or branches at joints.

Reproductive stems produce cones at their tips.

Ferns first appeared in the Devonian and have radiated extensively until there are over 12,000 species today.

Ferns often have horizontal rhizomes from which grow large megaphyllous leaves with an extensively branched vascular system.

Ferns produce clusters of sporangia, called sori, on the back of green leaves (sporophylls) or on special, non-green leaves.

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4. Seedless vascular plants formed vast "coal forests" during the Carboniferous period

The phyla Lycophyta and Pterophyta formed forests during the Carboniferous period about 290-360 million years ago.

These plants left not only living representatives and fossils, but also fossil fuel in the form of coal.

While coal formed during several geologic periods, the most extensive beds of coal were deposited during the Carboniferous period, when most of the continents were flooded by shallow swamps.

Dead plants did not completely decay in the stagnant waters, but accumulated as peat.

The swamps and their organic matter were later covered by marine sediments.

Heat and pressure gradually converted peat to coal, a "fossil fuel".

Coal powered the Industrial Revolution but has been partially replaced by oil and gas in more recent times.

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Course Pages maintained by
Dr. Graeme Lindbeck .