Chapter 29   Plant Diversity I: How Plants Colonized Land

Lecture Outline

Overview: The Greening of Earth

·        For the first 3 billion years of Earth’s history, the land was lifeless.

·        Thin coatings of cyanobacteria existed on land about 1.2 billion years ago.

·        About 500 million years ago, plants, fungi, and animals joined them.

·        More than 290,000 species of plants inhabit Earth today.

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

°        Some species, such as sea grasses, have returned to aquatic habitats.

·        The presence of plants has enabled other organisms to survive on land.

°        Plant roots have created habitats for other organisms by stabilizing landscapes.

°        Plants are the source of oxygen and the ultimate provider of food for land animals.

 

Concept 29.1 Land plants evolved from green algae

·        Researchers have identified a lineage of green algae called charophyceans as the closest relatives of land plants.

·        Many key characteristics of land plants also appear in a variety of algal clades.

·        Plants are multicellular, eukaryotic, photosynthetic autotrophs.

°        But red, brown, and some green algae also fit this description.

·        Plants have cell walls made of cellulose.

°        So do green algae, dinoflagellates, and brown algae.

·        Plants have chloroplasts with chlorophyll a and b.

°        So do green algae, euglenids, and a few dinoflagellates.

·        Land plants share four key features only with the charophyceans.

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

°        These complexes contrast with the linear arrays of cellulose-producing proteins in noncharophycean algae.

°        Also, the cell walls of plants and charophyceans contain a higher percentage of cellulose than the cell walls of noncharophycean algae.

2.     A second feature that unites charophyceans and land plants is the presence of peroxisome enzymes to help minimize the loss of organic products as a result of photorespiration.

°        Peroxisomes of other algae lack these enzymes.

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

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

°        These include the formation of a phragmoplast, an alignment of cytoskeletal elements and Golgi-derived vesicles, during the synthesis of new cross-walls during cytokinesis.

°        Over the past decade, researchers involved in an international initiative called “Deep Green” have conducted a large-scale study of the major transitions in plant evolution.

§        These researchers have analyzed genes from a wide range of plant and algal species.

§        Comparisons of nuclear and chloroplast genes support the hypothesis that the charophyceans are the closest living relatives of land plants.

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

·        In such environments, natural selection favors individuals that can survive periods when they are not submerged in water.

°        A layer of a durable polymer called sporopollenin prevents exposed charophycean zygotes from drying out until they are in water again.

°        This chemical adaptation may have been the precursor to the tough sporopollenin walls that encase plant spores.

·        The accumulation of such traits by at least one population of ancestral charophyceans enabled their descendents—the first land plants—to live permanently above the waterline.

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

°        The new frontier was spacious.

°        The bright sunlight was unfiltered by water and plankton.

°        The atmosphere had an abundance of carbon dioxide.

°        The soil was rich in mineral nutrients.

°        At least at first, there were relatively few herbivores or pathogens.

 

 

Concept 29.2 Land plants possess a set of derived terrestrial adaptations

·        A number of adaptations evolved in plants that allowed them to survive and reproduce on land.

·        What exactly is the line that divides land plants from algae?

·        We will adopt the traditional scheme, which equates the kingdom Plantae with embryophytes (plants with embryos).

°        Some botanists now propose that the plant kingdom should be renamed the kingdom Streptophyta and expanded to include the charophyceans and a few related groups.

°        Others suggest the kingdom Viridiplantae, which includes chlorophytes as well as plants.

·        Five key traits appear in nearly all land plants but are absent in the charophyceans.

°        We infer that these traits evolved as derived traits of land plants.

·        The five traits are:

1.     Apical meristems.

2.     Alternation of generations.

3.     Multicellular embryo that is dependent on the parent plant.

4.     Sporangia that produce walled spores.

5.     Gametangia that produce gametes.

Apical meristems

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

°        Light and carbon dioxide are mainly aboveground.

°        Water and mineral resources are found mainly in the soil.

·        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.

°        Cells produced by meristems differentiate into various tissues, including surface epidermis and internal tissues.

Alternation of generations

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

°        This life cycle also occurs in various algae.

°        However, alternation of generations does not occur in the charophyceans, the algae most closely related to land plants.

·        In alternation of generations, one of the multicellular bodies is called the gametophyte and has haploid cells.

·        Gametophytes produce gametes, egg and sperm, by mitosis.

°        Fusion of egg and sperm during fertilization form a diploid zygote.

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

°        Meiosis in a mature sporophyte produces haploid reproductive cells called spores.

°        A spore is a reproductive cell that can develop into a new organism without fusing with another cell.

·        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.

°        For example, humans do not have alternation of generations because the only haploid stage in the life cycle is the gamete, which is single-celled.

Walled spores produced by sporangia

·        Plant spores are haploid reproductive cells that grow into gametophytes by mitosis.

°        Sporopollenin makes the walls of spores very tough and resistant to harsh environments.

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

·        Within sporangia, diploid cells called sporocytes 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.

Multicellular gametangia

·        Plant gametophytes produce gametes within multicellular organs called gametangia.

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

°        The egg is retained within the base.

·        Male gametangia, called antheridia, produce and release sperm into the environment.

·        In many major groups of living plants, the sperm have flagella and swim to the eggs though a water film.

·        Each egg is fertilized within an archegonium, where the zygote develops into the embryo.

·        The gametophytes of seed plants are so reduced in size that archegonia and antheridia have been lost in some lineages.

Multicellular, dependent embryos

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

·        The multicellular, dependent embryo of land plants is such a significant derived trait that land plants are also known as embryophytes.

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

°        The embryo has specialized placental transfer cells that enhance the transfer of nutrients from parent to embryo.

°        These are sometimes present in the adjacent maternal tissues as well.

°        This interface is analogous to the nutrient-transferring embryo-mother interface of placental mammals.

·        Additional derived traits have evolved in many plant species.

·        The epidermis of many plants has a cuticle consisting of polymers called polyesters and waxes.

°        The cuticle waterproofs the epidermis, preventing excessive water loss, and offers protection from microbial attack.

·        Many land plants produce secondary compounds, so named because they are the products of secondary metabolic pathways that branch from primary metabolic pathways.

°        Alkaloids, terpenes, and tannins defend against herbivores and parasites.

°        Flavonoids absorb harmful UV radiation and may act as signals in symbiotic relationships with beneficial soil microbes.

°        Phenolics deter attack by pathogenic microbes.

 Land plants have diversified since their origin from algal ancestors.

·        Fossils of plant spores have been extracted from 475-million-year-old rocks in Oman.

·        These spores were embedded in plant cuticle material that is similar to spore-bearing tissue in living plants.

°        These fossils clearly belong to plants.

·        A 2001 study of the “molecular clock” of plants suggests that the common ancestor of living plants existed 700 million years ago.

·        A 2003 study suggests a new date of 490 to 425 million years, roughly the same age as the spores found in Oman.

·        Land plants can be informally grouped based on the presence or absence of an extensive system of vascular tissue, cells joined into tubes that transport water and nutrients throughout the plant body.

°        Plants that do not have an extensive transport system are described as “nonvascular plants,” although some mosses do have simple vascular tissue.

°        Nonvascular plants are informally called bryophytes.

°        There is some uncertainty about whether or not bryophytes are monophyletic and represent a clade.

·        Vascular plants form a clade consisting of 93% of all land plants.

°        Three smaller clades are found within the vascular plants.

§        Lycophytes include club mosses and their relatives.

§        Pterophytes include the ferns and their relatives.

§        These two clades are called the seedless vascular plants.

°        A third clade of vascular plants includes the seed plants, the vast majority of living plants.

·        A seed is an embryo packaged with a supply of nutrients within a protective coat.

·        Seed plants can be divided into two groups: gymnosperms and angiosperms.

·        Gymnosperms are called “naked seed plants” because their seeds are not enclosed in chambers.

·        Angiosperms are a huge clade including all flowering plants.

 

Concept 29.3 The life cycles of mosses and other bryophytes are dominated by the gametophyte stage

·        Bryophytes are represented by three phyla:

°        Phylum Hepatophyta—liverworts

°        Phylum Anthocerophyta—hornworts

°        Phylum Bryophyta—mosses

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

·        It has not been established whether the diverse bryophytes form a clade.

·        Systematists continue to debate the sequence in which the three phyla of bryophytes evolved.

·        Bryophytes acquired many unique adaptations after their evolutionary split from the ancestors of modern vascular plants.

°        They also possess some ancestral traits characteristic of the earliest plants.

·        In bryophytes, gametophytes are the largest and most conspicuous phase of the life cycle.

°        Sporophytes are smaller and are present only part of the time.

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

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

·        A protonema has a large surface area that enhances absorption of water and minerals.

·        In favorable conditions, protonema generate gamete-producing structures, the gametophores.

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

°        Unlike roots, rhizoids are not composed of tissues, lack specialized conducting cells, and do not play a primary role in water and mineral absorption.

·        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.

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

°        It is not clear if conducting tissues in mosses are analogous or homologous to the xylem and phloem of vascular plants.

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

·        The mature gametophores of bryophytes produce gametes in gametangia.

°        Each vase-shaped archegonium produces a single egg.

°        Elongated antheridia produce many flagellated sperm.

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

°        They swim into the archegonia and fertilize the eggs.

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

°        Layers of placental nutritive cells transport materials from parent to embryos.

 Bryophyte sporophytes disperse enormous numbers of spores.

·        While the bryophyte sporophyte does have photosynthetic plastids when young, it cannot live apart from the maternal gametophyte.

·        A bryophyte sporophyte remains attached to its maternal gametophyte throughout the sporophyte’s lifetime.

°        It depends on the gametophyte for sugars, amino acids, minerals, and water.

·        Bryophytes have the smallest and simplest sporophytes of all modern plant groups, consistent with the hypothesis that larger and more complex sporophytes evolved only later in vascular plants.

°        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.

§        One capsule can generate more than 50 million spores.

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

°        This is lost when the capsule is ready to release spores.

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

·        Liverworts have the simplest sporophytes among the bryophytes.

°        They consist of a short stalk bearing round sporangia that contain the developing spores, and a nutritive foot embedded in gametophyte tissues.

·        Hornwort and moss sporophytes are larger and more complex.

°        Hornwort sporophytes resemble grass blades and have a cuticle.

°        The sporophytes of mosses start out green and photosynthetic, but turn tan or brownish red when ready to release their spores.

°        The sporophytes of hornworts and mosses have epidermal stomata, like those of vascular plants.

§        These pores support photosynthesis by allowing the exchange of CO₂ and O₂ between the outside air and the interior of the sporophyte.

°        The fact that stomata are present in mosses and hornworts but absent in liverworts has led to three hypotheses for their evolution.

1.     If liverworts are the deepest-branching lineage of land plants, then stomata evolved once in the ancestor of hornworts, mosses and vascular plants.

2.     If hornworts are the deepest-branching lineage of land plants, then stomata evolved once and were lost in the liverwort lineage.

3.     Perhaps hornworts acquired stomata independently of mosses and vascular plants.

 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 such as mountaintops, tundra, and deserts.

°        Phenolic compounds in moss cell walls absorb damaging levels of radiation present in deserts and at high altitudes and latitudes.

·        Many mosses can exist in very cold or dry habitats because they are able to lose most of their body water and then rehydrate and reactivate their cells when moisture again becomes available.

°        Few vascular plants can survive the same degree of desiccation.

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

°        It forms extensive deposits of undecayed organic material, called peat.

°        Wet regions dominated by Sphagnum or peat moss are known as peat bogs.

°        Its organic materials do not decay readily because of resistant phenolic compounds and acidic secretions that inhibit bacterial activity.

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

·        Sphagnum has been used in the past for diapers and as 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.

·        Worldwide, an estimated 400 billion tons of organic carbon are stored as peat.

 

 

 

 

Concept 29.4 Ferns and other seedless vascular plants formed the first forests

 Ferns and other seedless vascular plants flourished in the Carboniferous period.

·        Bryophytes were the prevalent vegetation for the first 100 million years that terrestrial communities existed.

°        Then vegetation began to take on a taller profile with the evolution of vascular plants.

·        Modern seedless vascular plants provide insights into plant evolution during the Carboniferous period, when vascular plants began to diversify, but most groups of seed plants had not yet evolved.

·        The sperm of ferns and all other seedless vascular plants are flagellated and must swim through a film of water to reach eggs.

·        Due to the swimming sperm and their fragile gametophytes, modern seedless vascular plants are most common in damp environments.

·        Fossils of the ancestors of today’s vascular plants date back about 420 million years.

·        Unlike bryophytes, these plants had branched sporophytes that did not remain dependent on gametophytes for growth.

 Five main traits characterize modern vascular plants.

·        Five main traits characterize modern vascular plants:

1.     Life cycles with dominant sporophytes.

2.     Transport in xylem and phloem.

3.     Evolution of roots.

4.     Evolution of leaves.

5.     Sporophylls and spore variations.

Life cycles with dominant sporophytes

·        Fossils suggest that the ancestors of vascular plants had life cycles characterized by gametophytes and sporophytes that were about equal in size.

·        Among living vascular plants, the sporophyte generation is the larger and more complex plant.

°        For example, the leafy fern plants that you are familiar with are sporophytes.

°        The gametophytes are tiny plants that grow on or just below the soil surface.

°        This reduction in the size of the gametophytes is even more extreme in seed plants.

Transport in xylem and phloem

·        Vascular plants have two types of vascular tissue: xylem and phloem.

·        Xylem conducts most of the water and minerals.

°        The xylem of all vascular plants includes tracheids, tube-shaped cells that carry water and minerals up from roots.

°        When functioning, these cells are dead, with only their walls providing a system of microscopic water pipes.

°        The water-conducting cells in vascular plants are lignified, strengthened by the phenolic polymer lignin.

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

°        Lignified vascular tissue permitted vascular plants to grow to greater heights than bryophytes.

Evolution of roots

·        Lignified vascular tissue also allowed the evolution of roots.

·        Roots are organs that anchor vascular plants and enable them to absorb water and nutrients from the soil.

·        Roots also allow the shoot system to grow taller.

·        Roots may have evolved from the subterranean portions of stems in ancient vascular plants.

·        It is not clear whether roots evolved once in the common ancestor of all vascular plants or independently in different lineages.

·        Studying genes that control root development may resolve this controversy.

Evolution of leaves

·        Leaves are organs that increase the surface area of vascular plants, capturing more solar energy for photosynthesis.

·        In terms of size and complexity, leaves can be classified as microphylls and megaphylls.

·        All lycophytes have microphylls, small leaves with only a single unbranched vein.

°        These leaves probably evolved as small outgrowths on the surface of stems, supported by single strands of vascular tissue.

·        All other vascular plants have megaphylls, leaves with a highly branched vascular system.

°        A branched vascular system can deliver water and minerals to the expanded leaf.

°        It can also export larger quantities of sugars from the leaf.

°        Megaphylls support more photosynthetic activity.

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

°        One hypothesis proposes that megaphylls evolved when the branch system flattened and a tissue webbing developed, joining the branches.

°        Under this hypothesis, true, branched stems preceded the origin of large leaves and roots.

Sporophylls and spore variations

·        Vascular plants have sporophylls, modified plants that bear sporangia.

·        Sporophylls vary greatly in structure.

·        Ferns produce clusters of sporangia called sori, usually on the underside of leaves.

·        In gymnosperms, groups of sporophylls form cone or strobili.

·        Another key variation among vascular plants is the distinction between homosporous and heterosporous species.

·        Most seedless vascular plants are homosporous, producing a single type of spore.

°        This spore develops into a bisexual gametophyte with both archegonia (female sex organs) and antheridia (male sex organs).

°        Most ferns are homosporous.

·        A heterosporous species produces two kinds of spores.

°        Megaspores develop into female gametophytes.

°        Microspores develop into male gametophytes.

°        All seed plants and a few seedless vascular plants are heterosporous.

 Classification of seedless vascular plants.

·        Living seedless vascular plants form two clades: lycophytes and pterophytes.

·        Lycophytes include club mosses, spike mosses, and quillworts.

·        Pterophytes include ferns, horsetails, whisk ferns, and their relatives.

·        Phylum Lycophyta: club mosses, spike mosses, and quillworts

°        Modern species of lycophytes are relicts of an eminent past.

°        By the Carboniferous, there were two evolutionary lineages of lycophytes: small, herbaceous plants and giant, woody trees standing 40 meters tall.

°        The giant lycophytes became extinct as the climate cooled and dried at the end of the Carboniferous.

°        The small lycophytes survived, and are now represented by 1,200 species.

·        Phylum Pterophyta: ferns, horsetails, and whisk ferns

°        Ferns radiated extensively from their Devonian origins and grew with lycophytes and horsetails in the Carboniferous swamp forests.

§        There are 12,000 species of living ferns.

§        They are most diverse in the tropics, thrive in temperate forests, and some can even survive arid conditions.

°        Horsetails grew up to 15 meters in height during the Carboniferous period.

§        Today, only 15 species of single genus Equisetum survives.

°        Psilotum, the whisk fern, and a close relative form a clade of terrestrial epiphytes.

§        Whisk ferns are the only vascular plants lacking true roots and leaves.

§        These plants have been considered “living fossils” because their dichotomous branching and lack of true leaves and roots seemed similar to early vascular plants.

°        However, comparisons of DNA sequences and details of sperm ultrastructure indicate that they are closely related to ferns.

°        The whisk fern’s ancestors lost true leaves and roots during evolution.

 The significance of seedless vascular plants.

·        The ancestors of modern lycophytes and ferns, along with their seedless vascular relatives, formed the first forests during the Carboniferous.

·        With the evolution of vascular tissue, roots, and leaves, these plants accelerated their rate of photosynthesis and dramatically increased the removal of CO₂ in the atmosphere.

·        Scientists estimate that CO₂ levels dropped by as much as a factor of five during the Carboniferous, causing global cooling and widespread glacier formation.

·        The first forests gave rise to modern-day coal.

·        In the stagnant waters of the Carboniferous, dead plants did not fully decay.

·        The organic material turned to thick layers of peat. Marine sediments piled up on top, and over millions of years, heat and pressure converted the peat to coal.

·        Humans still burn 6 billion tons of coal each year.