CPB729 LEC

Chapter 29: 

Chapter 29 Plant Diversity I How Plants Colonized Land

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Overview: The Greening of Earth Looking at a lush landscape It is difficult to imagine the land without any plants or other organisms

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For more than the first 3 billion years of Earth’s history The terrestrial surface was lifeless Since colonizing land Plants have diversified into roughly 290,000 living species

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Concept 29.1: Land plants evolved from green algae Researchers have identified green algae called charophyceans as the closest relatives of land plants

Morphological and Biochemical Evidence: 

Morphological and Biochemical Evidence Many characteristics of land plants Also appear in a variety of algal clades

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There are four key traits that land plants share only with charophyceans Rose-shaped complexes for cellulose synthesis

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Peroxisome enzymes Structure of flagellated sperm Formation of a phragmoplast

Genetic Evidence: 

Genetic Evidence Comparisons of both nuclear and chloroplast genes Point to charophyceans as the closest living relatives of land plants

Adaptations Enabling the Move to Land: 

Adaptations Enabling the Move to Land In charophyceans A layer of a durable polymer called sporopollenin prevents exposed zygotes from drying out The accumulation of traits that facilitated survival on land May have opened the way to its colonization by plants

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Concept 29.2: Land plants possess a set of derived terrestrial adaptations Many adaptations Emerged after land plants diverged from their charophycean relatives

Defining the Plant Kingdom: 

Defining the Plant Kingdom Systematists Are currently debating the boundaries of the plant kingdom

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Some biologists think that the plant kingdom Should be expanded to include some or all green algae Until this debate is resolved This textbook retains the embryophyte definition of kingdom Plantae

Derived Traits of Plants: 

Derived Traits of Plants Five key traits appear in nearly all land plants but are absent in the charophyceans Apical meristems Alternation of generations Walled spores produced in sporangia Multicellular gametangia Multicellular dependent embryos

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Apical meristems and alternation of generations Figure 29.5

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Walled spores; multicellular gametangia; and multicellular, dependent embryos WALLED SPORES PRODUCED IN SPORANGIA MULTICELLULAR GAMETANGIA MULTICELLULAR, DEPENDENT EMBRYOS Spores Sporangium Longitudinal section of Sphagnum sporangium (LM) Sporophyte Gametophyte Sporophyte and sporangium of Sphagnum (a moss) Female gametophyte Archegonium with egg Antheridium with sperm Male gametophyte Archegonia and antheridia of Marchantia (a liverwort) Embryo Maternal tissue 2 µm Wall ingrowths Placental transfer cell 10 µm Embryo and placental transfer cell of Marchantia Figure 29.5

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Additional derived units Such as a cuticle and secondary compounds, evolved in many plant species

The Origin and Diversification of Plants: 

The Origin and Diversification of Plants Fossil evidence Indicates that plants were on land at least 475 million years ago

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Fossilized spores and tissues Have been extracted from 475-million-year-old rocks

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Whatever the age of the first land plants Those ancestral species gave rise to a vast diversity of modern plants

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Land plants can be informally grouped Based on the presence or absence of vascular tissue

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An overview of land plant evolution Bryophytes (nonvascular plants) Seedless vascular plants Seed plants Vascular plants Land plants Origin of seed plants (about 360 mya) Origin of vascular plants (about 420 mya) Origin of land plants (about 475 mya) Ancestral green alga Charophyceans Liverworts Hornworts Mosses Lycophytes (club mosses, spike mosses, quillworts) Pterophyte (ferns, horsetails, whisk fern) Gymnosperms Angiosperms Figure 29.7

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Concept 29.3: The life cycles of mosses and other bryophytes are dominated by the gametophyte stage Bryophytes are represented today by three phyla of small herbaceous (nonwoody) plants Liverworts, phylum Hepatophyta Hornworts, phylum Anthocerophyta Mosses, phylum Bryophyta

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Debate continues over the sequence of bryophyte evolution Mosses are most closely related to vascular plants

Bryophyte Gametophytes: 

Bryophyte Gametophytes In all three bryophyte phyla Gametophytes are larger and longer-living than sporophytes

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The life cycle of a moss

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Bryophyte gametophytes Produce flagellated sperm in antheridia Produce ova in archegonia Generally form ground-hugging carpets and are at most only a few cells thick Some mosses Have conducting tissues in the center of their “stems” and may grow vertically

Bryophyte Sporophytes: 

Bryophyte Sporophytes Bryophyte sporophytes Grow out of archegonia Are the smallest and simplest of all extant plant groups Consist of a foot, a seta, and a sporangium Hornwort and moss sporophytes Have stomata

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Bryophyte diversity

Ecological and Economic Importance of Mosses: 

“Tolland Man,” a bog mummy dating from 405–100 B.C. The acidic, oxygen-poor conditions produced by Sphagnum canpreserve human or other animal bodies for thousands of years. Ecological and Economic Importance of Mosses Sphagnum, or “peat moss” Forms extensive deposits of partially decayed organic material known as peat Plays an important role in the Earth’s carbon cycle Gametophyte Sporangium at tip of sporophyte Living photo- synthetic cells Dead water- storing cells 100 µm Closeup of Sphagnum. Note the “leafy” gametophytes and their offspring, the sporophytes. (b) Sphagnum “leaf” (LM). The combination of living photosynthetic cells and dead water-storing cells gives the moss its spongy quality. (c) Peat being harvested from a peat bog (a) Figure 29.10 a–d (d)

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Concept 29.4: Ferns and other seedless vascular plants formed the first forests Bryophytes and bryophyte-like plants Were the prevalent vegetation during the first 100 million years of plant evolution Vascular plants Began to evolve during the Carboniferous period

Origins and Traits of Vascular Plants: 

Origins and Traits of Vascular Plants Fossils of the forerunners of vascular plants Date back about 420 million years

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These early tiny plants Had independent, branching sporophytes Lacked other derived traits of vascular plants

Life Cycles with Dominant Sporophytes: 

Life Cycles with Dominant Sporophytes In contrast with bryophytes Sporophytes of seedless vascular plants are the larger generation, as in the familiar leafy fern The gametophytes are tiny plants that grow on or below the soil surface

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The life cycle of a fern Fern sperm use flagella to swim from the antheridia to eggs in the archegonia. 4 Sporangia release spores. Most fern species produce a single type of spore that gives rise to a bisexual gametophyte. The fern spore develops into a small, photosynthetic gametophyte. 2 Although this illustration shows an egg and sperm from the same gametophyte, a variety of mechanisms promote cross-fertilization between gametophytes. 3 On the underside of the sporophyte‘s reproductive leaves are spots called sori. Each sorus is a cluster of sporangia. 6 A zygote develops into a new sporophyte, and the young plant grows out from an archegonium of its parent, the gametophyte. 5 MEIOSIS Sporangium Sporangium Mature sporophyte New sporophyte Zygote FERTILIZATION Archegonium Egg Haploid (n) Diploid (2n) Spore Young gametophyte Fiddlehead Antheridium Sperm Gametophyte Key Sorus Figure 29.12

Transport in Xylem and Phloem: 

Transport in Xylem and Phloem Vascular plants have two types of vascular tissue Xylem and phloem

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Xylem Conducts most of the water and minerals Includes dead cells called tracheids Phloem Distributes sugars, amino acids, and other organic products Consists of living cells

Evolution of Roots: 

Evolution of Roots Roots Are organs that anchor vascular plants Enable vascular plants to absorb water and nutrients from the soil May have evolved from subterranean stems

Evolution of Leaves: 

Evolution of Leaves Leaves Are organs that increase the surface area of vascular plants, thereby capturing more solar energy for photosynthesis

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Leaves are categorized by two types Microphylls, leaves with a single vein Megaphylls, leaves with a highly branched vascular system

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According to one model of evolution Microphylls evolved first, as outgrowths of stems

Sporophylls and Spore Variations: 

Sporophylls and Spore Variations Sporophylls Are modified leaves with sporangia Most seedless vascular plants Are homosporous, producing one type of spore that develops into a bisexual gametophyte

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All seed plants and some seedless vascular plants Are heterosporous, having two types of spores that give rise to male and female gametophytes

Classification of Seedless Vascular Plants: 

Classification of Seedless Vascular Plants Seedless vascular plants form two phyla Lycophyta, including club mosses, spike mosses, and quillworts Pterophyta, including ferns, horsetails, and whisk ferns and their relatives

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The general groups of seedless vascular plants

Phylum Lycophyta: Club Mosses, Spike Mosses, and Quillworts: 

Phylum Lycophyta: Club Mosses, Spike Mosses, and Quillworts Modern species of lycophytes Are relics from a far more eminent past Are small herbaceous plants

Phylum Pterophyta: Ferns, Horsetails, and Whisk Ferns and Relatives: 

Phylum Pterophyta: Ferns, Horsetails, and Whisk Ferns and Relatives Ferns Are the most diverse seedless vascular plants

The Significance of Seedless Vascular Plants: 

The Significance of Seedless Vascular Plants The ancestors of modern lycophytes, horsetails, and ferns Grew to great heights during the Carboniferous, forming the first forests

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The growth of these early forests May have helped produce the major global cooling that characterized the end of the Carboniferous period Decayed and eventually became coal