Plant Diversity I: How Plants Colonized Land Chapter 29 Plant Diversity I: How Plants Colonized Land

Overview: The Greening of Earth 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 Plants supply oxygen and are the ultimate source of most food eaten by land animals

Overview: Multicellularity Advantages Multicellular organisms can be larger and operate more efficiently Cells are specialized to perform specific functions which benefit the whole organism Disadvantages Multicellular organisms need to coordinate the activities of their individual cells They are restricted in their exploitation of habitats because of their size.

How did plants change the world? Fig. 29-1 How did plants change the world? Figure 29.1 How did plants change the world?

Concept 29.1: Land plants (Kingdom Plantae) evolved from green algae!!!!!! Green algae called charophytes are the closest relatives of land plants Lower plants and green algae have flagellated sperm that is similar in structure Homologous chloroplasts – chlorophyll b Mitosis/cytokinesis – similarities Genetic similarities in nuclear genes and rRNA Food reserves - starch

Concept 29.1: Land plants (Kingdom Plantae) evolved from green algae!!!!!! Multicellular, photosynthetic, and immobile, all plants have cellulose walls Because most plants cannot move around, obtaining light for photosynthesis and bringing gametes together for sexual reproduction have been dominant themes in their evolution. Plants originated in the sea, and some forms later colonized land. The most successful land plants have vascular tissues that transport food and water through the plant and support the body in the air. Most successful vascular plants have adaptations freeing their reproduction from dependence on water. (cuticle, stomates, reduced gametophytes that are dependent on sporophytes)

Living on Land compared to Water Advantages More light Higher concentration of CO2 Nutrients in soil Disadvantages no support for plant body Less water available for reproduction Segregated resources (light and air above, water and minerals below soil surface) Extremes of temperature

Derived Traits of Plants Four key traits appear in nearly all land plants but are absent in the charophytes: Alternation of generations (with multicellular, dependent embryos) Walled spores produced in sporangia Multicellular gametangia Apical meristems Additional derived traits such as a cuticle and secondary compounds evolved in many plant species Symbiotic associations between fungi and the first land plants may have helped plants without true roots to obtain nutrients

Alternation of Generations and Multicellular, Dependent Embryos Plants alternate between two multicellular stages, a reproductive cycle called alternation of generations The gametophyte is haploid and produces haploid gametes by mitosis Fusion of the gametes gives rise to the diploid sporophyte, which produces haploid spores by meiosis. These spores germinate into gametophytes. The gametophyte produces gametes by mitosis. The gametes fuse for form a diploid zygote which grows into the sporophyte In the evolutionary history of plants, the sporophyte generation has increased in size, number of cells and complexity while the gametophyte has decreased in size, number of cell and complexity. In higher plants, the sporophyte is the dominant form.

Alternation of generations Fig. 29-5a Gamete from another plant Gametophyte (n) Mitosis Mitosis n n n n Spore Gamete MEIOSIS FERTILIZATION Zygote 2n Figure 29.5 Derived traits of land plants Mitosis Sporophyte (2n) Alternation of generations

Longitudinal section of Sphagnum sporangium (LM) Fig. 29-5c Spores (haploid) Sporangium Longitudinal section of Sphagnum sporangium (LM) Figure 29.5 Derived traits of land plants Sporophyte (diploid) – produces haploid spores by meiosis Gametophyte (haploid) Sporophytes and sporangia of Sphagnum (a moss)

The Origin and Diversification of Plants Those ancestral species gave rise to a vast diversity of modern plants Land plants can be informally grouped based on the presence or absence of vascular tissue Most plants have vascular tissue; these constitute the vascular plants Nonvascular plants are commonly called bryophytes Seedless vascular plants can be divided into clades Lycophytes (club mosses and their relatives) Pterophytes (ferns and their relatives) A seed is an embryo and nutrients surrounded by a protective coat Seed plants form a clade and can be divided into further clades: Gymnosperms, the “naked seed” plants, including the conifers Angiosperms, the flowering plants

Figure 29.7 Highlights of plant evolution 1 Origin of land plants (about 475 mya) 2 Origin of vascular plants (about 420 mya) 3 Origin of extant seed plants (about 305 mya) Liverworts Nonvascular plants (bryophytes) Land plants ANCES- TRAL GREEN ALGA 1 Hornworts Mosses Lycophytes (club mosses, spike mosses, quillworts) Seedless vascular plants 2 Vascular plants Pterophytes (ferns, horsetails, whisk ferns) Figure 29.7 Highlights of plant evolution Gymnosperms 3 Seed plants Angiosperms 500 450 400 350 300 50 Millions of years ago (mya)

Concept 29.2: Mosses and other nonvascular plants have life cycles dominated by gametophytes Bryophytes are represented today by three phyla of small herbaceous (nonwoody) plants: Liverworts, phylum Hepatophyta Hornworts, phylum Anthocerophyta Mosses, phylum Bryophyta Mosses are most closely related to vascular plants Nonvascular plants (bryophytes) Seedless vascular plants Gymnosperms Angiosperms

Bryophytes - Mosses and other nonvascular plants have life cycles dominated by gametophytes The hapliod gametophyte is the dominant generation The sporphyte is dependent on the gametopyhte Must have water to reproduce so that the sperm can swim to the egg because no vascular tissue to carry water to plants parts, therefore, plants are always low to the ground Bryophyte sporophytes grow out of archegonia, and are the smallest and simplest sporophytes of all extant plant groups. Although bryophyte sporophytes are usually green and photosynthetic when young, they cannot live independently. They remain attached to their parental gametophytes, from which they absorb sugars, amino acids, minerals, and water.

Figure 29.8 The life cycle of a moss Raindrop Sperm “Bud” Antheridia Male gametophyte (n) Key Haploid (n) Protonemata (n) Diploid (2n) “Bud” Egg Spores Gametophore Archegonia Spore dispersal Female gametophyte (n) Rhizoid Peristome Sporangium FERTILIZATION Figure 29.8 The life cycle of a moss MEIOSIS (within archegonium) Seta Zygote (2n) Capsule (sporangium) Mature sporophytes Foot Embryo Archegonium Young sporophyte (2n) 2 mm Capsule with peristome (SEM) Female gametophytes

Life Cycle of Mosses - Animation http://www.sumanasinc.com/webcontent/animations/content/moss.html

Polytrichum commune, hairy-cap moss Sporophyte (a sturdy Capsule Fig. 29-9d Polytrichum commune, hairy-cap moss Sporophyte (a sturdy plant that takes months to grow) Capsule Seta Figure 29.9 Bryophyte diversity Gametophyte

The Ecological and Economic Importance of Mosses Moses are capable of inhabiting diverse and sometimes extreme environments, but are especially common in moist forests and wetlands Some mosses might help retain nitrogen in the soil Sphagnum, or “peat moss,” forms extensive deposits of partially decayed organic material known as peat – peat has long been a fuel source in Europe and Asia Sphagnum is an important global reservoir of organic carbon (good CO2 sink)

Annual nitrogen loss (kg/ha) Fig. 29-10 RESULTS 6 5 4 Annual nitrogen loss (kg/ha) 3 2 Figure 29.10 Can bryophytes reduce the rate at which key nutrients are lost from soils? 1 With moss Without moss

(a) Peat being harvested Fig. 29-11 (a) Peat being harvested Figure 29.11 Sphagnum, or peat moss: a bryophyte with economic, ecological, and archaeological significance (b) “Tollund Man,” a bog mummy – acidic, oxygen-poor conditions preserve organisms

Concept 29.3: Ferns and other seedless vascular plants were the first plants to grow tall Vascular tissue allowed these plants to grow tall Seedless vascular plants have flagellated sperm and are usually restricted to moist environments Nonvascular plants (bryophytes) Seedless vascular plants Gymnosperms Angiosperms

Origins and Traits of Vascular Plants (xylem & phloem tissue) Living vascular plants are characterized by: Life cycles with dominant sporophytes, however, the gametophytes are photosynthetic and are not dependent on the sporophyte for nutrition. The gametophytes are tiny plants that grow on or below the soil surface Have roots that take up water from the soil and vascular tissue that transports it to leaves and stems up in the sunlight Must live in somewhat moist areas because their sexual reproduction still depends on water for flagellated sperm to swim in.

Key Haploid (n) Diploid (2n) Spore (n) Antheridium Young gametophyte Fig. 29-13-3 Key Haploid (n) Diploid (2n) Spore (n) Antheridium Young gametophyte Spore dispersal MEIOSIS Sporangium Mature gametophyte (n) Sperm Archegonium Egg Mature sporophyte (2n) Sporangium New sporophyte Zygote (2n) FERTILIZATION Sorus Figure 29.13 The life cycle of a fern Gametophyte Fiddlehead

Transport in Xylem and Phloem Vascular plants have two types of vascular tissue: xylem and phloem Xylem conducts most of the water and minerals and includes dead cells called tracheids Phloem consists of living cells and distributes sugars, amino acids, and other organic products Water-conducting cells are strengthened by lignin and provide structural support Increased height was an evolutionary advantage

Evolution of Roots Evolution of Leaves Roots are organs that anchor vascular plants They enable vascular plants to absorb water and nutrients from the soil Roots may have evolved from subterranean stems Evolution of Leaves Leaves are organs that increase the surface area of vascular plants, thereby capturing more solar energy that is used for photosynthesis

Athyrium filix-femina, lady fern 25 cm Fig. 29-15f Figure 29.15 Seedless vascular plant diversity 25 cm

The Significance of Seedless Vascular Plants The ancestors of modern lycophytes, horsetails, and ferns grew to great heights during the Devonian and Carboniferous, forming the first forests Photosynthesis dramatically increased the removal of CO2 from the atmosphere Increased photosynthesis may have helped produce the global cooling at the end of the Carboniferous period The decaying plants of these Carboniferous forests eventually became coal Coal was crucial to the Industrial Revolution, and people worldwide still burn 6 billion tons a year. Ironic that coal, formed from plants that contributed to global cooling, now contributes to global warming by returning carbon to the atmosphere.

Fig. 29-16 Figure 29.16 Artist’s conception of a Carboniferous forest based on fossil evidence

You should now be able to: Describe four shared characteristics and four distinct characteristics between green algae and land plants Explain why most bryophytes grow close to the ground and are restricted to periodically moist environments Describe three traits that characterize modern vascular plants and explain how these traits have contributed to success on land Explain how vascular plants differ from bryophytes