Plant Diversity I How Plants Colonized Land Chapter 29 Plant Diversity I How Plants Colonized Land
Land Ho! The Greening of Earth For the first 3 billion years of Earth’s history, the land was lifeless Cyanobacteria existed on land about 1.2 billion years ago ~500 million years ago plants, fungi and animals joined them DiversificationOver 290,00 species of plants inhabit Earth today
Plant Evolution Land plants evolved from green algae Charophyceans (a type of green algae) are the closest relatives of land plants Accumulation of adaptations to the terrestrial environment allowed the first land plants to live permanently above the waterline Chara, a pond organism (a) 10 mm Coleochaete orbicularis (b) 40 µm
Defining the Plant Kingdom Systematists Are currently debating the boundaries of the plant kingdom Plantae Streptophyta Viridiplantae Red algae Chlorophytes Charophyceans Embryophytes Ancestral alga
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 (gametes produced here) Multicellular dependent embryos
Apical meristems and alternation of generations of shoot Developing leaves 100 µm Apical meristem of root Root Shoot . APICAL MERISTEMS Localized regions of cell division at tips of roots and shoots Haploid multicellular organism (gametophyte) Mitosis Gametes Zygote Diploid multicellular organism (sporophyte) Alternation of generations: a generalized scheme MEIOSIS FERTILIZATION 2n n Spores ALTERNATION OF GENERATIONS Generations Gametophyte (haploid) Sporophyte (diploid)
Walled spores; multicellular gametangia; and multicellular, dependent embryos WALLED SPORES PRODUCED IN SPORANGIA Sporangium Sporophyte and sporangium of Sphagnum (a moss) Longitudinal section of Sphagnum sporangium Sporophyte Gametophyte MULTICELLULAR GAMETANGIA (where gametes are produced) Female gametophyte Archegonium with egg Female gametangia (archegonia) Male gametangia (antheridia) Archegonia and antheridia of Marchantia (a liverwort) Antheridium with sperm Male gametophyte MULTICELLULAR, DEPENDENT EMBRYOS Embryo Embryo retained within tissues of female parent. The parental tissues provide the developing embryo with nutrients. Maternal tissue Land plants are also known as EMBRYOPHYTES Embryo and placental transfer cell of Marchantia Wall ingrowths Placental transfer cell
Fossilized spores and tissues Have been extracted from 475-million-year-old rocks Fossilized spores. Unlike the spores of most living plants, which are single grains, these spores found in Oman are in groups of four (left; one hidden) and two (right). Fossilized sporophyte tissue. The spores were embedded in tissue that appears to be from plants.
Diversity of Modern Plants Land plants can be informally grouped based on the presence or absence of vascular tissue
An overview of land plant evolution Land plants Vascular plants Bryophytes (nonvascular plants) Seedless vascular plants Seed plants Mosses Liverworts Hornworts Charophyceans Gymnosperms Angiosperms Pterophyte (ferns, horsetails, whisk fern) Origin of seed plants (about 360 mya) Lycophytes (club mosses, spike mosses, quillworts) Origin of vascular plants (about 420 mya) Origin of land plants (about 475 mya) Ancestral green alga
Bryophytes 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
Bryophytes…the video clip Plant Classifications: Bryophytes." Online . Encyclopædia Britannica Online. 14 Jan. 2008 <http://www.britannica.com/eb/art-68471>.
Bryophyte Gametophytes In all three bryophyte phyla Gametophytes are larger and longer-living than sporophytes
Moss Life Cycle…The Movie
The life cycle of a moss 4 3 8 6 5 7 1 2 Mature sporophytes Young sporophyte Male gametophyte Raindrop Sperm Key Haploid (n) Diploid (2n) Antheridia Female gametophyte Egg Archegonia FERTILIZATION (within archegonium) Zygote Archegonium Embryo Female gametophytes Gametophore Foot Capsule (sporangium) Seta Peristome Spores Protonemata “Bud” MEIOSIS Sporangium Calyptra Capsule with peristome Rhizoid Mature sporophytes Spores develop into threadlike protonemata. 1 The haploid protonemata produce “buds” that grow into gametophytes. 2 Most mosses have separate male and female gametophytes, with antheridia and archegonia, respectively. 3 A sperm swims through a film of moisture to an archegonium and fertilizes the egg. 4 Meiosis occurs and haploid spores develop in the sporangium of the sporophyte. When the sporangium lid pops off, the peristome “teeth” regulate gradual release of the spores. 8 The sporophyte grows a long stalk, or seta, that emerges from the archegonium. 6 The diploid zygote develops into a sporophyte embryo within the archegonium. 5 Attached by its foot, the sporophyte remains nutritionally dependent on the gametophyte. 7
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 Grow out of archegonia Are the smallest and simplest of all extant plant groups Consist of a foot, a seta (an elongated stalk), and a sporangium Hornwort and moss sporophytes Have stomata (like vascular plants) (an opening for gas exchange)
Bryophyte diversity LIVERWORTS (PHYLUM HEPATOPHYTA) Gametophore of HORNWORTS (PHYLUM ANTHOCEROPHYTA) MOSSES (PHYLUM BRYOPHYTA) Gametophore of female gametophyte Marchantia polymorpha, a “thalloid” liverwort Foot Sporangium Seta 500 µm Marchantia sporophyte (LM) Plagiochila deltoidea, a “leafy” liverwort An Anthoceros hornwort species Sporophyte Gametophyte Polytrichum commune, hairy-cap moss
Ecological and Economic Importance of Mosses Sphagnum, or “peat moss” Forms extensive deposits of partially decayed organic material known as peat Peatlands play an important role as carbon reservoirs an help stabilize atmospheric carbon dioxide levels. (a) Peat being harvested from a peat bog Sporangium at tip of sporophyte Gametophyte (b) Closeup of Sphagnum. Note the “leafy” gametophytes and their offspring, the sporophytes. Living photo- synthetic cells Dead water- storing cells 100 µm (c) Sphagnum “leaf” (LM). The combination of living photosynthetic cells and dead water-storing cells gives the moss its spongy quality. (d) “Tolland Man,” a bog mummy dating from 405–100 B.C. The acidic, oxygen-poor conditions produced by Sphagnum can preserve human or other animal bodies for thousands of years.
Origin and Diversity of Vascular Plants 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 Fossils of the forerunners of vascular plants date back about 420 million years
Seedless Vascular Plants These early tiny plants Had branching sporophytes that weren’t dependent on gametopyhtes for growth, but lacked other derived traits of vascular plants Dichotomous (Y-shaped) branching made possible multiple sporangia. This evolutionary development allowed for greater spore production. Also increased survival in the face of herbivory. If some sporangia were eaten others might survive.
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
Tracheophytes-Pterophyta (ferns) …the video clip fern." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 13 Jan. 2008 <http://www.britannica.com/eb/article-9110635>.
Fern Life Cycle…The Movie
The life cycle of a fern Sporangia release spores. Most fern species produce a single type of spore that gives rise to a bisexual gametophyte. 1 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 Key Haploid (n) Diploid (2n) Antheridium Spore Young gametophyte MEIOSIS Sporangium Archegonium Sperm Mature sporophyte Egg New sporophyte Zygote Sporangium FERTILIZATION Sorus On the underside of the sporophyte‘s reproductive leaves are spots called sori. Each sorus is a cluster of sporangia. 6 Fern sperm use flagella to swim from the antheridia to eggs in the archegonia. 4 Gametophyte Fiddlehead A zygote develops into a new sporophyte, and the young plant grows out from an archegonium of its parent, the gametophyte. 5
Transport in Xylem and Phloem Vascular plants have two types of vascular tissue Xylem and phloem Xylem Conducts most of the water and minerals Includes dead cells called tracheids. Their cell walls remain to provide the internal ‘pipe system’. Cell walls are strengthened by the polymer lignin. This allows vascular plants to grow to greater heights than bryophytes Phloem Distributes sugars, amino acids, and other organic products Consists of living cells
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 Leaves Leaves are categorized by two types Are organs that increase the surface area of vascular plants, thereby capturing more solar energy for photosynthesis Leaves are categorized by two types Microphylls, leaves with a single vein Megaphylls, leaves with a highly branched vascular system
Evolution of Leaves According to one model of evolution Microphylls evolved first, as outgrowths of stems Vascular tissue Microphylls may have originated as small stem outgrowths supported by single, unbranched strands of vascular tissue. (a) Megaphylls, which have branched vascular systems, may have evolved by the fusion of branched stems. (b)
Sporophylls and Spore Variations Are modified leaves with sporangia (may look like a “photosynthetic leaf” may not) Most seedless vascular plants Are homosporous, producing one type of spore that develops into a bisexual gametophyte 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 Seedless vascular plants form two phyla Lycophyta, including club mosses, spike mosses, and quillworts. Are small herbaceous plants Pterophyta, including ferns, horsetails, and whisk ferns and their relatives. Ferns are the most diverse seedless vascular plants
Groups of seedless vascular plants LYCOPHYTES (PHYLUM LYCOPHYTA) PTEROPHYTES (PHYLUM PTEROPHYTA) WHISK FERNS AND RELATIVES HORSETAILS FERNS Isoetes gunnii, a quillwort Selaginella apoda, a spike moss Diphasiastrum tristachyum, a club moss Strobili (clusters of sporophylls) Psilotum nudum, a whisk fern Equisetum arvense, field horsetail Vegetative stem Strobilus on fertile stem Athyrium filix-femina, lady fern
Significance of Seedless Vascular Plants The ancestors of modern lycophytes, horsetails, and ferns grew to great heights during the Carboniferous, forming the first forests The growth of these early forests may have helped produce the major global cooling that characterized the end of the Carboniferous period. They decayed and eventually became coal