Plant Evolution PDF

## THE PLANTS - **Do not possess vascular tissue:** - Non-vascular plants - **Possess vascular tissue:** - **Produce seeds:** - Gymnosperms - **Produce flowers and fruit:** - Angiosperms - **Vascular seedless plants** **Ancestral protist** ## Sporophytes and sporan...

## THE PLANTS - **Do not possess vascular tissue:** - Non-vascular plants - **Possess vascular tissue:** - **Produce seeds:** - Gymnosperms - **Produce flowers and fruit:** - Angiosperms - **Vascular seedless plants** **Ancestral protist** ## Sporophytes and sporangia of Sphagnum (a moss) - The sporophyte produces spores in organs called sporangia - Diploid cells called sporocytes undergo meiosis to generate haploid spores - Spore walls contain sporopollenin, which makes them resistant to harsh environments ## Apical meristems - Plants sustain continual growth in their apical meristems - Cells from the apical meristems differentiate into various tissues ## Multicellular Gametangia - Gametes are produced within organs called gametangia - **Female gametangia**, called **archegonia**, produce eggs and are the site of fertilization - **Male gametangia**, called **antheridia**, are the site of sperm production and release ## The Origin and Diversification of Plants - Fossil evidence indicates that plants were on land at least 475 million years ago - Fossilized spores and tissues have been extracted from 475-million-year-old rocks ## 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 ## Seed Plants - 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 "vessel seed" plants, the flowering plants ## Bryophytes - Bryophytes are photosynthetic eukaryotic autotrophs that have a cellulose cell wall and chlorophylls a and b - Bryophytes and bryophyte-like plants were the prevalent vegetation during the first 100 million years of plant evolution ## Origins and Traits of Vascular Plants - Vascular plants began to diversify during the Devonian and Carboniferous periods - Fossils of the forerunners of vascular plants date back about 420 million years - Living vascular plants are characterized by: - Life cycles with dominant sporophytes separate from the gametophyte, along with flagellated sperm - Vascular tissues called xylem (with tracheids) and phloem - Well-developed roots, not rhizoids (400 mya in Lycopod fossils) and leaves. No seeds ## Bryophyte Life Cycle - In all three bryophyte phyla, gametophytes are larger and longer-living than sporophytes - Sporophytes are typically present only part of the time ## Features of Ancient Vascular Plants - Cooksonia, one of the first vascular (Xylem and Phloem) land plant, appeared about 420 MYA, Late Silurian - Only a few centimeters tall - No roots or leaves - Homosporous (producing only one type of spore) ## 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 ## 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 ## Seedless 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) - Seedless vascular plants are paraphyletic, and are of the same level of biological organization, or grade ## Sporophylls and Spore Variations - Sporophylls are modified leaves with sporangia - Sori are clusters of sporangia on the undersides of sporophylls - Strobili are cone-like structures formed from groups of sporophylls ## Evolution of Leaves - Leaves are organs that increase the surface area of vascular plants, thereby capturing more solar energy that is used for photosynthesis - Leaves are categorized by two types: - Microphylls, leaves with a single vein - Megaphylls, leaves with a highly branched vascular system - According to one model of evolution, microphylls evolved first, as outgrowths of stems ## Spore Production - **Homosporous spore production:** - Sporangium on sporophyll - Single type of spore - Typically a bisexual gametophyte - Eggs - Sperm - **Heterosporous spore production:** - Megasporangium on megasporophyll - Megaspore - Female gametophyte - Eggs - Microsporangium on microsporophyll - Microspore - Male gametophyte - Sperm ## Bryophyte Reproduction - Rhizoids anchor gametophytes to substrate - Mature gametophytes produce flagellated sperm in antheridia and an egg in each archegonium - Sperm swim through a film of water to reach and fertilize the egg ## Seedless Vascular Plants - 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 - Heterosporous species produce megaspores that give rise to female gametophytes, and microspores that give rise to male gametophytes ## Evolution of Roots - 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 ## The Significance of Seedless Vascular Plants - The ancestors of modern lycophytes, horsetails,etc., and pterophytes, ferns and fern allies, grew to great heights during the Devonian and Carboniferous (416 – 299 mya), forming the first “forests” or more properly called “mats”, when there is no true wood in the plants - The decaying plants (lycophytes, pterophytes, including the whisk ferns, tree ferns, etc.) of these Carboniferous “forests” eventually became coal ## Classification of Seedless Vascular Plants - There are two phyla of seedless vascular plants: - Phylum Lycophyta includes club mosses, spike mosses, and quillworts - Phylum Pterophyta includes ferns, horsetails, and whisk ferns and their relatives - All of the above (including the bryophytes) have flagellated sperm ## Seedless Vascular Plants - Lycophytes -true ferns, club mosses - Pterodophytes - horsetails, whisk ferns ## Overview: Transforming the World - Seeds changed the course of plant evolution, enabling their bearers to become the dominant producers in most terrestrial ecosystems - A seed consists of an embryo and nutrients surrounded by a protective coat