AB130: Biology III spring 2003-2004 Chapter 29-Plant Diversity I

Introduction

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  More than 280,000 species of plants inhabit terrestrial environments, including deserts, grasslands, and forests. Some species, such as sea grasses, have returned to aquatic habitats. Land plants (including the sea grasses) evolved from a certain green algae, called charophyceans.
  

A. An Overview of Land Plant Evolution

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  Nonvascular bryophytes, vascular pteridophytes (ferns), gymnosperms (conifers), and angiosperms (flowering plants).
  

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  Bryophytes are liverworts (Hematophyta), hornworts (Anthocerophyta) and mosses (Bryophyta). Their offspring develop from multicellular embryos.
  

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  Pteridiophytes (Lycophyta and Pterophyta) are often called seedless plants.
  

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  Gymonsperms and angiosperms, seed plants, evolved from a common ancestor.
  
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  Evolution of the first seed plants gave rise to cone-bearing gymnosperms, Ginkophyta, Cycadophyta, Gnetophyta and Coniferophyta, ca. 250 MYA.
  

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  The flowering angiosperms, Anthophyta, evolved about 130 MYA.
  

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  Four great episodes in the evolution of land plants:
  

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  1) The origin of bryophytes from algal ancestors; 2) The origin and diversification of vascular plants; 3) The origin of seeds and 4) The evolution of flowers.
  

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  Land plants are multicellular, eukaryotic, photosynthetic autrotrophs.
  

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  They share rosette cellulose-synthesizing complexes and peroxisomes with their closet relatives, the algal group called charophyceans.
  

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  The flagellated sperm of certain land plants resembles the sperm of charophyceans.
  

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  Some chorophyceans and all land plants form a phragmoplast during cytokinesis.
  

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  Several characteristics separate the four land plant groups from their closest algal relatives, including:
  

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  Apical meristems - to capitalize on terrestrial resources
  

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  Multicellular embryos that are dependent on the female parent (embryophytes)
  

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  Alternation of generations (also in some algae but not in charophyceans)
  
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  gametophyte (haploid, dominant in bryophytes) and sporophyte (diploid)
  
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  multicellular sporangia on sporophyte (2n) produce walled spores by meiosis
  

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  mitosis of spores forms multicellular gametangia (1n), which make gametes (not in flowering plants)
  

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  archaegonia produce eggs and antheridia make sperm (some motile)
  

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  gametes (1n) fuse to produce zygote (2n); mitosis reproduces a sporophyte
  

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  Additional terrestrial adaptations include:
  

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  cuticle of polyesters and waxes, with stomata
  

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  true roots, stems, and leaves (not in bryophytes)
  
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  xylem (dead) and phloem (living)
  

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  Lignin, in “woody” and vascular tissues of vascular plants.
  

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  Secondary metabolites also play important roles.
  

1. Land plants evolved from charophycean algae over 500 million years ago

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  Several lines of evidence support the phylogeny:
  

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  Homologous chloroplasts
  

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  Homologous cell walls
  

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  Homologous peroxisomes
  

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  Phragmoplasts
  

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  Homologous sperm
  

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  Molecular systematics
  

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  The advanced charophyceans Chara and Coleochaeta are haploid organisms.
  

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  They lack a multicellular sporophyte but retain and nourish diploid zygotes in haploid tissue.
  
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  A mutation that delayed meiosis until after some mitosis of the zygote could produce a multicellular, diploid sporophyte tissue.
  

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  A layer of sporopollenin prevents exposed charophycean zygotes from drying out.
  

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  The first land-adapted plants moved into a habitat occupied only by biofilms!
  

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  Traditionally, only the bryophytes, pteridophytes, gymnosperms, and angiosperms are in kingdom Plantae (embryophytes).
  
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  Others promote expanding the boundaries to include charophyceans or other non-charophycean green algae in Streptophyta or Viridiplantae.
  

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  Differences in life cycles among land plants are likely reproductive adaptations.
  

C. Bryophytes

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  Bryophytes are not a monophyletic group but are represented by three phyla:
  

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  Hepatophyta—liverworts, Anthocerophyta—hornworts, and Bryophyta—mosses
  

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  Liverworts and hornworts likely the earliest; Bryophytes most related to vascular plants.
  

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  The gametophyte (1n) is a mass of one-cell-thick filaments, called a protonema.
  

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  The protonema may produce meristems that form gametophores.
  
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  archegonia produce a single egg antheridia produce many flagellated sperm.
  

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  placental nutritive cells interface parent to embryos
  

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

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  Most lack conducting tissues (all lack lignin). They have no true stems or leaves.
  

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  Bryophyte sporophytes are photosynthetic but are dependent on the gametophyte.
  

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  Liverwort sporophytes have short stalks with round sporangia containing the developing spores, and a nutritive foot embedded in gametophyte tissues.
  

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  Hornwort sporophytes resemble grass blades and have a cuticle.
  

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  Both hornwarts and mosses have stomata like vascular plants
  

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  Moss sporophytes have a foot, stalk (seta), and sporangium (the capsule).
  

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  the calyptra (gametophyte tissue) protects maturing spores in the capsule.
  
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  a peristome may guard their release.
  

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  Bryophytes likely dominated the first 100 M years that terrestrial communities existed
  

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  D. The Origin of Vascular Plants
  

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  The branched sporophyte is dominant and is independent of the parent gametophyte.
  
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  The first vascular plants, pteridophytes, were seedless.
  

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  Vascular plants built on the adaptations inherited from mosslike ancestors.
  

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  The protracheophyte polysporangiophytes demonstrate the first steps in the evolution of sporophytes.
  

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  The branched sporophytes (independent of gametophyte) had lignified cells
  

E. Pteridophytes: The First True Vascular Plants

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  Lycophyta—lycophytes
  

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  Pterophyta—ferns, whisk ferns, and horsetails
  

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  Most pteridophytes have true roots with lignified vascular tissue.
  

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  Lycophytes have microphylls (sm. leaves) with only a single unbranched vein.
  

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  other vascular plants have highly-branched megaphylls.
  

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  The reduction in the size of the gametophytes is most extreme in seed plants.
  

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  Ferns may be homosporous and heterosporous plants.
  
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  A homosporous plant produces a single bisexual spore (gametophyte).
  

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  A heterosporous plant makes megaspores (archegonia) and microspores (antheridia)
  

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  Lycophyta dominated the Carboniferous period; about 1,000 species survive.
  

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  Many modern tropical species grow on trees as epiphytes
  

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  Sporophytes have upright stems (with microphylls) and horizontal stems with roots.
  

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  Specialized leaves (sporophylls) bear clustered sporangia (club shaped)
  

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  Spores develop into tiny photosynthetic or nonphotosynthetic haploid gametophytes.
  

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  Pterophyta - ferns and their relatives.
  

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  Psilophytes (whisk ferns) have Y-like branching and lack true leaves and roots.
  

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  Sphenophytes (horsetails), about 15 species in the genus Equisetum survive.
  

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  Upright hollow stems (photosynthetic) and roots from horizontal rhizomes.
  

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  Reproductive stems produce clusters of sporophylls in cones at their tips.
  

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  Ferns, there are over 12,000 species today.
  

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  Many have rhizomes with megaphyllous leaves and highly branched vasculature.
  

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  Produce clusters of sporangia (sori) on the back of leaves (sporophylls).
  

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  The phyla Lycophyta and Pterophyta formed forests during the Carboniferous period about 290-360 million years ago.