[1] Viridiplantae

• Viridiplantae or Chlorobionta


  A monophyletic group of eukaryotic organisms that includes what have traditionally been called "green algae" plus the land plants or embryophytes
• Viridiplantae


  • Cells with membrane-bound organelles, including a nucleus, microtubules, mitochondria, an endoplasmic reticulum, vesicles, and golgi bodies
• Cellulosic cell wall


  A polysaccharide in which the glucose sugar units are bonded in the beta-1,4 position, forming a supportive meshwork
• Chloroplasts in Viridiplantae


  Contain chlorophyll b in addition to chlorophyll a, have thylakoids stacked into grana, and manufacture starch as a storage product
• Sister groups of Viridiplantae

  • Chlorophytes (Chlorophyceae)
  • Streptophytes (Streptophyceae)
• Haplontic life cycle


  Haploid multicelled stage, gametes fuse to form diploid zygote, zygote divides by meiosis to form haploid spores
• Oogamy


  One gamete (egg) is larger and nonflagellate, the other gamete is a sperm cell
• Plasmodesmata


  Pores in the primary cell wall that allow for transfer of compounds between cells
• Charales


  • Haplontic life cycle, consist of a central axis bearing whorls of lateral branches or "leaves", capable of precipitating calcium carbonate, have specialized male and female gametangia (antheridia and oogonia)
• Charales


  Genera Chara and Nitella, perhaps the closest living relatives to the land plants
• Charales


  • Fresh water, aquatic organisms
  • Have a haplontic life cycle
  • Consist of a central axis bearing whorls of lateral branches or "leaves" on the haploid body
  • Some are capable of precipitating calcium carbonate as an outer layer of the plant body (accounting for the common names "brittleworts" or "stoneworts")
  • Grow by means of a single apical cell, similar to that of some land plants and representing a possible synapomorphy with them
  • Differ from land plants in lacking true parenchyma
• Gametangia of Charales


  Specialized male and female gametangia, termed antheridia and oogonia
• Oogonia of Charales

  • Distinctive in having a spirally arranged group of outer "tube" cells
  • Fossilized casts of oogonia retain the outline of these tube cells
• Oogonia and antheridia of the Charales


  Resemble the archegonia and antheridia of land plants, but are generally thought not to be directly homologous due to major differences in structure and development
• Members of the Charales retain the egg and zygote (although the latter only briefly) on the plant body
• Retention of egg and zygote on the haploid body of Charales

  May represent a transition to their permanent retention on the gametophyte of land plants
• The Embryophyta, or embryophytes (commonly known as land plants), are a monophyletic assemblage within the green plants
• First colonization of plants on land during the Silurian period, ca. 400 million years ago


  • Concomitant with the evolution of several important features that enabled formerly aquatic green plants to survive and reproduce in the absence of a surrounding water medium
• Innovations of land plants

  • Evolution of the embryo and sporophyte
  • Evolution of cutin and the cuticle
  • Evolution of parenchyma tissue
• Sporophyte


  A separate diploid (2n) phase in the life cycle of all land plants
• Gametophyte


  The corresponding haploid, gamete-producing part of the life cycle of land plants
• Haplodiplontic (diplobiontic) life cycle

  The life cycle of land plants, having both a haploid gametophyte and a diploid sporophyte, also called alternation of generations
• Alternation of generations does not necessarily mean that the two phases occur at different points in time; at any given time, both phases may occur in a population
• Formation of the sporophyte


  1. Delay of meiosis and spore production

  2. Zygote undergoes numerous mitotic divisions, resulting in the development of a separate entity
• Embryo


  An immature sporophyte that is attached to or surrounded by the gametophyte
• Development of the sporophyte

  1. A portion of the sporophyte differentiates as the spore-producing region, called the sporangium

  2. The sporangium is enveloped by a sporangial wall, consisting of one or more layers of sterile, non-spore-producing cells

  3. The sporangium contains sporogenous tissue, which matures into sporocytes that undergo meiosis to produce four haploid spores
• Adaptive advantages of the sporophyte generation

  • Large increase in spore production
  • Potential to prevent expression of recessive, deleterious alleles
  • Increased genetic variability in the sporophyte generation upon which natural selection acts
• Cuticle


  A protective layer secreted to the outside of the cells of the epidermis, consisting of a thin, homogeneous, transparent layer of cutin, a polymer of fatty acids
• Cutin


  Impregnates the outer cellulosic cell walls of epidermal cells, known as a "cutinized" cell wall
• Adaptive advantage of cutin and the cuticle

  • Prevention of desiccation outside the ancestral water medium
• Parenchyma tissue


  A solid mass of tissue formed by the cells derived from the apical meristem region, consisting of cells that most resemble the unspecialized, undifferentiated cells of actively dividing meristematic tissue
• Characteristics of parenchyma cells

  • Elongate to isodiametric
  • Have a primary (1°) cell wall only (rarely a secondary wall)
  • Living at maturity and potentially capable of continued cell divisions
  • Function in metabolic activities such as respiration, photosynthesis, lateral transport, storage, and regeneration/wound healing
• It is not clear if the evolution of both apical growth and true parenchyma is an apomorphy for the land plants alone, as they may also occur in certain closely related green plants, including the Charales
• Middle lamella


  A pectic-rich layer that develops between the primary cell walls of adjacent cells, functioning to bind adjacent cells together
• Antheridium


  A type of specialized gametangium of the haploid (n) gametophyte, containing the sperm-producing cells, surrounded by a layer of sterile cells (the antheridial wall)
• The evolution of the surrounding layer of sterile wall cells in the antheridium was probably adaptive in protecting the developing sperm cells from desiccation
• In all of the nonseed land plants, the sperm cells are released from the antheridium into the external environment and must swim to the egg in a thin film of water, a vestige of their aquatic ancestry
• The antheridium of the Charales may not be homologous with that of the land plants due to differing anatomy
• Antheridium


  A type of specialized gametangium of the haploid (n) gametophyte that contains the sperm-producing cells
• Antheridium


  • Surrounded by a layer of sterile cells, the antheridial wall
  • The evolution of the surrounding layer of sterile wall cells was probably adaptive in protecting the developing sperm cells from desiccation