Plants

Basic Characteristics of Plants

• Multicellular photoautotrophic eukaryotes
• Chloroplasts contain chlorophyll a and b.
• Cell walls have cellulose.
• Almost exclusively terrestrial
  • Examples of some adaptations of some plants:
    • Roots: for absorbing water and minerals
    • Leaves: for obtaining CO₂ and light
      • Waxy cuticle helps prevent dessication.
      • Stomata provides pores for gas exchange.
  • Important Note: algae, which live exclusively in water and therefore have different internal anatomy, are not in the Plant Kingdom.

Plant Life Cycle: Alternations of Generations

All Plants exhibit a two generation life cycle.

• Gametophyte Stage or “Generation”
  • Multicellular individual with haploid cells (n)
• Sporophyte Stage or “Generation”
  • Multicellular individual with diploid cells (2n)

Figure 29.2, page 548, Campbell's Biology, 5th Edition

Alternation of Generations: A Special Case of the Haploid-Diploid Cycle

• All sexually reproducing species exhibit the Haploid-Diploid Cycle.
  • The nature of “sexual reproduction” is to reduce the chromosome number by half and then bring it back to “normal” by finding a “mate” who will contribute their half in restoring this balance.
    • Therefore all sexually reproducing species must go through a haploid (n) stage and a diploid (2n) stage.
    • Sometimes the entire haploid stage can be represented by nothing more than single cells, like in humans who produce single cell gametes—sperm and eggs.
• Alternation of Generations” has a special meaning.
  • This term is reserved for the special case in which both the haploid (n) and diploid (2n) stages consist of multicellular individuals.
  • This only occurs in plants and some species of algae.

Which “Generation” is Bigger?

It Depends…

• It might seem “intuitively” obvious that the diploid stage in the plant life cycle “should” be the biggest since that is the way it is in humans. That is pure chauvinism on our part and simply not true!
• Mosses
  • Gametophyte generation (haploid stage) is bigger
• Ferns, Conifers, Flowering Plants
  • Sporophyte generation (diploid stage) is bigger.
  • As we humans expect it should be!

Figure 30.1, page 562, Campbell's Biology, 5th Edition

Evolution and Alternation of Generations

• Gametophyte Dominant
  • Non-vascular Plants
    • Bryophytes (mosses, etc.)
• Sporophyte Dominant
  • Vascular, Seedless Plants
    • Ferns, horsetails, etc.
  • Vascular, Seeded Plants
    • Gymnosperms (conifers, cycads, etc.)
    • Angiosperms (flowering plants)

Figure 29.3, page 549, Campbell's Biology, 5th Edition

Highlights in Plant Evolution: A Drama in Four Stages

Stage 1: From Water to Land

• 500 million years ago
• Evolved from green algae (charophytes)
• Spores appear: Surrounded by sporopollenin which protected plant embryos from harsh terrestrial conditions
• Bryophytes
  • Earliest plant division to evolve
  • Includes mosses

Stage 2: Vascular Plants Appear

• Adaptive radiation of seedless vascular plants
• 400 million years ago
• New groups evolve
  • Ferns
  • Horsetails
  • Lycophytes
• Have vascular tissue
• No seeds

Stage 3: Seeds Appear

• Vascular plants with seeds
• 360 million years ago
• Seed: An embryo packaged with food inside a protective covering
• New group evolved: gymnosperms
  • Word derivation:
    • Gymno: Naked
    • Sperm: Seed
    • “Naked Seeds”
  • Seeds are not enclosed in any specialized chambers.
  • Conifers (pines, firs, redwoods, etc.)
    • Seed sits “nakedly” inside the cone.

Stage 4: Flowering Plants Appear

• Seeds are not “naked”; they are found in a protective structure called an ovary.
• 130 million years ago
• New group evolved
  • Angiosperms: “Seed containers”
    • Angio: Container (referring to the ovary)
    • Sperm: Seed
• Most common type of plant on earth today.