Plant Diversity and Evolution

Plant Diversity

• Three groups of nonvascular plants.
• Vascular plants.
  • Two groups of seedless plants.
  • Two groups of seed plants.

Plant Life Cycles

• Alternation of generations: Gametophyte (haploid, n) and Sporophyte (diploid, 2n) stages.
• Meiosis: Spores (n) are produced in the sporangium and dispersed.
• Young gametophyte develops from a spore; mature gametophyte has rhizoids, archegonium (eggs), and antheridium (sperm).
• Fertilization: Sperm and egg fuse to form a zygote (2n).
• Zygote develops into a new sporophyte.
• Sporophyte: Young sporophyte develops, matures into an adult with sporangium; in ferns, sori (clusters of sporangia) are on the underside of the leaf (fiddlehead).

Nonvascular Plants (Bryophytes)

• Mosses and other nonvascular plants have life cycles dominated by gametophytes.
  • Hepatophyta: Liverworts.
  • Bryophyta: Mosses.
  • Anthocerophyta: Hornworts.
• Key features of bryophytes:
  • Lack vascular tissue.
  • Bi-flagellate sperm.
  • Gametophyte is the persistent, dominant stage.
  • Protonema: Early gametophyte growth phase with green, photosynthetic strands.
  • Anchored by rhizoids (not as complex as roots).
  • Thalloid form vs. stem-leaf/leafy form (e.g., liverworts, mosses).
  • Sporophyte (diploid) is a minor, intermittent stage, typically "parasitic" on the gametophyte.
  • Foot: Embedded in parental archegonium, absorbs nutrients.
  • Seta: Stalk in most species.
  • Sporangium/capsule: Produces spores.

Hepatophyta - Liverworts

• Key features:
  • Oil bodies in cells: Organelles with many functions.
  • Elaters: Long, skinny, twisted cells in the sporangium; aid dispersal of spores.

Anthocerophyta - Hornworts

• Key features:
  • Sporophytes: Long, tapered, green.
  • Consist entirely of sporangium (no seta).
  • Semi-independent.
  • Persistent.
  • Single chloroplast per cell with a pyrenoid.
  • Pyrenoid: Aids photosynthesis and stores starch.

Bryophyta - Mosses

• Key features:
  • Blades usually one cell thick.
  • Long brownish sporophytes.
  • Protonema is highly branching.
  • Capsule with a peristome: Teeth that assist in the release of spores.

Ecological and Economic Importance of Mosses

• Some mosses help reduce nitrogen loss from the soil; others harbor nitrogen-fixing cyanobacteria.
• Sphagnum, or “peat moss,” forms peat, deposits of partially decayed organic material.
• Low temperature, pH, and oxygen level of peatlands inhibit decay.
• Peat can be used as a source of fuel.
• Peatlands cover approximately $3\%$ of Earth’s land surface yet contain approximately $30\%$ of the world’s soil carbon.

Seedless Vascular Plants

• Ferns and other seedless vascular plants were the first plants to grow tall.
• Bryophytes were prominent types of vegetation during the first 100 million years of plant evolution.
• The earliest fossils of vascular plants date to 425 million years ago.
• Vascular tissue allowed these plants to grow tall.
• Like bryophytes, seedless vascular plants have flagellated sperm and are usually restricted to moist environments.

Origins and Traits of Vascular Plants

• Living vascular plants are characterized by:
  • Dominant, independent, persistent sporophytes.
  • Gametophyte reduced and usually short-lived.
  • Vascular tissues, xylem and phloem.
  • Well-developed roots, stems, and leaves.
  • Sporangium-bearing leaves called sporophylls.

Transport in Xylem and Phloem

• Vascular plants have two types of vascular tissue: xylem and phloem.
  • Xylem: Conducts water and minerals using tube-shaped cells called tracheids, strengthened by lignin, which provides structural support for the entire plant.
  • Phloem: Tubes that distribute nutrients (sugars, amino acids, etc.).
  • Vascular tissue allowed for:
    • Increased height.
    • Specialization of body regions and division of labor.

Roots

• Organs that anchor vascular plants and enable uptake of water and nutrients from the soil.

Leaves

• Organs that increase the surface area of vascular plants; capture solar energy for photosynthesis.
• Basic kinds of leaves:
  • Microphylls: Small, often spine-shaped leaves with a single vein.
    • Found only in lycophytes.
  • Megaphylls: Larger, with branched vascular system.
    • Found in most vascular plants.

Sporophylls and Spore Variations

• Sporophylls: Modified leaves with sporangia.

  • Sori: Clusters of sporangia on the undersides of sporophylls found in ferns.
  • Strobili: Cone-like structures formed from groups of sporophylls found in lycophytes and gymnosperms.
  • Carpels and stamens: Sporophylls found in angiosperms.

• Most seedless vascular plants are homosporous; they produce one kind of spore:

  • Produce 1 type of sporophyll, 1 type of sporangium, 1 type of spore, that develops into 1 type of bisexual gametophyte.

• Some seedless vascular plants and all seed plants are heterosporous; they produce 2 kinds of spores:

• Produce 2 types of sporophylls:

  • Megasporophylls.
  • Microsporophylls.

Classification of Seedless Vascular Plants

• Lycophyta: Includes club mosses, spike mosses, and quillworts.
• Monilophyta: Includes ferns, horsetails, etc.

Lycophyta

• Living lycophytes are small plants.
• Leaves are microphylls (unique to Lycophyta).
• Have dichopodial roots (unique to Lycophyta).
• Dichotomous branching.
• Often with strobili – cone-shaped sporophylls.

Monilophyta

• Have megaphylls for leaves (also in seed plants).
• Monopodial roots – branch along the side of the parent root (also in seed plants).
• Multiflagellate sperm (also in some seed plants).
  • True ferns
    • Fronds – complex leaves.
    • Sori – sporangia clusters on the underside of fronds.
  • Horsetails
    • Once diverse, now one genus Equisetum.
    • Have distinctive strobili.
    • Minute leaves at joints.
    • Stem photosynthetic.

Significance of Seedless Vascular Plants

• Lycophytes, horsetails, and ferns grew tall during the Carboniferous period, forming the first forests.
• Early forests formed vast coal deposits due to arrested decomposition in swamps.
• Tree ferns still occur today in the tropics!