Transition to Land Plants and Plant Adaptations

Archaeplastida and the Transition to Land Plants

• Supergroup Archaeplastida includes red algae and green algae.
• Green algae are hypothesized to be the closest living relatives to plants.
• Specifically, the Charophytes are considered the closest relatives, inhabiting shallow freshwater ponds and lakes.
• The hypothesis suggests plants evolved as Charophytes adapted that allowed them to survive on land.
• During plant evolution, continents were subjected to periodic flooding and receding tides.
• Natural selection favored algal species that could survive longer periods unsubmerged.
• The Characeans, a group of green algae, are hypothesized to have spawned the lineage of modern land plants.

Adaptive Traits for Terrestrial Survival

• Spores: Haploid cells that can produce multicellular adults without fusion with another gamete.
• Produced by gametogenesis during the gametophyte generation.
• Sporangia: Structures that house spores during development until release.
• The presence of sporangia in early plant precursor organisms like Cooksonia suggests early adaptation.
• Fossilized spores date back approximately 475 million years.

Common Traits Uniting Characeans and Plants

• Rings of cellulose-synthesizing proteins in their cell membranes.
• Similarity of enzymes within their peroxisomes.
• Flagellated sperm structure.
• Formation of a phragmoplast (group of microtubules) between daughter nuclei in dividing cells.
• Molecular analysis of nuclear and chloroplast genes supports this relationship.

General Characteristics of Plants

• Plants share a unique combination of characteristics:
  • Multicellular.
  • Eukaryotic.
  • Photosynthetic autotrophs.
  • Cell walls made of cellulose.
  • Chloroplasts containing chlorophyll a and chlorophyll b.

Environmental Differences and Adaptations

• Support:
  • Aquatic: Buoyancy of water provides support, reducing gravitational force.
  • Terrestrial: Full force of gravity requires internal supporting structures.
• Access to Resources:
  • Aquatic: Minerals and nutrients are dissolved in the water, readily available to all cells.
  • Terrestrial: Requires systems for acquiring and transporting resources.
• Desiccation:
  • Aquatic: No risk of drying out.
  • Terrestrial: Adaptation to prevent water loss is essential.
• Photosynthesis:
  • Aquatic: Light is refracted and comes from all directions within the photic zone.
  • Terrestrial: Light comes primarily from above.
  • Photosynthesis concentrated at the top of the plant due to shading.
  • Competition for sunlight drove the development of vascular tissues in plants.
  • Vascular tissues transport water and nutrients.
• Reproduction:
  • Aquatic: Gametes can be released into the environment.
  • Terrestrial: Requires specialized structures such as spores, seeds, and flowers for reproduction and dispersal.

Plant Traits Facilitating Terrestrial Survival

• Sporangia: Produce spores for dispersal.
• Alternation of Generations: Life cycle includes separate haploid (gametophyte) and diploid (sporophyte) stages.
  • Gametophyte (haploid) produces gametes; sporophyte (diploid) produces spores.
  • One generation is dominant (larger, longer-lasting) in each major plant group.
• Gametangia: Structures similar to sporangia but for gamete production.
• Apical Meristems: Areas of cell division at the tips of stems and roots, facilitating lengthwise (primary) growth.

Alternation of Generations

• Basic Cycle
  • Sporophyte (2n) undergoes meiosis to produce spores (n).
  • Spores (n) develop into gametophytes (n).
  • Gametophytes (n) produce gametes (n).
  • Gametes (n) fuse during fertilization to form a zygote (2n).
  • Zygote (2n) develops into a new sporophyte (2n).
• Ferns: Sporophyte generation is dominant.
• Mosses and Bryophytes: Gametophyte generation is dominant.

Gametophytes and Gametangia in Mosses

• In mosses, the gametophyte generation is dominant, with the sporophyte generation confined to a small sporangium.
• Gametangia (antheridia in males, archegonia in females) produce gametes.
• Mosses rely on water for flagellated sperm to swim from the antheridium to the archegonium for fertilization.

Meristems and Plant Growth

• Apical Meristems: Regions of primary (lengthwise) growth in roots and shoots.
• Shoot Apical Meristems: Located at the tip of the shoot, driving upward growth.
• Root Apical Meristems: Located just behind the root tip, with cells dividing in two directions.
  • Cells pushed upwards contribute to root length.
  • Cells pushed downwards form the root cap, protecting the meristem as the root pushes through soil.

Bryophytes: Mosses, Liverworts, and Hornworts

• Simplest group of plants with dominant gametophyte generation.
• The sporophyte generation is a temporary stalk that depends on the gametophyte.
• Cells transfer nutrients from the gametophyte to the sporophyte.

Adaptations to Terrestrial Environment in Derived Plants

• Waxy Cuticle: Prevents desiccation by coating external cells; stomata regulate gas exchange.
• Roots: Absorb water and minerals from the soil and anchor the plant.
• Shoots: Support the plant body; leaves facilitate photosynthesis.
• Cell Walls: Lignin provides structural support to shoots.
• Vascular System: Transports water and nutrients from roots to shoots.
• Secondary Compounds: Attract pollinators or defend against herbivores.

Phylogenetic Tree of Plant Development

• Non-vascular plants (Bryophytes): Mosses, liverworts, and hornworts, the first land plants.
• Seedless Vascular Plants: Ferns, horsetails, club mosses, spike mosses, quillworts. Possess vascular tissues but no seeds.
• Seed Plants: Gymnosperms (conifers) and Angiosperms (flowering plants).