Strand 1 Topic 2 - Biodiversity of plants

Strand 1: Topic 2 - Biodiversity of Plants

UNIT 1: Evolutionary Development of Major Plant Groups

Phylogenetic Trees

• Species today have evolved from pre-existing species through gradual changes over time.

• All species are connected through a common ancestor.

• Classification of species is based on shared characteristics.

  • Organisms with many shared traits are more closely related.

• Cladograms illustrate the evolutionary relationships between species.

  • When branches represent a scale reflecting time/change, it is referred to as a phylogenetic tree.

Evolution of Plant Life

• Plants are multicellular eukaryotes with cellulose cell walls.

• They all have multicellular embryos developing within the plant's tissues.

• All possess chloroplasts for photosynthesis.

• Originated from a common ancestral type of aquatic algae.

• Evolution led to better adaptation of plants to terrestrial life.

• The Plantae kingdom is divided into four main divisions based on evolutionary history:

  • Bryophytes (mosses)

  • Pteridophytes (ferns)

  • Gymnosperms (cone-bearing plants)

  • Angiosperms (flowering plants)

Early Evolutionary Events

• Early plant evolution saw a branching event:

  • One branch led to today's bryophytes.

  • The other led to ferns, pines, and flowering plants.

• Each node in the branching diagram represents a common ancestor.

• The closeness of relation between two groups is determined by how recently they shared a common ancestor.

Features Used to Group Plants

• Major structural changes over time contributed to current plant groupings.

• Key features include the development of:

  • Vascular tissue

  • Seeds or spores

  • Flowers and fruits

• Plants vary in their dependence on water for fertilization and the size/dominance of spore- and gamete-producing phases in their life cycles.

• Alternation of Generations: a unique plant lifecycle feature.

  • Asexual generation (sporophyte) is diploid (2n), produces haploid spores through meiosis.

  • Sexual generation (gametophyte) is haploid (n), produces gametes.

• Gametes fuse during fertilization to form a diploid zygote that grows into a new sporophyte generation.

Diagram of Alternation of Generations

• Zygote → Sporophyte Generation (2n) → Gametophyte Generation (n)

  • Female gamete + Male gamete + Fertilization → Zygote

  • Illustrates the life cycle stages, including meiosis and spore development.

Unit 2: Bryophytes

• Represent the most primitive terrestrial plants, adapted features for life on land.

• Non-vascular, absorbing water directly from soil and air via osmosis.

• Dependent on water for reproduction (male sperm swim to eggs).

• Only group with a dominant gametophyte generation, forming green carpets of moss.

• Features include:

  • Separate male and female shoots

  • Photosynthetic leaf-like structures

Gametophyte Generation of Bryophytes

• Plant body called thallus (similar to algae).

• Short height due to lack of vascular tissue for support.

• No true roots, stems, or leaves; have root-like rhizoids for anchoring and water absorption.

• Sex organs: female archegonia (contain eggs) and male antheridia (produce sperm).

  • Sperm swim towards eggs due to flagella.

• Fertilized eggs develop into small multicellular embryos, depending on the parent plant for nourishment.

Sporophyte Generation of Bryophytes

• Sporophytes (2n) grow out of and are attached to gametophytes.

• Lacks chlorophyll; relies on gametophyte for food.

• Comprises a foot, stalk (seta), and spore case (sporangium).

• Spores are produced via meiosis in sporangia and released to develop into new gametophyte plants.

Unit 3: Pteridophytes (Ferns)

• This abundant group includes seedless vascular plants like ferns, horsetails, and club mosses.

  • Advanced traits include:

    • Dominant sporophyte generation

    • Presence of vascular tissue for larger growth

    • True roots

• Primitive traits involve water dependency for sperm movement to eggs.

  • Gametophyte structures resemble those of mosses.

Sporophyte Generation of Ferns

• Fern sporophytes feature a horizontal underground stem known as a rhizome.

• Roots extend from the rhizome for anchoring.

• Leaves, called fronds, have a waxy cuticle on both surfaces.

• Spores are produced in grouped structures (sori) on the undersides of fronds, protected by a membrane (indusium).

Gametophyte Generation of Ferns

• Spores develop into a prothallus, a flattened, heart-shaped structure.

• Rhizoids grow to anchor and absorb water and nutrients.

• Male and female sex organs are located on the prothallus' underside, with fertilization needing water.

Gymnosperms (Cone-Bearing Plants)

• Comprised of four groups: conifers, cycads, gnetophytes, and ginkgoes.

• Conifers make up the largest group.

• First to produce seeds (naked seeds, no fruits).

• Dominant sporophyte generation; these seeds provide protection to embryos and require no water for fertilization.

Reproduction in Gymnosperms

• Male and female cones present; male cones produce pollen grains.

• Pollen grains contain the male gametophyte.

• Female cones contain woody structures with ovules housing female gametophytes (eggs).

Fertilization in Gymnosperms

• Pollen is dispersed by wind to the ovules in female cones.

• Pollen tubes facilitate sperm transport to ovules and fertilization occurs without water.

• The zygote develops into an embryo and forms a protective seed coat.

Advantages of Seeds Over Spores

• Tougher outer coats, resistant to drying out.

• Seeds contain food reserves for embryos; spores must germinate quickly to survive.

• Seeds have fully developed embryos ready to grow under favorable conditions; spores lack this initial development.

Angiosperms - Flowering Plants

• Most recently evolved and the largest group, with significant agricultural importance.

• Classified into two classes:

  • Dicotyledons (two seed leaves)

  • Monocotyledons (one seed leaf)

Features of Angiosperms

• Well-developed vascular systems, enabling efficient water and nutrient transport.

• Vascular cambium allows for secondary growth.

• Possess a waxy cuticle on leaves for moisture retention.

• Dominant sporophyte generation with a greatly reduced gametophyte generation housed inside flowers.

  • Pollen carries male gametes; ovules carry female gametes.

• Unique flowers serve as complex reproductive structures.

Comparison of Plant Groups

Summary of Phylogenetic Development

• Evolved from an ancestral type of algae through major structural changes reflected in their classification today.