L9b Biodiversity

Plant Overview and Characteristics

Introduction to Plants

Plants are a diverse group of multicellular, eukaryotic organisms that play a crucial role in Earth's ecosystems. They are primarily autotrophic, producing their own food through photosynthesis, and possess unique adaptations for life on land.

• Multicellular and Eukaryotic: Plants are composed of multiple cells with membrane-bound organelles.

• Autotrophic: They synthesize organic molecules from inorganic substances using sunlight (photosynthesis).

• Cell Wall: Plant cells have a rigid cell wall mainly composed of cellulose.

• Sessile: Most plants are stationary and do not move from place to place.

• Alternation of Generations: Plants have a unique life cycle alternating between multicellular diploid (sporophyte) and haploid (gametophyte) stages.

Alternation of Generations

Plant Life Cycle

The plant life cycle alternates between two multicellular stages: the diploid sporophyte and the haploid gametophyte. This process is known as alternation of generations and is distinct from the animal life cycle, which only has a multicellular diploid stage.

• Sporophyte (2n): The diploid stage that produces haploid spores via meiosis.

• Gametophyte (n): The haploid stage that produces gametes (sperm and egg) via mitosis.

• Fertilization: Fusion of gametes forms a diploid zygote, which grows into a new sporophyte.

Evolution and Phylogeny of Plants

Origin and Major Adaptations

Plants are believed to have evolved from green algae, specifically charophytes. Over time, they developed several key adaptations for terrestrial life:

• Waxy Cuticle: Prevents water loss and protects against desiccation.

• Stomata: Pores that regulate gas exchange for photosynthesis.

• Vascular Tissue (Xylem & Phloem): Specialized tissues for transporting water, minerals, and nutrients throughout the plant body.

Classification of Plants

Major Plant Groups

Plants are classified based on the presence of vascular tissue, seeds, and flowers. The main groups include bryophytes, lycophytes, pterophytes, gymnosperms, and angiosperms.

Bryophytes (Non-Vascular Plants)

• Examples: Mosses, liverworts, hornworts

• Characteristics: Lack vascular tissue, small size, no true roots or leaves, reproduce via spores, require water for sexual reproduction.

• Dominant Generation: Gametophyte

Lycophytes and Pterophytes (Seedless Vascular Plants)

• Examples: Clubmosses (lycophytes), ferns (pterophytes)

• Characteristics: Have vascular tissue (xylem and phloem), reproduce via spores, contain lignin for structural support, have roots, stems, and leaves (fronds in ferns).

• Symbiotic Relationships: Often form mycorrhizal associations with fungi.

Gymnosperms (Seed Plants without Flowers)

• Examples: Conifers (pines, spruces, cedars), cycads, ginkgoes

• Characteristics: Produce seeds not enclosed in fruit ("naked seeds"), dominant in boreal forests, cones as reproductive structures, wind pollination, seeds can remain dormant.

Angiosperms (Flowering Plants)

• Characteristics: Largest group of living plants, reproductive structure is the flower, seeds enclosed within fruit, highly diverse, pollination by wind or animals.

• Fruit: Develops from the ovary after fertilization, aids in seed dispersal.

Seed Structure and Types of Angiosperms

Monocots vs. Dicots (Eudicots)

Angiosperm seeds contain one or two cotyledons, which store and supply nutrients to the embryo.

• Monocots: One cotyledon (e.g., grasses, lilies, orchids)

• Dicots (Eudicots): Two cotyledons (e.g., beans, sunflowers, oaks)

• Cotyledon Function: Provides "food supply" (starch, oils, protein) for the developing embryo before photosynthesis begins.

Plant Reproduction

Asexual Reproduction in Seed Plants

Many plants can reproduce asexually, producing genetically identical offspring (clones) through mitosis. This process is energy-efficient but reduces genetic diversity.

• Methods: Rhizomes, stolons, corms, tubers, fragmentation, grafting, cuttings

• Benefits: High survival rate, no need for pollinators, rapid colonization

• Costs: Lack of genetic diversity, reduced adaptability

Sexual Reproduction in Seed Plants

Sexual reproduction increases genetic diversity and involves the production of seeds, which protect and nourish the embryo and aid in dispersal.

• Gymnosperms: Pollination by wind, cones as reproductive organs, pollen tube delivers sperm to ovule

• Angiosperms: Pollination by wind or animals, flowers as reproductive organs, double fertilization produces zygote and endosperm

• Fruit: Mature ovary containing seeds, aids in dispersal

Pollination and Fertilization in Angiosperms

• Anther: Produces pollen

• Stigma: Sticky surface to catch pollen

• Style: Stalk connecting stigma to ovary

• Ovary: Contains ovules (eggs)

• Double Fertilization: One sperm fertilizes the egg (zygote), another fuses with polar nuclei (endosperm)

Benefits and Costs of Reproductive Strategies

Asexual Reproduction

• Benefits: Efficient, rapid, high survival rate, no need for pollinators

• Costs: Low genetic diversity, less adaptability

Sexual Reproduction

• Benefits: High genetic diversity, better adaptability, seeds can be dispersed and remain dormant

• Costs: Requires more energy and resources

Threats to Plant Diversity

Human Impact

• Climate Change: Alters habitats and growing conditions

• Overharvesting: Reduces plant populations

• Pollution: Can interfere with photosynthesis and plant health

Summary Table: Major Plant Groups and Adaptations

Plant Group	Vascular Tissue	Spores	Seeds	Fruit	Flowers
Peatmoss (Bryophytes)	✗	✓	✗	✗	✗
Ferns (Pterophytes)	✓	✓	✗	✗	✗
Conifers (Gymnosperms)	✓	✗	✓	✗	✗
Flowering Plants (Angiosperms)	✓	✗	✓	✓	✓

Additional info: Table entries inferred from standard plant biology knowledge and the provided homework prompt.