Chapter 19: Diversity of Plants

Importance of Plants to Life on Earth

Plants are fundamental to the survival of animals and the maintenance of ecosystems. They provide food, oxygen, shelter, medicines, and contribute to soil formation and fertility.

• Food Source: Plants are primary producers, supporting herbivores and higher trophic levels.

• Oxygen Production: Through photosynthesis, plants release oxygen, which is essential for aerobic organisms and forms ozone to protect from UV radiation.

• Water Detoxification: Aquatic plants purify water bodies.

• Shelter: Plants offer habitats for various animal species.

• Medicinal and Economic Value: Many medicines and products are derived from plants.

• Aesthetic Value: Plants enhance landscapes and human well-being.

• Soil Formation: Decomposed plant material enriches soil, and roots prevent erosion.

General Characteristics of Plants

Plants are classified in Kingdom Plantae and exhibit unique adaptations for terrestrial life.

• Sessile: Plants are immobile.

• Evolution: Originated from green algae; developed a waxy cuticle to prevent water loss.

• Vascular Tissues: Some evolved internal tissues (xylem and phloem) for transport.

• Photosynthesis: Autotrophic, producing carbohydrates using sunlight.

• Reproduction: Can reproduce sexually (via gametes) or asexually (runners, spores, seeds).

• Cell Structure: Cell walls contain cellulose and lignin; cells have chlorophyll.

Major Groups of Plants

Plants are divided into two main groups: non-vascular (bryophytes) and vascular (tracheophytes).

• Non-vascular plants: Bryophytes (mosses, liverworts)

• Vascular plants: Tracheophytes (ferns, gymnosperms, angiosperms)

Alternation of Generations

Plant Reproductive Strategy

Plants exhibit alternation of generations, alternating between two distinct forms: the diploid sporophyte and the haploid gametophyte.

• Sporophyte (2n): Produces haploid spores via meiosis; spores develop into gametophytes.

• Gametophyte (n): Produces gametes (sperm and egg) via mitosis; fertilization forms a diploid zygote, which grows into a sporophyte.

Bryophytes (Mosses and Liverworts)

Characteristics and Lifecycle

Bryophytes are non-vascular plants lacking specialized tissues for water and nutrient transport. They rely on osmosis and diffusion and require moist environments for fertilization.

• Reproduction: Use spores; gametophyte is the dominant, visible form.

• Structure: Lack roots, stems, and leaves; anchored by rhizoids.

• Habitat: Mostly moist environments; some adapted to dry habitats.

• Ecological Role: Peat moss (Sphagnum) forms peat bogs, which preserve organic material.

Peat Bogs and Preservation

Peat bogs are low-oxygen, low-bacteria environments formed by peat moss, capable of preserving organic remains.

• Example: Tollund Man, a Bronze Age mummy, was preserved in a peat bog.

Vascular Plants (Tracheophytes)

General Features

Vascular plants possess specialized tissues (xylem and phloem) for water and nutrient transport and have true roots, stems, and leaves. They are divided into three groups:

• Seedless vascular plants: Ferns, club mosses, horsetails

• Gymnosperms: Non-flowering seed plants

• Angiosperms: Flowering seed plants

Seedless Vascular Plants

These plants reproduce via spores and have fronds (leaves). The sporophyte is the dominant form.

• Examples: Ferns, club mosses, horsetails

• Lifecycle: Spores produced on fronds; gametophyte is small and short-lived.

Gymnosperms

Gymnosperms are seed plants that do not produce flowers. Seeds develop inside cones or fleshy structures, not fruits.

• Conifers: Cone-bearing plants with needle-shaped leaves; examples include pines, spruces, firs, cypresses, sequoias.

• Reproduction: Ovules fertilized by pollen; wind disperses pollen from male to female cones.

Angiosperms

Angiosperms are flowering plants that produce seeds enclosed in fruits. They are the most recently evolved and diverse group.

• Adaptations: Broad leaves for sunlight capture, flowers for protection and pollinator attraction, fruits for seed dispersal.

• Dispersal: Seeds spread by animals, wind, or water.

Seed Structure

Components of Seeds

Seeds are composed of an embryo, stored food (endosperm), and a protective seed coat. Cotyledons are the first leaves to emerge from the embryo.

• Seed coat: Prevents drying out.

• Endosperm: Provides nutrition for the developing embryo.

• Cotyledons: First leaves; number varies (monocots vs. dicots).

Comparison of Major Plant Groups

Features of Bryophytes, Ferns, Conifers, and Flowering Plants

The following table summarizes key features of the major plant groups, including their reproductive strategies, dispersal mechanisms, and presence of transport structures.