Plant Diversity and Structure: Colonization, Evolution, and Development

Chapter 29 – Plant Diversity I: How Plants Colonized Land

Evidence Linking Charophytes to Land Plants

Charophytes, a group of green algae, are considered the closest relatives of land plants due to several structural and genetic similarities.

• Cellulose-synthesizing complexes: Both charophytes and land plants have rosette-shaped cellulose-synthesizing complexes in their cell membranes.

• Peroxisome enzymes: Similar enzymes help minimize photorespiration.

• Structure of flagellated sperm: Sperm structure is similar in both groups.

• Formation of phragmoplast: Both form a phragmoplast during cell division.

• Genetic evidence: DNA sequence data supports a close relationship.

Example: Chara and Coleochaete are charophyte genera closely related to land plants.

Derived Traits of Land Plants

Land plants possess several derived traits that distinguish them from their algal ancestors.

• Alternation of generations: Life cycle alternates between multicellular haploid (gametophyte) and diploid (sporophyte) stages.

• Multicellular dependent embryos: Embryos develop within female gametophyte tissues, receiving nutrients from parental tissues.

• Walled spores produced in sporangia: Sporopollenin-rich walls protect spores from desiccation.

• Apical meristems: Regions of cell division at tips of roots and shoots enable growth in length.

Challenges of Terrestrial Life and Adaptations

Transitioning to land posed challenges such as desiccation, support, and reproduction. Plants evolved several adaptations:

• Cuticle: A waxy covering that prevents water loss.

• Stomata: Pores that regulate gas exchange and water loss.

• Vascular tissues: Xylem and phloem transport water, minerals, and nutrients.

Major Groups of Seedless Plants

Seedless plants include bryophytes, lycophytes, and pterophytes, each with distinct life cycles and structures.

• Bryophytes: Nonvascular plants (mosses, liverworts, hornworts); dominant gametophyte stage; lack true roots and leaves.

• Lycophytes: Club mosses and relatives; have vascular tissue but reproduce via spores.

• Pterophytes: Ferns and relatives; vascular tissue present; dominant sporophyte stage.

Alternation of Generations

Plants alternate between haploid and diploid multicellular stages.

• Gametophyte (n): Produces gametes by mitosis.

• Sporophyte (2n): Produces spores by meiosis.

• Mosses: Gametophyte is dominant; sporophyte is dependent.

• Ferns: Sporophyte is dominant; gametophyte is independent but small.

Example: In mosses, the green carpet is the gametophyte; in ferns, the leafy plant is the sporophyte.

Chapter 30 – Plant Diversity II: The Evolution of Seed Plants

Evolutionary Significance of Seeds

Seeds represent a major evolutionary innovation, enhancing survival and dispersal.

• Protection: Seed coat shields the embryo from harsh conditions.

• Dispersal: Seeds can be transported by wind, water, or animals.

• Dormancy: Seeds can remain inactive until conditions are favorable.

• Nutrient support: Stored food supports early growth of the embryo.

Seed Plants vs. Seedless Plants

Seed plants differ from seedless plants in several reproductive adaptations.

• Reduced gametophytes: Gametophytes are microscopic and develop within sporophyte tissues.

• Heterospory: Production of two types of spores: megaspores (female) and microspores (male).

• Pollen: Male gametophyte enclosed in a tough wall; enables fertilization without water.

• Ovules: Structures that develop into seeds after fertilization.

Innovations in Seed Plant Evolution

• Reduced gametophytes: Protects gametophytes from environmental stress.

• Heterospory: Distinct male and female spores.

• Pollen and pollination: Pollen grains carry sperm to ovules, often via wind or animals.

• Ovules and seeds: Ovule develops into a seed after fertilization.

Gymnosperms vs. Angiosperms

Seed plants are divided into gymnosperms ("naked seeds") and angiosperms ("enclosed seeds").

Feature	Gymnosperms	Angiosperms
Seed enclosure	Exposed on cones	Enclosed within fruits
Pollination	Mainly wind	Wind or animals
Reproductive structure	Cones	Flowers
Examples	Pine, fir	Oak, rose

Gymnosperm Life Cycle

• Male gametophytes: Develop in pollen cones from microspores.

• Female gametophytes: Develop in ovulate cones from megaspores.

• Pollination: Pollen is transferred by wind to ovules.

• Cones: Pollen cones produce pollen; ovulate cones produce ovules and seeds.

• Seed formation: Fertilized ovule develops into a seed.

Angiosperm Life Cycle

• Flower structure: Includes stamen (anther, filament), carpel (stigma, style, ovary), petals, and sepals.

• Pollination: Can occur via wind or animals.

• Double fertilization: One sperm fertilizes the egg, another fuses with two nuclei to form endosperm.

• Seed and fruit formation: Ovule becomes seed; ovary becomes fruit.

• Seed structure: Seed coat, embryo, and food reserves (endosperm or cotyledons).

• Pollen grain: Contains male gametophyte.

• Ovule anatomy: Contains female gametophyte and egg cell.

Chapter 35 – Plant Structure, Growth, and Development

Hierarchical Organization of Plants

Plants are organized into organs, tissues, and cells.

• Organs: Roots, stems, leaves.

• Tissues: Dermal (protection), vascular (transport), ground (storage, support, photosynthesis).

• Cells: Specialized for various functions (e.g., parenchyma, collenchyma, sclerenchyma).

Three Basic Plant Organs and Their Functions

• Roots: Anchor plant, absorb water and minerals, store carbohydrates.

• Stems: Support leaves and flowers, transport fluids, elevate reproductive structures.

• Leaves: Main site of photosynthesis and gas exchange.

Primary vs. Secondary Growth

Plants grow in length (primary growth) and thickness (secondary growth).

• Primary growth: Occurs at apical meristems (tips of roots and shoots).

• Secondary growth: Increases girth via lateral meristems (vascular cambium and cork cambium).

Types of Plant Tissues

• Dermal tissue: Protective outer covering (epidermis, periderm).

• Vascular tissue: Xylem (water transport) and phloem (sugar transport).

• Ground tissue: Functions in storage, photosynthesis, and support.

Cell Differentiation and Morphogenesis

Cell differentiation leads to specialized cell types, contributing to the plant's form (morphogenesis).

• Totipotency: Many plant cells can de-differentiate and become any cell type.

• Pattern formation: Spatial organization of tissues and organs.

Developmental Plasticity

Plants can alter their growth and development in response to environmental conditions.

• Plasticity: Ability to change form (e.g., leaf shape, root growth) based on environment.

• Example: Aquatic plants may develop different leaf shapes when submerged vs. exposed to air.

Additional info: Developmental plasticity allows plants to optimize resource use and survive in variable environments.