Plant Evolution and Structure

Plant Evolution

• Plants evolved from an ancestral protist, gradually developing features like vascular tissue, seeds, flowers, and fruits.
• Non-vascular plants lack vascular tissue and are typically small and low-growing.
• Vascular plants possess vascular tissue (xylem and phloem), enabling them to transport water and nutrients, and grow taller and more complex.
• Seedless vascular plants, including lycophytes and pterophytes, reproduce via spores and lack seeds.
• Seed plants, including gymnosperms (e.g., conifers) and angiosperms (flowering plants), reproduce using seeds. A seed contains an embryo, nutrients, and a protective coat.

Plant Structure and Function

• Apical meristems: Sites of continuous growth in plants. Cells in apical meristems differentiate into various tissues.
• Sporophytes: The diploid plant generation in plants' life cycle, producing spores.
• Spores: Haploid reproductive cells produced by sporophytes.
• Sori: Clusters of sporangia on the undersides of sporophylls.
• Sporophylls: Modified leaves bearing sporangia.
• Strobili: Cone-shaped structures formed from groups of sporophylls (e.g., in conifers).
• Roots: Organs that anchor plants and absorb water and nutrients.
• Leaves: Increase the surface area for capturing solar energy (photosynthesis). Microphylls have a single vein, while megaphylls have a highly-branched vascular system.
• Gametangia: Structures producing gametes (reproductive cells): archegonia (female) and antheridia (male).
• Xylem: Transports water and minerals.
• Phloem: Transports sugars, amino acids, and other organic products. Tracheids are dead cells within xylem.
• Chlorophylls a and b: Essential pigments for photosynthesis found in some plants.

Plant Life Cycles

• Bryophytes (non-vascular plants) have life cycles dominated by the gametophyte generation.
• Seedless vascular plants exhibit dominant sporophyte generation with smaller, independent gametophytes.
• Seed plants exhibit either a dominant sporophyte or a reduced gametophyte that exists within the sporophyte.

Plant Classification

• Nonvascular plants are commonly called bryophytes.
• Seedless vascular plants are paraphyletic, divided into lycophytes (club mosses, etc.) and pterophytes (ferns, etc.). All seedless vascular plants have flagellated sperm.
• Important plant groups include, lycophytes, pterophytes, gymnosperms, and angiosperms.

Plant Origins

• Fossil evidence indicates plants existed on land at least 475 million years ago (mya).
• Spores and fossilized tissues from 475 million-year-old rocks provide evidence.

Significance of Seedless Vascular Plants

• Ancestors of modern lycophytes, horsetails, and pterophytes grew to great heights in Devonian and Carboniferous periods (416–299 mya) creating the earliest forests or mats.
• Decaying plant matter contributed to coal formation.