Rucete ✏ Campbell Biology In a Nutshell
Unit 6 PLANT FORM AND FUNCTION — Concept 35.1 Plants Have a Hierarchical Organization Consisting of Organs, Tissues, and Cells
Vascular plants display a highly organized structure made up of organs, tissues, and cells, each serving specialized functions. This hierarchical organization supports the plant's ability to absorb nutrients, grow, and adapt to its environment.
Plant Organs: Roots, Stems, and Leaves
Plants have two main systems:
- Root system: Below ground; absorbs water and minerals, stores nutrients, and anchors the plant.
- Shoot system: Above ground; includes stems and leaves involved in photosynthesis, support, and reproduction.
Roots
- Taproot system: Main vertical root with lateral branches; typical of tall plants for strong anchorage and vertical growth.
- Fibrous root system: Mat of thin roots; common in monocots; helps prevent soil erosion.
- Root hairs: Increase surface area for absorption.
- Specialized roots:
- Prop roots (support)
- Storage roots (e.g., beets)
- Pneumatophores (air roots in swamps)
- Strangling aerial roots, buttress roots (tropical adaptations)
Stems
- Main function: elevate leaves and reproductive structures.
- Made of:
- Nodes: points where leaves attach
- Internodes: segments between nodes
- Apical buds: primary growth sites
- Axillary buds: can grow into branches or flowers
- Modified stems:
- Rhizomes (underground horizontal shoots)
- Stolons (surface runners for asexual reproduction)
- Tubers (e.g., potatoes; storage organs)
Leaves
- Main site of photosynthesis and gas exchange
- Composed of:
- Blade (flattened part)
- Petiole (joins leaf to stem)
- Monocots: parallel veins; Eudicots: branched veins
- Leaf types:
- Simple (single blade)
- Compound (blade divided into leaflets)
- Modified leaves:
- Tendrils (climbing)
- Spines (defense)
- Storage leaves (e.g., onion)
- Reproductive leaves (e.g., Kalanchoë plantlets)
Tissue Systems
All plant organs contain three basic tissue types:
1. Dermal Tissue
- Outer protective layer
- Epidermis (nonwoody plants): reduces water loss (cuticle)
- Periderm (woody plants): replaces epidermis
- Specialized cells:
- Root hairs (absorption)
- Guard cells (gas exchange)
- Trichomes (defense, water retention)
2. Vascular Tissue
- Transports materials and supports plant body
- Xylem: conducts water and minerals from roots
- Phloem: transports sugars throughout the plant
- Stele:
- Root stele: vascular cylinder
- Stem/leaf stele: vascular bundles
3. Ground Tissue
- Functions: storage, photosynthesis, support, transport
- Pith: internal to vascular tissue
- Cortex: external to vascular tissue
Cell Specialization
Plant development involves cell differentiation, producing specialized cells with structural and functional roles. These changes occur in both cytoplasm and cell walls as tissues form within each plant organ.
In a Nutshell
Vascular plants have a hierarchical structure built from three main organs—roots, stems, and leaves—and three tissue systems—dermal, vascular, and ground tissue. These structures evolved to meet the demands of life in both air and soil. Root systems anchor and absorb, shoots maximize photosynthesis and reproduction, and tissues coordinate protection, transport, and storage.