Secton 2A

35.1: Plants have a hierarchical organization consisting of organs, tissues, and cells.
35.2: Different meristems generate cells for primary and secondary growth.
35.3: Primary growth lengthens roots and shoots.
35.4: Secondary growth increases the diameter of stems and roots in woody plants.

The primary plant organs are roots, stems, and leaves.
These organs are organized into two main systems:
- Root System: Functions primarily underground, anchoring the plant and extracting water and nutrients from the soil.
- Shoot System: Functions above ground, consisting of stems and leaves, responsible for photosynthesis and reproduction.
Nutritional Dependence:
- Roots rely on sugars produced by photosynthesis in shoots.
- Shoots rely on water and minerals absorbed by roots.
Vascular Tissue: Connects the root and shoot systems, forming a continuous network throughout the plant.

In exploring plant organs, examples will be drawn from two major groups of angiosperms: monocots and eudicots.
Cotyledons: Seed leaves crucial for the plant's early growth phases.

The primary root emerges first from a germinating seed.
Eudicots and Gymnosperms: Typically develop a taproot system, characterized by:
- Taproot: Main vertical root extending deep into the soil.
- Lateral Roots: Branching roots that arise from the taproot.
Monocots: Develop a fibrous root system, characterized by:
- A mass of thin, fibrous roots spreading out just below the soil surface.
Adventitious Roots: Organs that arise from unusual sources (e.g. bases of stems), aiding in stability and nutrient acquisition.

Absorption mainly takes place at the root tips, where root hairs increase surface area for more efficient uptake of water and minerals.
Types of root adaptations include:
- Prop Roots: Aerial roots that support tall trees (e.g. hala trees).
- Storage Roots: Roots that store food and water, as in beets.
- Green Roots: Some plants have photosynthetic roots (e.g. certain aerial plant species).
- Pneumatophores: Air roots found in mangroves that help roots access oxygen in tidal environments.
- Aerial Roots: Roots that descend from above-ground branches (e.g. strangler figs).

Support: Raises leaves for better sunlight exposure.
Separation: Provides spacing for leaves to optimally access light.
Reproductive Structures: Elevates reproductive organs to facilitate pollen dispersal and fruit production.

Composed of:
- Nodes: Points where leaves attach.
- Internodes: Segments between nodes.
At the shoot tip, the apical bud initiates elongation of the shoot, while axillary buds can form branches, maintaining apical dominance.

Examples of modified stems include:
- Rhizomes: Horizontal shoots that grow just below the surface, producing vertical shoots from axillary buds.
- Bulbs: Rounded underground stems for storage.
- Stolons: Horizontal shoots on the surface (e.g. strawberry plants), facilitating asexual reproduction.
- Tubers: Swollen ends of rhizomes or stolons for food storage (e.g. potatoes).

Main Photosynthetic Organ: Leaves carry out photosynthesis, usually structured with:
- Blade: The flattened portion of the leaf.
- Petiole: The stalk connecting the leaf to the stem.
- In monocots, the leaf base forms a sheath around the stem.
Vein Arrangement:
- Most monocots exhibit parallel vein arrangements.
- Most eudicots showcase branching veins.

Classification:
- Simple Leaf: A single undivided blade.
- Compound Leaf: A blade divided into multiple leaflets without axillary buds at the base of each leaflet.
Functional Modifications:
- Support: Some leaves are adapted as tendrils (e.g. pea plant).
- Protection: Certain leaves evolve into spines (e.g. cacti).
- Storage: Leaves like onion store food and water.
- Reproduction: Some leaves (e.g. Kalanchoe daigremontiana) produce adventitious plantlets that root in soil.

Each organ comprises three main tissue types:
- Dermal Tissue: Protective cover for the plant.
- Vascular Tissue: Transports materials through the plant body.
- Ground Tissue: Responsible for metabolic activities, located between dermal and vascular tissues.

In nonwoody plants, dermal tissue features the epidermis with a cuticle for water conservation.
In woody plants, the periderm replaces the epidermis in older regions.
Trichomes: Outgrowths of the epidermis aiding in defense and reducing water loss.

Comprises xylem and phloem:
- Xylem: Moves water and dissolved minerals from roots to shoots.
- Phloem: Transports synthesized sugars from leaves to roots and growing organs.
The central vascular cylinder in roots is known as the stele, while in stems and leaves, xylem and phloem are organized into vascular bundles.

Comprises all tissues not classified as dermal or vascular.
Regions within ground tissue:
- Pith: Inside the vascular tissue.
- Cortex: Outside the vascular tissue.
Includes cells specialized for various functions like storage, photosynthesis, and support.

Indeterminate Growth: Continuous growth throughout a plant's life.
Determinate Growth: Growth that stops after reaching a certain size (e.g., leaves and flowers).

Definition: Meristems are undifferentiated tissues that enable indeterminate growth in plants.
Types of Meristems:
- Apical Meristems: Located at roots and shoots' tips, facilitating primary growth (lengthening).
- Lateral Meristems: Located in sections of stems and roots, facilitating secondary growth (increasing diameter).

Primary Growth: Extends the roots through the soil and helps shoots reach for light.
Secondary Growth: Adds girth in woody plants through two lateral meristems:
- Vascular Cambium: Generates secondary xylem (wood) and phloem.
- Cork Cambium: Produces protective periderm, replacing the epidermis in older structures.

In deciduous trees, primary and secondary growth occur simultaneously but in different locations. Annual growth reveals alternating zones corresponding with primary (new growth) and secondary growth (previous year's structure).