Lesson 2: Plant Anatomy

Plants follow a structural hierarchy similar to animals:

Cells: Basic building blocks distinguished by vacuoles, cell wall thickness, and vitality at maturity.
Tissues: Groups of organized cells performing a shared function.
Organs: Made of multiple tissues (Leaves, Roots, Stems).
Organ Systems:
- Root System: Generally below ground; anchors the plant and absorbs water/minerals.
- Shoot System: Stems, leaves, and flowers.

Angiosperms are categorized into two groups based on morphological differences:

Monocots: Characteristics include scattered vascular bundles in stems and ringed vascular bundles in roots.
Eudicots: Characteristics include vascular bundles arranged in rings in stems and a star-shaped xylem core in roots.

Growth occurs at localized regions called meristems, which contain undifferentiated cells that divide by mitosis.

Located at the tips of roots and shoots.
Responsible for primary growth (extension in length).
Give rise to three primary meristems:
1. Protoderm: Forms dermal tissue.
2. Procambium: Forms primary vascular tissue.
3. Ground Meristem: Forms ground tissue.
Protective structures include leaf primordia (shoots) and root caps (roots).

Found in plants like corn; allow for the lengthening of the internode (space between leaves).

All plant cells have a primary cell wall made of cellulose. Some have a secondary cell wall containing lignin for added strength.

Epidermis: Usually one cell layer thick; protected by a waxy cuticle.
Guard Cells: Pair of cells flanking stomata (pores) to regulate gas exchange and water loss.
Trichomes: Hair-like outgrowths that protect against light or herbivores.
Root Hairs: Increase surface area for water absorption.

Parenchyma: Most common; thin walls, large vacuoles, living at maturity. Function in photosynthesis and storage.
Collenchyma: Living at maturity; irregularly thickened primary walls. Provide flexible support (e.g., celery strings).
Sclerenchyma: Dead at maturity; thick secondary walls with lignin.
- Sclereids: Gritty texture in pears; found in seed coats.
- Fibers: Long strands often associated with vascular tissue.

Xylem: Conducts water and minerals.
- Tracheids: Dead at maturity; tapered ends.
- Vessels: Larger diameter; stack end-to-end; more efficient than tracheids.
Phloem: Conducts food (sucrose).
- Sieve Tube Members: Living but lack a nucleus; stack into tubes.
- Companion Cells: Help maintain metabolic functions for sieve tube members.

Structure: Composed of a blade and a petiole.
Mesophyll: Ground tissue for photosynthesis.
- Palisade Mesophyll: Elongated cells near the upper surface.
- Spongy Mesophyll: Loosely arranged cells for gas exchange.
Veins: Contain xylem (brings water) and phloem (transports sugar).

Root Cap: Protects the tip; secretes slime for lubrication.
Zones of Growth:
1. Zone of Cell Division: Apical meristem location.
2. Zone of Elongation: Cells increase in length; vacuoles fill $90\%$ of cell volume.
3. Zone of Maturation: Cells specialize into distinct types.
Internal Anatomy:
- Endodermis: Innermost layer of cortex; contains the Casparian strip (made of suberin) to force water through cell membranes.
- Stele: Tissues inside the endodermis, including the pericycle (gives rise to branch roots).

Monocot vs. Eudicot:
- Eudicots: Vascular bundles in a ring; contains pith (center) and cortex (outer).
- Monocots: Vascular bundles scattered; no vascular cambium (no secondary growth).
Secondary Growth in Woody Stems:
- Annual Rings: Formed by seasonal variations in secondary xylem cell size.
- Periderm: Composed of cork, cork cambium, and phelloderm; replaces epidermis.
- Lenticels: Unsuberized patches in bark allowing for gas exchange.

Annuals: Live, reproduce, and die in one year.
Biennials: Two-year cycle; vegetative growth in year one, reproduction in year two.
Perennials: Live and grow year after year; can be herbaceous or woody.