Plant Structures: Anatomy and Function

Plant Structures Introduction

• Common plant structures include stems, roots, and leaves
• Activities and exercises can increase knowledge of how plants transport water, minerals and food
• The different types of tissues comprising the plant body are covered

Intended Learning Outcomes

• Demonstrate understanding of plant structure and function
• Describe the shoot and root organ systems
• Distinguish between meristematic vs permanent tissue and simple vs complex plant tissue
• Summarize roles of dermal, vascular, and ground tissue

Lesson Objectives

• Identify different plant structures and their functions
• Discuss shoot and root organ systems
• Compare meristematic and permanent tissues, simple and complex plant tissues
• Explain the roles of dermal, vascular, and ground tissue

Online Activities

• Face-to-face discussion on plant tissues and organ systems

Learning Guide Questions

• What are the different types of plant organ systems?
• How are water and minerals transported in plants?

Plant Organ Systems

• Vascular plants have two distinct organ systems for conducting food and water.
• Shoot system: grows above ground, absorbs light for photosynthesis

Shoot System

• The non-reproductive parts (leaves and stems) are called the vegetative part
• The reproductive parts (flowers and leaves) also form the shoot system

Root System

• Root System is usually underground
• Supports the plant
• Absorbs water and minerals

Plant Tissues

• Two general types of plant tissues: meristematic and permanent.

Meristematic Tissue

• Meristematic tissue consists of undifferentiated cells in regions of continuous cell division and growth.
• These cells continue to divide and contribute to plant growth.

Types of Meristems

• Apical meristems: located at tips of stems and roots, for lengthwise extension
• Lateral meristems: facilitate growth in thickness/girth
• Intercalary meristems: found in monocots at the base of leaf blades/nodes, enabling leaf blade lengthening after mowing

Dermal Tissue

• Covers and protects the plant.
• Simple tissue composed of similar cell types.
• Covers the outer plant surface and controls gas exchange

Vascular Tissue

• Transports water, minerals, and sugars.
• Complex tissue composed of different cell types.
• Made of two specialized conducting tissues: xylem and phloem.

Xylem Tissue

• Transports water/nutrients from roots
• Includes vessel elements, tracheids (both conduct water), and xylem parenchyma across three cell types.

Phloem Tissue

• Transports organic compounds from photosynthesis sites
• Consists of sieve cells, companion cells, phloem parenchyma, and phloem fibers across four cell types.

Ground Tissue

• Serves as a site for photosynthesis.
• Provides a supporting matrix for vascular tissue.
• Helps store water and sugars.

Permanent Tissue

• Cells that are no longer actively dividing

Stems

• Stems can be herbaceous (soft) or woody.
• Main function: provide support for leaves, flowers, and buds.
• In some cases stems store food.
• Stems connect roots to leaves, transporting water and minerals.

Stem Structures

• Nodes: points of attachment for leaves, aerial roots, and flowers
• Internode: stem region between two nodes
• Petiole: stalk from stem to the leaf base
• Axillary bud: in the axil (leaf/stem base area), can become a branch or flower
• Apical bud: apex (tip) of the shoot, containing the apical meristem

Stem Anatomy

• Stems are composed of simple tissue types

Parenchyma cells

• Parenchyma cells are responsible for metabolic functions like photosynthesis.
• Help repair and heal wounds
• Store starch.

Collenchyma cells

• Collenchyma cells are elongated with unevenly thickened walls.
• Provide structural support
• Alive at maturity
• Commonly found below the epidermis.

Sclerenchyma cells

• Sclerenchyma cells provide support and are often dead at maturity.

Types of Sclerenchyma cells

• Sclereids: small, give pears a gritty texture
• Sclerenchyma fibers: used to make linen and rope

Dermal Tissue of the Stem

• Primarily consists of epidermis
• A single layer of cells covers/protects underlying tissue

Leaf Epidermis

• Contains stomata, which are openings for gas exchange
• Each stoma is surrounded by guard cells that regulate opening and closing.
• Trichomes are hair-like structures which reduce transpiration, increase solar reflectance and store defensive compounds.
• Woody plants' outer layer consists of a tough, waterproof cork cell layer called bark.

Vascular Tissue Arrangement

• In dicot stems, vascular bundles are arranged in a ring.
• In monocot stems, vascular bundles are randomly scattered

Xylem Tissue

• Xylem tissue in stems is composed of xylem parenchyma, tracheids, and vessel elements.
• Tracheids – xylem cells with thick secondary cell walls that are lignified
• Vessel elements – xylem cells with thinner walls; they are shorter than tracheids.

Phloem Tissue

• Consists of phloem parenchyma, phloem fibers, sieve-tube cells, and companion cells
• Sieve-tube cells: transport substances and are arranged in long tubes.
• Companion cells: provide metabolic support and have more ribosomes/mitochondria than sieve-tube cells.

Ground Tissue

• Ground tissue in stems consists of parenchyma cells, collenchyma and sclerenchyma cells
• Pith: Ground tissue towards is on the interior of the vascular tissue in a stem or root
• Cortex: layer of tissue between the vascular tissue and the epidermis

Stem Growth

• Primary growth: increases shoot/root length via cell division in the shoot apical meristem
• Secondary growth: increases thickness/girth, caused by cell division in the lateral meristem

Herbaceous vs Woody Plants

• Herbaceous plants have primary growth but lack notable secondary growth (thickness).
• Secondary growth ("wood") occurs in woody plants (some dicots, rarely monocots).

Stem Modifications

• Rhizomes: Modified stems grow horizontally underground, possessing nodes and internodes; vertical shoots may sprout from buds.
• Corms: Resemble rhizomes but are rounded and fleshy; they store food for winter survival.
• Stolons: Stems run parallel or just under the surface, giving rise to new plants at the nodes.
• Runners: Stolons that run above ground, producing clone plants at the nodes
• Tubers: Modified stems store starch (e.g., potatoes); swollen stolon ends with adventitious buds.
• Bulbs: Underground storage units; stems modified into fleshy leaves around the stem base.
• Tendrils: Slender, twining strands for climbing support.
• Thorns: Modified branches as sharp protective outgrowths (e.g., roses, orange).

Typical Leaf Structure

• Leaves are the primary sites for photosynthesis.
• Lamina: leaf blade, the widest part
• Petiole: attaches the leaf to the plant stem
• Sessile leaves: leaves without a petiole
• Stipules: small green appendages found at the base of the petiole
• Midrib: runs the leaf's length, branching to produce veins
• Margin: the edge of the leaf

Leaf Venation

• Venation: the arrangement of veins
• Monocots have parallel venation
• Dicots have reticular venation

Phyllotaxy

• Phyllotaxy: the arrangement of leaves on a stem
• Alternate: one leaf per node, alternating on the stem
• Spiral: leaves arranged in a spiral along the stem
• Opposite: two leaves at the same point, connecting opposite each other
• Whorled: three or more leaves connected at a node

Leaf Form

• Simple leaf: blade is completely undivided
• Compound leaf: blade is completely divided into leaflets attached to the rachis

Compound Leaf Types

• Palmately compound: leaflets radiating outwards from one point
• Pinnately compound: feather-like, leaflets arranged along the midrib

Root Systems

• Tap root system: a primary root grows vertically downwards with smaller lateral roots, penetrating deeply
• Fibrous root system: located closer to the surface, forming a dense network to prevent erosion, but does not penetrate deeply

Root Growth

• Plant embryo emerges, giving rise to the root system within the radicle

Root Cap

• Root cap protects the tip and can be divided into three zones

Root Cap Zones

• Zone of cell division: nearest to the root tip, actively dividing root meristem cells
• Zone of elongation: newly formed cells increase in length
• Zone of cell maturation: root cells differentiate into special cell types

Root Epidermis

• Epidermis: the outer layer of cells surrounding ground and vascular tissue
• Provides protection
• Helps with absorption

Root Hairs

• Root hairs are extensions of epidermal cells
• Increase surface area
• Contribute to water and mineral absorption

Ground Tissue

• Within the root, ground tissue forms the cortex and pith

Root Stele

• Stele: is the root's inner portion with vascular tissue
• In dicots, xylem and phloem are arranged in an X shape
• In monocots, vascular tissue forms a ring around the pith

Root Endodermis

• Endodermis: a cell layer that separates the stele from ground tissue
• Exclusive to roots
• Serves as a checkpoint for materials
• Suberin: waxy substance on endodermal cell walls

Casparian Strip

• Casparian strip: waxy region that forces water and solutes to cross endodermal cell membranes
• Pericycle: outermost vascular tissue cell layer that can give rise to lateral roots

Root modifications:

• Bulb: roots which can store starch
• Aerial/prop: roots that provide additional support to anchor the plant
• Tap: modified for food storage
• Epiphytic: enables a plant to grow on another plant
• Plays a significant role in photosynthesis
• Pneumatophores: "air roots" that enable root systems to capture oxygen