Bio 127 Lec Lesson 4.4 (Plant Anatomy)

Root, Stem, and Leaf Anatomy

Primary Root (Radicle)

The first organ to emerge from a germinating seed.
Function: Anchors the plant and absorbs water/minerals.
Fate:Taproot system in dicots and gymnosperms.
Fibrous root system in monocots (e.g., grasses).

Taproot System

A root system where the primary root persists and gives rise to lateral roots.
Characteristic of: Dicots and gymnosperms.

Fibrous Root System

A root system where the primary root degenerates, replaced by many thin, branching roots from the base of the stem.
Characteristic of: Monocots (e.g., Zea mays, grasses).

Zone of Cellular Division (Meristematic Zone)

Located just above the root cap, this region contains actively dividing cells from the Root Apical Meristem (RAM).
Function: Generates new cells for root elongation and tissue differentiation.

Zone of Cellular Elongation

Region where newly formed and undifferentiated cells enlarge and stretch, contributing to root lengthening.
Mechanism: Small vacuoles coalesce and fill with water to expand cells.
Cellular expansion is responsible for pushing the root cap into the soil.

Zone of Cellular Maturation (Differentiation)

Region where cells fully differentiate into specialized types such as:
Xylem and phloem (central layer for transport)
Epidermis (outer layer for protection)
Also where: Root hairs form to increase absorption surface area.

Root Cap

A thimble-shaped group of living parenchyma cells covering the RAM.
Functions:Protects meristem as root pushes through soil.
Secretes mucilage to reduce friction.
Perceives gravity (via statocytes and statoliths).

Root Gravitropism

The growth response of roots to gravity.
Positive gravitropism: Roots grow downward.
Crucial during early seedling development.

Statocytes

Gravity-sensing cells located in the columella of the root cap.
Function: House statoliths (starch-filled amyloplasts) that detect gravity direction.

Statoliths

Amyloplast-containing dense starch granules that settle in response to gravity, triggering auxin redistribution and root bending.

Starch-Statolith Hypothesis

Hypothesis stating that gravity perception occurs due to statolith sedimentation in statocytes, triggering a cascade that leads to auxin redistribution and directional root growth.

Amyloplast

Large non-pigmented starch-storing plastids, abundant in shoots, roots and seeds and has a density relative to cytosol.

Auxin in Root Gravitropism

A plant hormone that redistributes to the lower side of a root in response to gravity.
Effect:
High auxin on lower side → inhibits elongation.
Lower auxin on upper side → stimulates elongation.
Result: Root bends downward.

Vertical Root Orientation

In normal upright roots, auxin is symmetrically distributed via:
Basipetal flow from tip to elongation zone.
Promotes even growth → downward root growth.

Horizontal Root Orientation

When a root is laid horizontally:
Statoliths shift to one side → auxin redistributes.
More auxin on lower side inhibits growth there.
Upper side elongates faster, causing the root to bend downward and reorient vertically.

Epidermis (Root)

The root’s outermost uniseriate layer of elongated, compact cells.
Functions:Protection
Absorption of water and minerals
Feature: Thin or absent cuticle to allow easy water entry.

Root Hair Development

Root hairs emerge from trichoblasts (specialized epidermal cells).
Functions:Increase surface area
Enhance water and nutrient absorption
Development influenced by nuclear migration, cytoplasmic reorganization, and positional cues.

Exodermis

A specialized hypodermal layer in roots with suberized cell walls and Casparian bands.
Function: Acts as a barrier regulating water and solute movement before reaching inner tissues.

Cortex (Proper)

The region of the root between the epidermis and vascular tissue, made mostly of parenchyma cells.
Functions:Storage of starch
Facilitates water movement via intercellular spaces
Tissue layers: Exodermis → Cortex proper → Endodermis

Endodermis

The root’s innermost cortical layer composed of compactly arranged cells.
Function:Forms a selective barrier between the cortex and vascular system
Regulates what enters the xylem and phloem

Casparian Strip

A band of suberized material in the radial and transverse walls of endodermal cells.
Function:Blocks apoplastic flow (through cell walls)
Forces water and solutes into the symplast pathway (through cytoplasm) for selective uptake.

Water and Solute Uptake (Root Hairs)

Water enters by osmosis
Solutes enter via active transport (e.g., ions) or facilitated diffusion
Root hairs absorb water and nutrients between soil particles, initiating uptake.

Apoplast Pathway

Water and solutes move through cell walls and intercellular spaces, avoiding the cytoplasm.
Blocked by: Casparian strip at the endodermis.

Symplast Pathway

Water and solutes enter the cytoplasm and travel from cell to cell via plasmodesmata.
Function:Allows selective absorption
Becomes the only pathway across the endodermis due to Casparian strip.

Crossing the Endodermis (Checkpoint)

Due to the Casparian strip, water and solutes must enter the cytoplasm (symplast) to pass.
This checkpoint filters nutrients before entry into the vascular tissue.

Entry into the Xylem

After passing through the pericycle, water and solutes enter the xylem vessels for upward transport.
Transport driven by:Transpiration pull
Cohesion and adhesion (water molecule properties)

Pericycle

A layer of parenchyma cells located just inside the endodermis, surrounding the vascular tissue.
Functions:Initiates lateral root formation
Can become multiseriate (multiple cell layers)
May contribute to vascular cambium and cork cambium during secondary growth

Lateral Root Founder Cells

Specialized cells in the pericycle that retain meristematic activity and give rise to lateral roots.
Function: They divide and form a lateral root primordium, which eventually penetrates through outer root tissues.

Vascular Cylinder (Stele)

The central conducting core of a root, composed of:
Xylem
Phloem
Pericycle
Sometimes pith (in monocots)
Enclosed by: The endodermis

The Stellar Theory

Proposes that the stele (vascular cylinder) has evolved across plant lineages and its structural variations correspond to evolutionary adaptations.

Protostele

The most primitive type of stele with a solid core of xylem surrounded by phloem.
Found in: Dicot roots and early vascular plants.

Haplostele

A stele with a circular cross-section of xylem with phloem directly outside of it.

Siphonostele

A type of stele with a central pith surrounded by vascular tissue (xylem and phloem).
More advanced than protostele; common in ferns and seed plants.
Two types:Amphiphloic and Ectophloic

Amphiphloic Siphonostele

A type of stele with its phloem present on both sides of the xylem.

Ectophloic Siphonostele

A type of stele with its phloem present only on the outside of the xylem.

Eustele

A stele with discrete vascular bundles arranged in a ring around the pith.
Found in: Most dicot stems and some roots.

Actinostele

A subtype of protostele where the xylem core is star-shaped, often surrounded by phloem.
Typical in: Dicot roots

Plectostele

A stele with multiple lobes of xylem, giving the appearance of separate plates with phloem located between them.

Atactostele

A stele with advanced stelar type in seed plants, bundles are scattered throughout the tissue.

Solenostele

A stele with a vascular tissue that encircles the pith while maintaining a continuous vascular cylinder.

Dictyostele

A stele with a segmented vascular cylinder due to interruptions created by leaf gaps.
Found in: Ferns

Root Classification Based on Poles

A root classification system wherein the roots are classified by the number of xylem arms (poles) radiating from the center of the vascular cylinder.

Monarch Roots

Has a single xylem pole
Found in certain primitive plants

Diarch Roots

Has two xylem poles
Found in some species of monocots, such as certain grasses, as well as some dicots

Triarch Roots

Has three xylem poles
Found in dicot roots, specifically in the Fabaceae family

Tetrarch Roots

Has four xylem poles
Less common but can be found in certain monocots

Polyarch Roots

Roots with five or more xylem poles.
Characteristic of monocots, such as grasses.
Associated with larger steles and often presence of pith.

Stem (Definition & Functions)

The aerial axis of a plant that bears leaves, buds, flowers, and fruits.
Functions:Support for above-ground plant parts
Conduction of water, nutrients, and photosynthates
Storage, sometimes photosynthesis (green stems)

Epidermis (Stem)

The outermost single cell layer within a stem, usually covered with a cuticle.
Functions:Protection against water loss and pathogens
May contain trichomes (hairs) and lenticels for gas exchange

Cortex (Stem)

Region beneath the stem epidermis composed mainly of parenchyma cells.
May include:Collenchyma (support)
Chlorenchyma (photosynthesis in green stems)
Function: Storage, support, and in some stems, photosynthesis.

Vascular Bundles (Stem)

Strands of xylem and phloem tissues responsible for conduction of water, nutrients, and food.

Pith (Medulla)

The central part of the stem made of large, loosely arranged parenchyma cells.
Functions:Storage of nutrients and water
Can also provide support in young stems

Epidermis - Herbaceous Dicot & Monocot

The epidermis type with a single layer of cells often with a protective cuticle.

Epidermis - Woody Dicot

The epidermis type with a structure similar to herbaceous dicots but may develop a
thicker protective layer due to secondary growth
.

Cortex - Herbaceous Dicot

A cortex system comprised of
collenchyma for flexible support
and
parenchyma for storage
.

Cortex - Woody Dicot

A cortex system with a similar structure to herbaceous dicots, but can be
thicker and may include cork cambium
in older plans that develop a
periderm
.

Cortex - Monocot

A cortex that typically
consists of parenchyma cells
and may have
aerenchyma in aquatic species
.

Vascular Presence & Arrangement - Herbaceous Dicot

Vascular system with vascular bundles that are arranged in a
ring pattern
.

Vascular Presence & Arrangement - Woody Dicot

Vascular system with vascular bundles forming a
continuous cylinder
.

Vascular Presence & Arrangement - Monocot

Vascular system with vascular bundles that are
scattered/dispersed throughout the stem
.

Vascular Cambium - Herbaceous Dicot

Vascular cambium system that have some that can exhibit
limited secondary growth
, but generally
do not have a continuous vascular cambium
(some).

Vascular Cambium - Woody Dicot

Vascular cambium system that is present
between the layer of xylem and phloem that produces new xylem and phloem during
secondary growth
.

Vascular Cambium - Monocot

Group containing closed/absent vascular cambium with no secondary growth.

Ground Tissue (Pith) - Herbaceous Dicot

Pith system with a
central core of parenchyma;
serving as a storage and a water reserve.

Ground Tissue (Pith) - Herbaceous Dicot

A present pith system, often serving as storage.

Ground Tissue (Pith) - Monocot

A pith system that is absent
or
minimal
; ground tissue is often referred to as
ground parenchyma
.

Secondary Growth - Herbaceous Dicot

Non-woody plants
that typically grow in a season and
do not develop secondary growth (wood)
.

Secondary Growth - Woody Dicot

Plants that
develop secondary growth
, forming woody stems and branches.

Secondary Growth - Monocot

Generally exhibits
primary growth only
with
little to no secondary growth
.

Secondary Growth

An increase in girth (width) of stems and roots, especially in woody plants.
Caused by:Vascular Cambium (produces secondary xylem and phloem)
Cork Cambium (Phellogen) (produces protective outer layers)

Vascular Cambium

A lateral meristem that forms a cylinder of dividing cells between xylem and phloem.
Function:
Adds secondary xylem (wood) inward
Adds secondary phloem (inner bark) outward

Radial Divisions

An accommodation to the increase in girth caused by the growth from the vascular cambium.
Happens parallel to the stem radius.

Fusiform Initials

Elongated cells that form the vertical vascular tissues (tracheary elements, sieve tubes, fibers).

Ray Initials

Short, cuboidal cells that form horizontal rays—parenchyma cells for storage and lateral transport.

Cork Cambium (Phellogen)

A lateral meristem that replaces the epidermis during secondary growth.
Produces:Phellem (cork) to the outside
Phelloderm to the insideTogether they form the periderm (protective outer tissue).

Phellem Cells

Also called cork cells, these are part of what make up the periderm.
Dead at maturity
Provides a barrier against physical damage and water loss

Phelloderm

Produced along its inner side, these are part of what make up the periderm.
Consists of living cells
Contributes to the plant’s storage and metabolic functions

Bark (Woody Stem)

All tissues external to the vascular cambium, including:
Secondary phloem
Cork cambium (phellogen)
Cork (phellem)
Sometimes phelloderm

Cambial Activity

Happens in the vascular cambium, a layer of meristematic tissue between the xylem and phloem, is responsible for producing new cells
In temperate regions:Starts: Spring → Summer
Ceases: Autumn → Winter

Annual Rings (Growth Rings)

Visible rings in secondary xylem caused by alternating periods of rapid and slow growth:
Early wood (spring wood): Larger cells & thin walls because of more xylem cells.
Late wood (summer wood): Smaller cells & thicker walls because cambial activity slows down.
Used to estimate age of trees.

Source-Sink Concept

Describes the movement of photosynthates (mainly sucrose) from source to sink.
Source: Site of sugar production (e.g., mature leaves)
Sink: Site of sugar utilization or storage (e.g., roots, fruits, young leaves)

Source

Sites of sugar production, like in mature photosynthesizing leaves.
Produce more resources than they consume.
Often the site of Phloem loading.

Sink

Sites of sugar utilization or storage, like in growing tissues, storage organs, or developing flowers and seeds.
Import and utilizes resources for growth and storage
Often the site of Phloem unloading.

Pressure-Flow Hypothesis (Mass Flow Model)

Explains phloem transport of sugars from source to sink via bulk flow.
Steps:

1. Sugar loaded into sieve tubes at source → lowers water potential
2. Water enters from xylem → builds turgor pressure
3. Sugars move down pressure gradient to sink
4. Sugars unloaded; water exits phloem to xylem

Phloem Loading

Movement of sugars (e.g., sucrose) from source cells into sieve tube elements.
Apoplastic: Active transport via H⁺-sucrose symporters.
Symplastic: Passive movement via plasmodesmata.
Result: Lowers water potential, drawing in water and generating turgor pressure.

Phloem Transport (Pressure-Flow)

Bulk flow of sugar solution from high pressure (source) to low pressure (sink).
Driven by water uptake at source and release at sink.
Requires no energy for long-distance movement—just the pressure gradient.

Phloem Unloading

Sugars exit sieve tubes into sink tissues (e.g., roots, fruits).
Symplastic: Through plasmodesmata.
Apoplastic: Active uptake by sink cells.
Result: Increases water potential, causing water to return to xylem.

Leaf Anatomy (Overview)

Leaves are a flattened, green, dorsiventral structure that serve as the primary photosynthetic organs of most plants.
Main tissues:

1. Epidermis (protection)
2. Mesophyll (photosynthesis)
3. Vascular bundles (conduction of water, nutrients, and sugars)

Epidermis (Leaf)

The outermost single cell layer on both upper (adaxial) and lower (abaxial) leaf surfaces.
Key Features:
Covered in cuticle to prevent water loss
Contains stomata (mainly on lower side) for gas exchange
May bear trichomes for protection or secretion

Trichomes

Specialized cellular appendages, also called plant hairs, that grow from epidermal cells and vary greatly in appearance.
Function: Protection from UV light or herbivory, and in carnivorous plants, even in digestion.

Stomata

Pores in the epidermis flanked by guard cells that regulate gas exchange and transpiration.
More numerous on the abaxial (lower) surface
Open/close in response to turgor changes in guard cells

Mesophyll

Ground tissue between upper and lower epidermis, site of photosynthesis.
Divided into:
Palisade Mesophyll: Tightly packed, columnar cells rich in chloroplasts (adaxial side).
Spongy Mesophyll: Loosely packed with air spaces (abaxial side); facilitates gas diffusion.

Monocot Stomata

Stomata that are generally arranged in parallel rows along the leaf’s length.
Dumbbell-shaped, with swollen ends and a narrow central region.
Amphistomatic, meaning they are present on both upper and lower surfaces of leaves but more common in the lower.

Dicot Stomata

Randomly scattered on the leaf’s surface, reflecting the net-like venation pattern on dicot leaves.
Bean or kidney-shaped.
Hypostomatic, meaning they are more abundant on the lower leaf surface, though some can have stomata on both.

Vascular Bundles (Leaf Veins)

Strands of xylem and phloem embedded in mesophyll, often surrounded by a bundle sheath.
Orientation:
Xylem faces adaxial (upper) side
Phloem faces abaxial (lower) side
Function: Transport and support

Bundle Sheath Cells

Compact parenchyma cells surrounding vascular bundles, forming a boundary between veins and mesophyll.
Function:Regulates exchange between vascular tissue and mesophyll
In C4 photosynthetic plants, involved in Kranz anatomy

Bifacial (Dorsiventral) Leaf

A typical dicot leaf with differentiated mesophyll layers:
Palisade mesophyll on top
Spongy mesophyll beneath
Common in: Dicotyledonous plants

Unifacial (Isobilateral) Leaf

Leaf with undifferentiated mesophyll, where both surfaces look alike.
Features:Mesophyll lacks clear palisade-spongy division
Stomata on both sides
Common in: Monocotyledons (e.g., grasses)

Monocot Leaf - Overall

Mesophyll: Undifferentiated
Stomata: Both surfaces
Veins: Parallel venation
Bundle sheath: Prominent, often with extensions
Leaf symmetry: Often isobillateral

Dicot Leaf - Overall

Mesophyll: Differentiated (palisade + spongy)
Stomata: Mostly lower surface (abaxial)
Veins: Reticulate venation
Bundle sheath: May or may not be prominent
Leaf symmetry: Typically dorsiventral