Chapter 35:

Plant Structure, Growth &

Development

2. Vascular Plant Growth

1. Vascular Plant Structure

3. Vascular Plant Development

1. Vascular Plant Structure

Lateral

(branch)

roots

Taproot

Stem

Axillary bud

LeafBlade

Petiole

Vegetative

shoot

Apical bud

Internode

Node

Apical bud

Reproductive shoot (flower)

Shoot

system

Root

system

“Roots & Shoots”

• stems

• leaves

• flowers, fruits

• taproot (if present)

• lateral roots

SHOOT System(above ground)

ROOT System(below ground)

Overall Organization of

Vascular Plants

Plants have a hierarchical organization

consisting of organs, tissues, and cells:

ORGANS

• distinct functional structure consisting of

multiple types of tissues

TISSUES

• collection of 1 or more cell types that

performs a specific function within an organ

3 Basic Plant Organs

ROOTS• absorb water, minerals and other nutrients from

the soil

• anchor & support plant in the ground

STEMS• structural support of plant above ground

• transport of water & nutrients throughout the

plant

LEAVES• harvesting light & CO2 for photosynthesis

Plant organs evolved to obtain nutrients, water

and energy on land – below & above ground

CO2

O2

H2O

Minerals

Root Function

• anchorage in the soil

Roots supply the plant

with:

• water

• mineral nutrients

• carbohydrate storage

• roots rely on shoot

system for carbohydrates

*over-watering can suffocate a plant!

• roots also need access

to O2

Root Structures

The first root to emerge during plant development,

the primary root, will then give rise to:

• lateral roots to increase absorption and anchorage

• tiny root hairs to maximize

surface area for absorption

In may plants, the primary

root develops into a

prominent tap root which

provides:

• support for a large,

vertical (tall) shoot system

• storage for carbohydrates

Fibrous Root Structures

In some plants, usually monocots, the primary

root disappears and a fibrous root system

forms which:

• retains topsoil

• increases survival from

grazing animals since

plant can grow back from

remaining roots

Prop roots Buttress roots Pneumatophores

In other plants, adventitious roots develop from

unusual sources (stems, leaves) which may provide:

• greater structural

support

• greater O2 access in watery

environments

Evolutionary Adaptations of Roots

Stems support and position the photosynthetic

structures (leaves) and reproductive structures (e.g.,

flowers, cones) to maximize their success.

• points of leaf attachment called

nodes

Stem Structure and Function

Stem structures include:

• internodes – the stems

between each node

• apical buds at the shoot tips

where growth occurs

• axillary buds which give rise to

lateral branches, thorns or

flowers

Lateral

(branch)

roots

Taproot

Stem

Axillary bud

LeafBlade

Petiole

Vegetative

shoot

Apical bud

Internode

Node

Apical bud

Reproductive shoot (flower)

Shoot

system

Root

system

Evolutionary Adaptations of Stems

Stems can be modified to serve a variety of

functions:

• rhizomes which grow just beneath

the soil surface and give rise to

vertical shoots from axillary buds

• stolons that function as “runners”

along the soil surface giving rise to

new plantlets

• tubers that serve

as storage “sinks”

for carbohydrates

Root

Rhizome

Rhizomes

Stolons

Stolon

Tubers

Leaf Structure & Function

Leaves are the primary photosynthetic organs.

PetioleAxillary

bud

Leaflet

Compound leaf

PetioleAxillary

bud

Simple leaf

Leaf structures include:

• one or more blades

• a stalk called a petiole that

connects the leaf to a stem

• simple leaves have 1 blade

• compound leaves have

multiple blades called leaflets

• veins that have a branched

(dicots) or parallel (monocots)

arrangement

Storage leaves

Stem

Evolutionary Adaptations of Leaves

Leaves can be modified for a variety of functions:

• tendrils which cling

to larger support

structures

• spines to repel

herbivores

• bulbs that store

nutrients

• reproductive leaves

that detach and give

rise to a new plant

(asexual)

Spines

Tendrils

Reproductive leaves

3 Basic Plant Tissue TypesDermal tissue

• outer, protective covering

of the plant

Vascular tissue

• transports water, nutrients

& gives structural support

Ground tissue

• everything else!

Dermal

tissueGround

tissue Vascular

tissue

each of these tissues forms

a continuous tissue system

throughout the plant

There is also a type of

undifferentiated tissue called

meristem which we will

address later in this chapter.

More on Dermal Tissue…

In nonwoody plants and structures (e.g., leaves) the

dermal tissue is epidermis.

• epidermis is frequently covered with a waxy cuticle to

minimize water loss

• some plants also have trichomes in epidermal tissue

which provide protection from water loss, intense light

and insects

In woody plants the epidermis develops into a

protective laver called periderm (part of the bark).

Trichomes

300 μ

m

More on Vascular Tissue…

Plant vascular tissue consists of phloem & xylem.

Xylem

• transports water & minerals upward from the root

system to the organs and tissues of the shoot system

Phloem

• transports photosynthetic

products (e.g., sugars)

downward to the roots and

other parts of the plant

Phloem & xylem are organized

into vascular bundles or

cylinders called steles.

More on Ground Tissue…

Tissues that are not dermal or vascular are ground

tissue which come in 2 general types.

Pith

• ground tissue found

internal to the

vascular tissue

Cortex

• ground tissue found

between the dermal

and vascular tissue

Ground tissues include cells involved in storage,

transport, structural support and photosynthesis.

Basic Plant Cell Types

• Parenchyma

• Collenchyma

• Sclerenchyma

• Water-conducting cells of xylem

• Water-conducting cells of xylem

Plant cells fall into one of 5 general types:

Parenchyma cells in a privet(Ligustrum) leaf (LM)

25 μm

Parenchyma Cells

• have thin primary (1o) cell walls without a

secondary (2o) cell wall

• the least differentiated plant cell type

• the most

metabolically active

plant cell type

• are capable of

undergoing cell

division and further

differentiation

Collenchyma Cells

• provide flexible support in newly formed

shoot structures without restraining

growth

• flexible 1o cell

walls with

irregular

2o wall

thickening

Collenchyma cells

(in Helianthus stem) (LM) 5 μm

Sclerenchyma Cells

• provide rigid support due to thick 2o cell walls

containing lignin that are dead at maturity

5 μm

Sclereid cells in pear (LM)

Cell wall

Fiber cells (cross section from ash tree) (LM)

25 μm

• 2 types of

sclerenchyma

cells:

• sclereid cells

with very thick

2o cell walls

• long and

slender fiber

cells arranged

in threads

Water-Conducting Xylem Cells

2 types of xylem

cells, both of

which are dead at

maturity:

100 μm

Tracheids and vessels

(colorized SEM)

TracheidsVessel

Perforation

plate

Vessel

element

Vessel elements, with

perforated end wallsTracheids

Pits

TRACHEIDS

• found in all xylem

vessels

• long, thin with

tapered ends

VESSEL ELEMENTS

• wider, less tapered

• perforated ends

Sugar-Conducting Phloem Cells

2 types of phloem cells, both of which are

alive at maturity:

SIEVE CELLS

• found in seedless

vascular plants &

gymnosperms

SIEVE-TUBE ELEMENTS

• cells that form sieve

tubes in angiosperms

• have sieve plates

between elements &

supporting

companion cells

2. Vascular Plant Growth

Meristem Tissue

Unlike animals, plants are capable of indeterminate

growth – growth throughout the life of the plant.

This unlimited growth potential is due to meristem

tissue – a special, undifferentiated tissue with

unlimited replicative potential.

• in contrast, animals and some plant structures (e.g.,

flowers, thorns) exhibit determinate growth in which

they stop growing when they reach a certain size

There are 2 types of meristems:

• APICAL MERISTEM • LATERAL MERISTEM

Root apical

meristems

Axillary bud

meristem

Shoot tip

(shoot apical

meristem and

young leaves)

Apical Meristem

Apical meristem is located at

the tips of roots and shoots

and is responsible for growth

in length – what is called

primary growth.

• in non-woody (herbaceous)

plants, most if not all growth is

due to apical meristem

• in woody plants (e.g., trees),

there is also growth in width,

what is referred to as

secondary growth…

Vascular

cambium

Cork cambium

Lateral

meristems

Primary

xylem

Secondary

xylem

Pith

Periderm

Vascular

cambium

Secondary

phloem

Primary

phloem

Cortex

Cork cambium

Secondary growth in stems

Pith

Primary xylem

Primary phloem

Cortex

Epidermis

Primary growth in stems

Lateral MeristemSecondary growth in width is due to

2 types of lateral meristem:

• vascular cambium which adds new

layers of phloem & xylem

• cork cambium which replaces the

epidermis with protective periderm

Primary Growth of Roots

100 μm

Mitotic

cells

Root cap

Zone of cell

division

(including

apical

meristem)

Zone of

elongation

Zone of

differentiation

Dermal

Ground

Vascular

Vascular cylinderCortex

Epidermis

Root hair

Root tips have a protective, non-dividing root cap.

Just underneath the

root cap is the

Zone of Cell Division

which contains the

apical meristem cells.

Beyond the Zone of

Cell Division are 2

zones in successive

developmental stages:

Zone of Elongation• pushes root into soil

Zone of Differentiation• cells adopt specific fates

100 μm

(a) Root with xylem and phloem in

the center (typical of eudicots)

Xylem

Phloem

Ground

Vascular

Dermal

Pericycle

Core ofparenchymacells

Vascular cylinder

Endodermis

Cortex

Epidermis

Endodermis

Pericycle

Xylem

Phloem

70 μm

In most eudicot

roots, there is a

central vascular

cylinder (stele)

with a “X-shaped”

arrangement of

xylem as seen in

cross section with

phloem filling in

between the

“arms” of the X.

Eudicot

Roots

100 μm

Xylem

Phloem

Ground

Vascular

Dermal

Pericycle

Core of

parenchyma

cells

Vascular cylinder

Endodermis

Cortex

Epidermis

(b) Root with parenchyma in the

center (typical of monocots)

In most monocot

roots, there is a

core of parenchyma

cells surrounded by

a ring of alternating

phloem and xylem

vessels.

Monocot

Roots

Lateral Root Growth100 μm Epidermis

Lateral root

Emerginglateralroot

Cortex

Vascularcylinder Pericycle

1 2 3

Lateral root growth occurs from the meristematic

pericycle, the outermost layer of cells in the

vascular cylinder just inside the endodermis, the

innermost layer of cortex.

Primary Growth of Shoots

Leaf primordia

Young leaf

Shoot apical

meristem

Developing

vascular

strand

Axillary bud

meristems

0.25 mm

Primary growth of shoot structures occurs from:

• apical meristem

which lengthens

the stem and

gives rise to leaf

primordia

• axial meristem

which gives rise

to new branches

from the main

stem

Organization of Eudicot Stems

1 mm

Vascular

Ground

Dermal(a) Cross section of stem with

vascular bundles forming a

ring (typical of eudicots) (LM)

Cortex

Pith

Vascular

bundle

Epidermis

Xylem

Phloem

Sclerenchyma

(fiber cells) Ground tissueconnectingpith to cortex

In most eudicot

stems, the vascular

tissue consists of

bundles of phloem

and xylem arranged

in a ring around the

central pith tissue.

• the xylem is always

located inside the

phloem adjacent to

the pith

1 mm

Vascular

Ground

Dermal(b) Cross section of stem with

scattered vascular bundles

(typical of monocots) (LM)

Epidermis

Vascular

bundles

Ground

tissue

Organization of Monocot Stems

In most monocot

stems, the vascular

tissue consists of

bundles of phloem

and xylem scattered

throughout the

ground tissue.

Leaf Structure

50

μm

10

0 μ

m

Guard

cells

VeinCuticle

Dermal

Ground

Vascular

Lower

epidermis

Spongy

mesophyll

Palisade

mesophyll

Upper

epidermis

Phloem

Xylem

Bundle-

sheath

cell

(a) Cutaway drawing of leaf tissues

Stoma

Sclerenchyma

fibersCuticle

Guard

cells

Epidermal

cell

Stomatal

pore

(b) Surface view of a spiderwort

(Tradescantia) leaf (LM)

(c) Cross section of a lilac

(Syringa) leaf (LM)

Vein Air spaces Guard cells

Epidermis

• outer cell layer on both sides of leaf

• secrete waxy cuticle to waterproof the leaf

Mesophyll (ground tissue of leaf)

• loosely packed photosynthetic parenchyma cells

• palisade or spongy arrangement

Vascular Bundles

• phloem & xylem

• surrounded by bundle sheath cells

Stomata (singular = “stoma”)

• openings for gas exchange, transpiration

• regulated by guard cells

Epidermis

Pith

Primary xylem

Vascular cambium

Primary phloem

Cortex

Pith

Primary

xylem

Vascular

cambium

Primary

phloem

Cortex

Epidermis

(a) Primary and secondary growth

in a two-year-old woody stem

Periderm

(mainly

cork

cambia

and cork)

Secondary

phloem

Secondary

xylem

Vascular ray

Secondary xylem

Secondary phloem

First cork cambium

Cork

Cork

Most recent

cork cambium

Bark

Layers of

periderm

Secondary phloem

Vascular cambium Bark

Cork

cambium

Cork

PeridermLate wood

Early wood

Secondary

xylem

Growth ringVascular ray

1.4 mm

1 m

m

(b) Cross section of a three-year-

old Tilia (linden) stem (LM)

Secondary

(2o) Growth

of Stems &

Roots

All gymnosperms and most eudicots undergo

growth in diameter or width – 2o growth.

• most monocots undergo primary growth only

VASCULAR CAMBIUM

CORK CAMBIUM

• a single-celled ring of meristem between primary xylem

and phloem

• produces new (secondary) xylem toward the inside and

new (secondary) phloem toward the outside

• produces cork cells periderm in place of the original

epidermis to produce a protective outer layer

More on Vascular Cambium…

C

After one yearof growth

After two yearsof growth

Vascularcambium Growth

Secondaryxylem

Secondaryphloem

Vascularcambium

CX

CX P

CX PX

CX PX P

2o phloem and xylem cells form adjacent to

the vascular cambium cells, pushing earlier

layers further away from the vascular

cambium.

Year

1600 1700 1800 1900 2000

Rin

g-w

idth

ind

exes

0

0.5

1

1.5

2

Growth Rings Reveal Past Climates

In woody stems, spring 2o xylem (spring wood)

differs from summer 2o xylem (summer wood),

giving the appearance of annual growth rings.

• warm & wet = wider ring • cold & dry = narrower ring

More on Woody Stems…

Growth

ring

Vascular

ray

Heartwood

Sapwood

Secondary

xylem

Secondary phloem

Vascular cambium

Layers of periderm

Bark

• older xylem that no longer transports fluid = hardwood

• newer, active xylem = sapwood

• 2o phloem + periderm = bark

3. Vascular Plant Development

Arabidopsis – A model Plant

Much of what we know about plant

development comes from studying

a tiny weed – Arabidopsis thaliana.

• small size

• grows fast

• small genome size

Arabidopsis has several advantages

that make it very practical to use as

a model plant organism:

• easy to genetically modify

Genetic Modification of Arabidopsis

Plant with

new trait

Agrobacterium tumefaciens

Recombinant

Ti plasmid

DNA with

the gene

of interest

Site where

restriction

enzyme cuts

Ti

plasmid

1

2

3

T DNA

Agrobacterium tumefaciens, a plant pathogen, is the key:

• contains the

Ti plasmid with a

“T DNA” region

that is integrated

randomly into the

host plant cell

genome

• DNA of interest

can be “cloned”

into the T DNA

region and then

introduced into

the host plant

genome

Asymmetrical Cell Division

& Cell Fate in Plants

Unspecialized

epidermal cell

Developing

guard cells

Asymmetrical

cell division

Guard cell

“mother cell”

Asymmetrical

or “uneven”

cell division

has been

shown to

precede the

adoption of

distinct cell

fates in plants

as shown in

this example.

An Arabidopsis mutant

called gnom demonstrates

the importance of

asymmetric cell division in

early plant development:

normal

gnom

mutant

• the 1st division of normal plant

zygotes is asymmetric and

determines the polarity of the

plant (i.e., root vs shoot

systems)

• the 1st division of gnom mutant

zygotes is symmetrical and the

embryo develops without any

polarity – no roots or shoots

Pe

Se

Normal Arabidopsis flower

StCa

MutantArabidopsis flower

Pe

Pe

Pe

Se

Se

Genetic Control of Flowering

Environmental cues such as day

length and temperature trigger flower

development in plants such as

Arabidopsis.

Mutants such as the one shown here

have led to the ABC hypothesis of

flower development:

• the inner whorls of

the mutant flower

develop into petals

and sepals instead

of stamens and a

carpel

The ABC Hypothesis of Flowering

(b) Side view of flowers with organ identity mutations

Wild type Mutant lacking A Mutant lacking B Mutant lacking C

Sepal

Petal

CarpelStamen

Whorls:

Activegenes: A A A AC C C C

B B B BC C C CC C C C

B B B BA A A AC CC C A B B AB A A B

A A A A

(a) A schematic diagram of the

ABC hypothesis

Sepals

Petals

Stamens

CarpelsAB

C

Sepal

Stamen

Petal

CarpelC geneactivity

A geneactivity

B + Cgene

activityA + Bgene

activity