1

The Plant Body

Today’s topics:

• Genetics of PlantDevelopment

– Flower Development

• Organization of stems androots– Xylem and Phloem

• Secondary growth

1 April 2009

Cell-Cell Signals

• e.g. Root Hair

Development

Genetic control of Flower

Development

Apetala

Class AAgamous

Class C

Pistillata

Class B

“ABC Model”

These genes allcode fortranscriptionfactors

Normal

Flower

Fig. 35-34Sepals

Petals

Stamens

Carpels (a) A schematic diagram of the ABC hypothesisA

A + B

gene

activity

BC

A gene

activity

B + C

gene

activity

C gene

activityCarpel

Petal

Stamen

Sepal

Activegenes:

Whorls:

Stamen

Carpel

Petal

Sepal

Wild type Mutant lacking A Mutant lacking B Mutant lacking C

A A A A

B B B B

C C C C

B B B B

C C C C C C C C A A A AC C C C A A A AB B B B

A A A A

(b) Side view of flowers with organ identity mutations

Which class of mutant is this?

Normal

2

The evolutionary determination of sepal

and petal.

A CB2

B1

B3

MADS box genes and Plant

Evolution

http://www.biologie.uni-hamburg.de/b-online/e28_2/phylogeny.htm

The Plant Body

• All ofthese are

made ofthe same

cells andtissues

All Cells of the Plant Body

Originate in a Meristem

(a) Planes of cell division

Developingguard cells

Guard cell“mother cell”

Unspecializedepidermal cell

(b) Asymmetrical cell division

Pattern of cell division affects form

Cellulosemicrofibrils

Nucleus Vacuoles 5 !m

Microfibrils control cell wall expansion

3

Major Tissue

Systems

• Dermal

• Ground

• Vascular

Dermaltissue

Groundtissue Vascular

tissue

Perforationplate

Vesselelement

Vessel elements, withperforated end walls Tracheids

Pits

Tracheids and vessels(colorized SEM)

Vessel Tracheids 100 !mVessels and Tracheids

Ground

Dermal

Keyto labels

Vascular

Root hair

Epidermis

Cortex Vascular cylinder

Zone ofdifferentiation

Zone ofelongation

Zone of celldivision

Apicalmeristem

Root cap

100 !m

Roots

Root with xylem and phloem in the center(typical of eudicots)

(a)

100 !m

Epidermis

Cortex

Endodermis

Vascularcylinder

Pericycle

Xylem

Phloem

Dermal

Ground

Vascular

Key

to labels

Root Cross Section

Shoot apical meristem Leaf primordia

Youngleaf

Developingvascularstrand

Axillary budmeristems

0.25 mm

Shoot GrowthSclerenchyma(fiber cells)

Phloem Xylem

Ground tissueconnecting

pith to cortex

Pith

CortexEpidermis

Vascularbundle

1 mm

Cross section of stem with vascular bundles forminga ring (typical of eudicots)

(a)

Dermal

Ground

Vascular

Keyto labels

Stem x.s.

4

Secondary Growth

Fig. 35-19a3

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Primary and secondary growthin a two-year-old stem

(a)

Periderm (mainly

cork cambia

and cork)

Secondary phloem

Secondaryxylem

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Vascular ray

Secondary xylem

Secondary phloem

First cork cambium

Cork

Growth

Cork

Bark

Most recent corkcambium

Layers ofperiderm

Vascular cambium Growth

Secondaryxylem

After one yearof growth

After two yearsof growth

Secondaryphloem

Vascularcambium

X X

X X

X

X

P P

P

P

C

C

C

C

C

The Vascular Cambium always produces

• Xylem toward the inside

• Phloem toward the outside

Annual Rings