1 Image – the Angel Oak Lecture #4 – Plant Structure, Growth And Development.

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Image – the Angel Oak

Lecture #4 – Plant Structure, Growth And Development

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Key Concepts:

• What is a kingdom?• Why study plants?• What makes a plant a plant?• The hierarchy of structure – plant cells,

tissues and organs• Growth• Primary growth – elongation• Secondary growth – diameter expansion• Morphogenesis occurs during growth

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Image – Linnaeus

Carolus Linnaeus

(1707-1778)

The founder of modern taxonomy defined kingdoms by morphological

similarity

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Linnaeus’ Taxonomic Hierarchy

Taxonomic Category Example (taxon)

Kingdom Plantae, also Metaphyta = all plants

Division (phylum) Magnoliophyta = all angiosperms

Class Liliopsida = all monocots

Order Asparagales = related families (Orchidaceae, Iridaceae, etc)

Family Orchidaceae = related genera (Platanthera, Spiranthes, etc)

Genus Platanthera = related species (P. ciliaris, P. integra, etc)

Specific name/epithet ciliaris = one species

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Linnaeus’ Taxonomic Hierarchy

Taxonomic Category Example (taxon)








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Images – the yellow fringed orchid

Platanthera ciliaris

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Images – the 3 multicellular kingdoms, animals, fungi and plants

Linnaeus recognized only 2 kingdoms• If it moved – animal; if it didn’t – plant• Fungi were lumped with plants• The microscopic world was largely unknown

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Diagram – the 5 kingdom system

The 5 kingdom system – developed in the 1960’s and used until recently

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Diagram – 3 domain system of classification

Molecular data supports 3 domain classification scheme

Kingdoms are defined by monophyletic lineage

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Diagram – transition from 5 kingdom to 3 domain system indicating dynamic nature of classification

Classification is Dynamic!

Multicellular eukaryotes remain fairly well defined – the plants, fungi and animals. Classification of single

celled organisms is still underway.

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Current Taxonomic HierarchyTaxonomic Category Example (taxon)

Domain Eukarya = all eukaryotic organisms








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Why Plants?

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Image – shooting stars

Why Plants?

• Food• Pharmaceuticals• Building materials• Furniture• Paper• Chemicals• Horticulture/Floriculture• etc…..

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What makes a plant a plant???• multicellular, eukaryotic organisms with extensive specialization

• almost all are photosynthetic, with chloroplasts (= green)– some obtain additional nutrition through parasitism or carnivory– some are saprophytic, entirely without chlorophyll (eat dead OM)

• excess carbohydrates stored as starch (coiled, branched polymer of glucose)

• cell walls of cellulose = fibrous (not branched) polysaccharide = accounts for the relative rigidity of the cell wall

• cell division by formation of cell plate

• most extant plant species are terrestrial (many characteristics that are adapted for terrestrial life)

• separated from cyanobacteria by chloroplasts

• separated from green algae by various adaptations to terrestrial life

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Images and diagrams – characteristics that separate plants from other kingdoms

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What makes a plant a plant???• Multicellular, eukaryotic organisms with extensive

specialization • Almost all are photosynthetic, with chloroplasts (= green)

Some obtain additional nutrition through parasitism or carnivory Some are saprophytic, entirely without chlorophyll (absorb dead

OM)• Excess carbohydrates stored as starch (coiled, branched

polymer of glucose)• Cell walls of cellulose = fibrous (not branched)

polysaccharide = accounts for the relative rigidity of the cell wall

• Cell division by formation of cell plate• Most extant plant species are terrestrial (many

characteristics that are adapted for terrestrial life)• Separated from cyanobacteria by chloroplasts• Separated from green algae by various adaptations to

terrestrial life Read this later….

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Plants were the first organisms to move onto land

• Occurred about 475mya• Very different conditions from former

marine habitat• Many new traits emerged in adaptation to

life on dry land• Extensive adaptive radiation into many

new ecological niches

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Diagram – phylogeny of land plants; same on next slide

Four major groups of plants have emerged

since plants took to land

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We will focus on

angiosperms

Next semester in 211 you will learn more about the transition from water to land,

and the evolution of reproductive strategies in all

plants

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Images – flowering plants

Angiosperms – the flowering plants:90% of the Earth’s modern flora

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Diagram – plant cell; same on next slide

Basic Structure of the Plant Cell – what’s unique???

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Basic Structure of the Plant Cell

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Critical Thinking

• Do all plant cells have chloroplasts???• How can you tell???

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Critical Thinking

• Do all plant cells have chloroplasts???• NO!!!• How can you tell???

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Image – chloroplast free white bracts on white-top sedge

Critical Thinking

• Do all plant cells have chloroplasts???

• NO!!!• How can you tell???• Chlorophyll reflects

green lightGreen tissues have

chloroplastsNon-green tissues

don’t

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Diagram – primary and secondary cell walls; same on next slide

More on the cell wall:

• All cell walls are produced by the cell membrane, outside

• Primary wall is produced firstMostly cellulose

• Secondary walls are produced laterLignified, so ???

• Secondary walls are interior to primary walls

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More on the cell wall:

• All cell walls are produced by the cell membrane

• Primary wall is produced firstMostly cellulose

• Secondary walls are produced laterLignified, so rigid!

• Secondary walls are interior to primary walls

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Micrographs – plant cell types

Five Major Plant Cell

Types

• Parenchyma

• Collenchyma

• Sclerenchyma

• Xylem elements

• Phloem elements

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Micrographs – parenchyma cells

Parenchyma

• Thin primary wall• No secondary wall• Many metabolic and storage functions• Bulk of the plant body

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Micrograph – collenchyma cells; same on next slide

Collenchyma

• Thick primary wall

• No secondary wallImplications???

• Support growing tissues

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Collenchyma

• Thick primary wall

• No secondary wall Extensible – no

lignin means they can elongate

• Support growing tissues

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Micrograph – sclerenchma cells; same on next slide

Sclerenchyma

• Thick secondary wall• Secondary walls are

lignifiedImplications???

• Support mature plant parts

• Often dead at maturity

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Sclerenchyma

• Thick secondary wall• Secondary walls are

lignifiedLignified cells are rigid

and fixed in size• Support mature plant

parts• Often dead at maturity

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Micrographs – collenchyma and sclerenchyma cell comparison

Collenchyma vs. Sclerenchyma• Both provide structural support• Both have thick walls• Collenchyma = thick primary wall, no lignin• Sclerenchyma = thick secondary wall, lignified

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Diagrams and micrograph – tracheids and vessel elements

Xylem Elements

• Lignified secondary walls

• Always dead at maturity (open)

• Function to transport water and dissolved nutrients, and to support the plant

• Tracheids and vessel elements

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Micrograph – rings of lignin in developing vessel element; same on next slide

Critical Thinking

• Vessel elements and the convergent evolution of rings

• What else looks like this????

• What is the function????

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Critical Thinking

• Vessel elements and the convergent evolution of rings

• What else looks like this????

• What is the function????

• Stiff rings hold the “tube” openTrachea in both

vertebrates and inverts

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Micrograph – phloem elements

Phloem Elements

• Sieve tube members + companion cells

• STM lack nucleus, ribosomes – their metabolism is controlled by the companion cells

• Function to transport the products of metabolism

• Non-angiosperms have more primitive phloem elements

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Diagram – phloem elements

Critical Thinking

• What might be the functional advantage of a cell with no nucleus???

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Critical Thinking

• What might be the functional advantage of a cell with no nucleus???

• Sieve plates are very open• Plus, function is to move large volumes of

sap around the plantNucleus and other organelles get in the way

• But, phloem transport requires ATP and thus a living cell

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Plants are Simple

Only Five Major Cell Types

• Parenchyma

• Collenchyma

• Sclerenchyma

• Xylem elements

• Phloem elements

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Hands On• Use thin sections and stains to see

different plant cells (Page 13)• Sections must be VERY thin to allow light

to pass through• Use toluidine blue to increase contrast• With a fresh section, use phloroglucinol to

see lignified areas of the tissues• Follow instructions for staining in manual,

and take notes to answer questions on handout – label and keep your samples

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Five Major Plant Cell

Types

• Parenchyma

• Collenchyma

• Sclerenchyma

• Xylem elements

• Phloem elements

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Diagram – plant tissue types

Tissue Systems

• Epidermis• Vascular• Ground• Meristem

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Micrograph and diagram – epidermis

Epidermis Tissue:• Covers the outer surface of all

plant parts• Shoot surfaces covered with

waxy cuticleHelps to protect the plant and

prevent desiccation• Usually a single, transparent

cell layer• Tight joints; stomata allow for

gas exchange

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Critical Thinking

• Do roots have a waxy cuticle???• Why or why not???

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Critical Thinking

• Do roots have a waxy cuticle???• No• Why or why not???• Wax is waterproof

Roots absorb water from the soilA waxy coating would be a functional

DISadvantage

Never forget the importance of natural selection!!!!!

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Hands On

• Look at your leaf cross sections• Can you see the epidermis?• Can you see the waxy cuticle?

Diagram of leaf tissue arrangement

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Micrograph – vascular bundle in cross section

Vascular Tissue:

• Transports water, solutes, and metabolic products throughout the plant

• Confers structural support• Includes xylem elements,

phloem elements, parenchyma and sclerenchyma fibers

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Critical Thinking

• Why does vascular tissue give structural support to a plant???

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Critical Thinking

• Why does vascular tissue give structural support to a plant???

• LIGNIN• Xylem and sclerenchyma fibers are

lignified!

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Hands On

• Look at your cross sections – leaf and stem

• Can you see the vascular tissues?

Diagram of leaf tissue arrangement

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Micrograph and diagram – ground tissues in stems and leaves

Ground Tissue:

• Bulk of the plant body – pith, cortex and mesophyll

• Mostly parenchyma• Most metabolic,

structural and storage functions

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Hands On

• Look at the stem cross sections• Can you see the ground tissues?• The potatoes are mostly ground tissue

What characteristics do they share with other stems?

What differences?What function???

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Micrograph – herbaceous dicot stem

Critical Thinking

• Is this what the inside of a tree looks like???

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Micrograph of herbaceous eudicot stem; image of woody stem; diagram of woody stem tissue organization

Critical Thinking

• Is this what the inside of a tree looks like???

• No – wood is xylem tissueThe bulk of a tree is wood, not ground tissue

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Image – new growth at tip of stem

Meristem Tissue:

• How the plant grows• Cells divide constantly during the growing

season to make new tissues• More details later

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Diagram – plant tissue systems

Plants are Simple

Only Four Major Tissue Types

• Epidermis• Vascular• Ground• Meristem

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Tissues Make Organs:

• Roots – anchor the plant, absorb water and nutrients

• Stems – support the leaves• Leaves – main site of photosynthesis• Reproductive organs (flowers, cones, etc –

more later)

All organs have additional functions – hormone synthesis, transport, etc…

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Diagram – root and shoot systems

Plant Organ Systems

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Hands On

• Show ‘n’ Tell• What plant parts did you bring???• Discuss your plants with your team• Focus on visible tissues and organs• Be prepared to demonstrate your findings

to the whole class

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ancestral

paleoherbs magnoliids eudicots monocots

Modern molecular evidence indicates four classes of angiosperms

Not all plants have the same tissue organization in their organs

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Images – water lily and magnolia

Paleoherbs and Magnoliids comprise about 3% of angiosperms

Paleoherbs• Aristolochiaceae, Nymphaeaceae, etc

Magnoliids• Magnoliaceae, Lauraceae, nutmeg, black

pepper, etc

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ancestral

paleoherbs magnoliids eudicots monocots

Modern evidence indicates 4 classes of angiosperms

~ 97% of angiosperms

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Images – monocots

Monocots include grasses, sedges, iris, orchids, lilies, palms, etc…..

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Critical Thinking

• Grasses are arguably the most important plant family

• Why???

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Critical Thinking

• Grasses are arguably the most important plant family

• Why???• They feed the world

Direct nutrition for most of the world – grains such as rice, wheat and corn

Indirect nutrition by feeding the animals we eat

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Images – eudicots

Eudicots include 70+% of all angiosperms:

• Most broadleaf trees and shrubs• Most fruit and vegetable crops• Most herbaceous flowering plants

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Monocots vs. Eudicots

Monocots• Flower parts in multiples of 3• Parallel leaf venation• Single cotyledon• Vascular bundles in a ring in the roots• Vascular bundles in complex arrangement in the

stem

• ~90,000 species

Eudicots• Flower parts in multiples

of 4 or 5• Netted leaf venation• Two cotyledons• Vascular tissues in a

solid core in the roots• Vascular bundles in a

ring around the stem• Modern classification

indicates 2 small primitive groups + eudicots

• 200,000+ species

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Micrographs – cross sections of eudicot and moncot roots; same on next 3 slides

Root System Tissue OrganizationEudicots Monocots

Epidermis, ground, endodermis, pericycle, vascular tissues

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Eudicot root – closeup

Epidermis

Cortex

Endodermis

Pericycle

Vascular tissues – in solid core

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Monocot root – closeup

Epidermis

Cortex

Endodermis

Pericycle

Vascular tissues – in ring

Pith in the very center

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Critical Thinking

• Where do branch roots form???

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Micrograph – root emerging from pericycle

Critical Thinking

• Where do branch roots form???• The pericycle is the meristem tissue• Roots branch from the inside and push

their way out

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Micrograph – eudicot and monocot stem tissue organization; same on next 4 slides

Stem System Tissue Organization

Eudicots Monocots

Epidermis, ground, vascular tissues

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Eudicot stem – closeup

Epidermis

Cortex

Vascular tissues – bundles in a ring

Pith

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Monocot stem – closeup

Epidermis

Cortex

Vascular tissues – bundles are scattered

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Wood forms from a meristem that links the vascular bundles:

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Stem System Tissue Organization

Eudicots Monocots

Monocots cannot make woodMore on wood formation later

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Various images and a micrograph of a monocot stem – an example of one influence of plants on American history

Monocots, Palmetto Trees, Ft. Moultrie and the SC State Flag

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Hands On

• Examine the micrographs and discuss with your team (switch PowerPoints)

• What is the tissue organization in each slide, and how does that tell you what plant part is represented?

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Micrograph – cross-section of leaf tissue arrangement

Leaf Tissue Arrangement

Epidermis, ground, vascular tissues

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Diagram – leaf tissue arrangement

Leaf closeup

Epidermis

Cortex – palisade mesophyll

Cortex – spongy mesophyll

Vascular tissues

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Micrograph – epidermis tissue showing stomata

Stomata – pores to allow for gas exchange and transpiration

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Hands On

• Make a cross section of both monocot and eudicot leaves

• Stain with T-blue• Position both on the slide for side-by-side

comparison• Note the similarities and differences in

tissue organization

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Diagram – shoot and root systems

See, plants really are simple

• 5 cell types• 4 tissue types• 4 organ types

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Plant Growth

• Remember, most plants are anchored by roots

• They can’t move to escape or take advantage of changes in their environment

• Plants adjust to their environment• Simple structure + lots of developmental

flexibility allow plants to alter when and how they grow

Developmental flexibility comes from meristems

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Meristem Tissues

• Actively dividing cells that generate all other cells in the plant body

• Cause indeterminate growthStems and roots elongate throughout the

plant’s life (indeterminate primary growth)Trees continually expand in diameter

(indeterminate secondary growth)Branches form in roots and stems

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Not all plant parts have indeterminate growth patterns

Indeterminate:Rootsand

StemsThese parts grow

throughout the life of the plant, exploring

new environments or responding to

damage

Determinate:LeavesFlowersFruits

These parts grow to a genetically +/-

predetermined size and shape and then stop – cannot repair

damage

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Some mature cells can de-differentiate to become meristematic once more!!!

• Primarily occurs in the indeterminate partsStems and roots

• A process that very seldom occurs in other kingdoms

• Allows stems and roots to repair damage and form branches and sprouts

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Critical Thinking

• Not all stem and root cells can de-differentiate….

• What would control this???

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Critical Thinking

• Not all stem and root cells can de-differentiate…

• What would control this???• Lignin!!!

Lignin is strong and rigidOnce a cell is lignified, it cannot expand or

divide

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Growth in Plants:an irreversible increase in size due to

metabolic processes(processes that use ATP energy)

• Cell division produces new cells = function of meristem

• Cell expansion increases the size of the new cells = up to 80% of size increase

• Cell differentiation occurs during and after expansion

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Diagram – planes of cell division and the effect on morphogenesis

The plane of cell division contributes to morphogenesis

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Division in one plane results in files of cells

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Division in two planes results in sheets of cells

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Division in three planes results in 3-D masses of cells

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Critical Thinking

• What tissues are files of cells???• What tissues are sheets of cells???• What tissues are 3-D bulky???

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Critical Thinking

• What tissues are files of cells???Primary vascular tissues, sclerenchyma fibers

• What tissues are sheets of cells???Epidermis, secondary vascular tissues

• What tissues are 3-D bulky???Ground tissues – pith and cortex

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Hands On

• Use pasta wheels to build all three tissue types

• Each wheel = one cell

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Diagram – how auxin works to promote cell expansion

Auxin-mediated cell expansion

ATP is usedUse the index to find the figure on the

acid growth hypothesis

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Diagram – cellulose orientation in primary wall and the effects on morphogenesis

The direction of cell expansion depends on cellulose orientation, and contributes to morphogenesis

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Diagram – patterns of growth in roots

Expansion and differentiation occur in an overlapping

zone in all plant parts

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REVIEW: Growth in Plants:an irreversible increase in size due to





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Diagram – location of meristems on the plant body; next slide also

Location of the meristems

determines the pattern of plant

growth

Most common meristems:

apical, axillary and lateral

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Apical meristems

cause elongation of

roots and stems

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Micrograph – longitudinal section showing distribution of tissues in root

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Root Cap• Protects the meristem• Determines geotropism• Secretes mucigel

Eases movement of roots through soilSecretes chemicals that enhance nutrient

uptake• Constantly shedding cells

Mechanical abrasion as roots grow through soil

• Constantly being replenished by meristem

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Images – root cap and mucigel

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Diagram – longitudinal section of root showing zones of growth; same on next 2 slides

Primary Growth in Roots

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Micrograph – root hairs extending from epidermis; same on next few slides

Root Hairs• Form as the epidermis

fully differentiates• Extensions off epidermal

cellsNOT files of cellsPart of an epidermal cell

• Hugely increase the surface area of the epidermis

• 10 cubic cm (double handful) of soil might contain 1 m of plant rootsMostly root hairs

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Critical Thinking

• What is the selective advantage of root hairs???

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Critical Thinking

• What is the selective advantage of root hairs???

• Increased surface area allows for more absorption of water and nutrients

• Fine diameter allows roots to ramify throughout the soil environment

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Root Hairs• By contrast, 10 cc of soil

may contain up to 1000 m of fungal hyphae (1km!)These serve a similar

function for the fungusRamify throughout the

substrate for maximum absorption

Some fungi form symbiotic associations with plant roots and both organisms benefit from this huge absorptive surface area!

More in 211…..

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Diagram – location of apical meristems

Apical meristems

cause elongation of

roots and stems

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Micrograph – longitudinal section of stem showing apical and axillary meristems

Apical Meristems in Shoots

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Critical Thinking

• There is no “shoot cap” – why not???

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Critical Thinking

• There is no “shoot cap” – why not???• No selective advantage! Shoots “push”

through air – essentially no friction

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Diagram – meristem locations

Axillary meristems allow for

branching – similar in

structure and function to

apical meristems

Remember, pericycle in roots has same function

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Micrograph – longitudinal section of stem showing apical and axillary meristems; same on next two slides

Axillary Meristems in Shoots

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Primary Growth in Shoots

• Apical meristem• Leaf primordia• Axillary buds

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As with roots – cell division occurs first; zones of expansion and differentiation

overlap

Axillary buds may activate to make branches, or may remain dormant

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Diagram – how stems elongate during primary growth

Primary growth of a shoot – elongation from the tip

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Hands On

• Start some seedsDampen a paper towelAdd seedsKeep lightly covered – why???

• Keep a “journal”

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Diagram – primary vs. secondary growth

Remember:

Elongation is primary growth

Diameter expansion is

secondary growth

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Diagram – meristem locations

Lateral meristems

cause diameter

expansion

Roots also expand in diameter, but it’s more complicated – we’ll save that for

BIOL 300

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Diagram – lateral meristems

Lateral Meristems = Cambiums

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Images – cross section of wood and whole tree

Secondary growth – diameter

expansion

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Micrograph – cross section of a eudicot stem; same on next 2 slides

Eudicot Stem – recall the arrangement of vascular bundles

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Vascular cambium

forms here:

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Vascular cambium

forms here: a cylinder of

meristem tissue between

the xylem to the interior and the phloem to the exterior

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Diagram – location of the vascular cambium relative to other tree tissues

Secondary xylem and phloem form through cell division by the vascular cambium

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Diagram – transition from primary growth to secondary growth; same on next slide

During primary growth the vascular tissues form in bundles from the apical meristem

During secondary growth the vascular tissues form in cylinders from the vascular cambium

2o xylem to the inside

2o phloem to the outside

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Secondary xylem

accumulates

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Micrograph – cross section of woody plant showing secondary tissues; same on next slide

Secondary Xylem = Wood!

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Annual growth rings are accumulating rings of secondary xylem

Vascular cambium divides essentially in two planes and remains only a single cell layer thick

Divisions make 2o xylem and 2o phloem and also increase the diameter of the cambium itself

One layer of cambium, continuously increasing in

diameter

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Year 1 Year 2 Year 3 Year 4

Wood accumulates with each year’s elongationStep 1: Primary growth elongates the tip

Step 2: Vascular cambium forms connecting the bundlesStep 3: Secondary growth builds diameter

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Diagram – pattern of accumulation of secondary xylem as a tree grows; same on next slide

Critical Thinking

• Why do eudicot trees taper???

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Critical Thinking• Why do eudicot trees taper???• Elongation occurs from the tip• Every year adds height to the stem• Each new section of stem has just

one layer of secondary growthThe section below that has +1 layersThe section below that has +2 layersThe section below that has +3 layersetc, etc, etcThe bottom of the tree has as many

rings as the tree is old

144

Bark• All tissues external to the vascular

cambium• Diameter expansion splits original

epidermisBark structurally and functionally replaces

epidermis• Inner bark

Functional secondary phloem• Outer bark

Composition varies as tree matures

145

Micrograph – cross section of a tree showing bark formation

Bark Formation

146

Cork Cambium

• Meristematic tissue• Forms in a cylinder during 2o growth• Divides to produce cork cells

Cells filled with waxy, waterproof suberin• Eventually cork cambium becomes cork

itself

147

More on cork cambium

• First layer develops from cortexDe-differentiation!!!

• Second layer forms from cortex – same process

• Third layer forms from cortex…..• Cortex eventually runs out• Then what???

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150





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Diagram – lateral meristems and the secondary tissues in a tree; same on next slide

Critical Thinking

• What is the next available layer of tissue???

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Critical Thinking

• What is the next available layer of tissue???

• Secondary phloem!

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• Third layer forms from cortex…..• Cortex eventually runs out• Then what???• Cork cambium forms from 2o phloem once

all the cortex is used up

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• Third layer forms from cortex…..• Cortex eventually runs out• Then what???• Cork cambium forms from 2o phloem

2o phloem does NOT accumulate like 2o xylem

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Diagram – how undifferentiated cells develop into the tissues of the plant body

Stem Tissue Derivations and Fates:

Cells divide, expand and differentiate

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Review: Key Concepts:

• What is a kingdom? • Why study plants?• What makes a plant a plant?• The hierarchy of structure – plant cells,

tissues and organs• Growth• Primary growth – elongation• Secondary growth – diameter expansion• Morphogenesis occurs during growth

157

Hands On

• Go downstairs and find a living woody plant

• Snap off a twig – be gentle!• Locate bark – peel off and describe

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1 Image – the Angel Oak Lecture #4 – Plant Structure, Growth And Development.

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