Lecture #7

Angiosperms: Form & Function

Plant cells

• contain all the usual eukaryotic “suspects”– mitochondria, Golgi, ER, vacuoles

etc….– double phospholipid plasma

membrane with embedded proteins and carbohydrates

– BUT they also possess a cell wall– cytoplasm and the organelles are

sometimes referred to as the protoplasm

– nucleus is similar to the animal cell nucleus

Plant cells• unlike animal cells – plant

cells are capable of storing large quantities of many substances

• some substances are stored within the cytoplasm

– other materials are stored within vacuoles• primary central vacuole –

water and salts and wastes• spherosomes or lipid

bodies - lipid

Plastids• plant cells also possess specialized structures

– plastids – group of dynamic organelles that are able to perform many functions• made of an inner membrane and an outer membrane with a stroma in

between• several types within a plant cell - develop from immature proplastids• 1. chloroplasts – forms from immature proplastids once the cell is

exposed to light– contain the photosynthetic pigment chlorophyll and all the accessory proteins

and complexes required for photosynthesis– inner membrane is elaborately folded into membrane sheets called thylakoid

membranes – in certain areas the thylakoid membranes is stacked together – grana

(transport of H+ ions required in photosynthesis)

chloroplast

Plastids• 2. amyloplasts – in plants tissues that can’t

photosynthesize– roots, bark and wood cells– accumulate sugar and store it as starch

• 3. chromoplasts – e.g. in tomatoes and yellow squash– bright red, yellow and orange lipids accumulated here

• 4. leucoplasts – large and unpigmented plastids– no chlorophyll or lipid pigments– involved in the synthesis of fats and phospholipids

chloroplast

The Cell Wall

• all cells of the plant have cell walls (except the sperm)

• it is an active, dynamic organelle with many metabolic functions• contains large amounts of

cellulose– made by rosette protein complexes

• all cells have a thin primary cell wall– made of cellulose– also contains pectin – complex

polysaccharide that allows for plant growth

middle lamina – “cement”-likelayer of pectin found betweentwo plant cells

The Cell Wall

• all cells of the plant have primary cell walls

• in cells that require strength – there is a thicker secondary wall – laid down in between the primary

wall and the plasma membrane– the secondary wall is almost always

impregnated with lignin– lignin is strong and resists fungal

and bacterial attacks– both the primary and secondary

walls are permanent and are not degraded or depolymerized

middle lamina – “cement”-likelayer of pectin found betweentwo plant cells

• plant cells are in communication with one another – despite the presence of the cell wall

• connections between cells = plasmodesma

– the plasma membrane of one cell passes through the plasmodesma and is continuous with the PM of the adjacent plant cell

– often plant cells have clusters of the PDs – called pit fields– plant cells with high levels of transport will have many of these pit fields

• exists a diffusional space from one cell wall to another = apoplast• the transport of materials from one cell to another through this

cell wall space = apoplastic• the transport of materials from the inside of one cell through the

plasmodesma into the neighbouring cell = symplastic

• the presence of these PDs means that the individuality of the plant cell is eliminated

• the plant becomes one interconnected mass of cells and protoplasm = symplast

Plant Cells

• three types of plant cells:• classified based on the nature of their cell walls

• 1. parenchyma – only have thin primary walls– undifferentiated cells – differentiation leads to the other two cell types

• 2. collenchyma – primary cell walls thin in some areas, thick in others

• 3. sclerenchyma – primary and secondary walls containing lignin

1. Parenchyma

• only possess thin primary cell walls• these cells comprise parenchyma tissue –

ground tissue that fills the gaps in between other tissue types

• most common type of cell and tissue within the plant

• relatively undifferentiated• metabolically active• most usually remain alive once they mature• capable of dividing even after they mature =

known as meristematic

spongy

palisade

1. Parenchyma

• some parenchyma cell subtypes are specialized for specific tasks– a. chlorenchyma cells – parenchymal

cells containing chloroplasts – b. glandular cells – secrete – c. transfer cells – mediate short distance

transport of material • parenchyma tissues can be classified

according to their function– spongy parenchyma – round loosely

packed parenchyma cells in the leaf – surround air spaces

– palisade parenchyma – columnar shaped parenchyma cells found beneath the epidermis of the leaf

spongy

palisade

– thin primary cell wall in some areas• in other areas the cell wall thickens –

most often the corners of the cell• cells exhibit plasticity – the ability to

become deformed by pressure or tension and to retain this new shape once this force is removed• collenchyma tissue - usually only

produced in elongating shoot tips – give the tips strength as it elongates but it can be stretched– found just underneath the epidermis

2. Collenchyma

sclerenchyma

collenchyma

– has both primary and secondary cell walls • these cell walls are lignified• have the property of elasticity- the ability

to become deformed by pressure or tension and to return to normal shape once this force is removed

• most sclerenchyma cells die once they mature

• because they only need to provide strength to the plant

• some can remain alive and metabolically active

3. Sclerenchyma

• two types of sclerenchyma cells – conducting and mechanical– mechanical – comprised of long fibers

and short sclereids with thick secondary walls

– conducting – makes up vascular tissues• sclerenchyma tissue: develops

mainly in organs that have stopped growing and have achieved their final shape• the absorption of water by

collenchyma and parenchyma tissues can cause swelling – sclerenchyma tissues prevent the cell from expanding

3. Sclerenchyma

Plant Tissues

• found throughout the plant– i.e. in roots, stems and leaves– so each forms a tissue system that is continuous through

all parts of the plant

• 1. dermal • 2. vascular• 3. ground

Plant Tissues• 1. dermal tissue - plant’s outer protective

covering– forms the first line of defense– usually a single tissue – epidermis– epidermis = single layer of parenchyma

cells – cells are tightly joined together• forms specialized structures in roots, stems and

leaves– e.g. root hairs, trichromes

• in leaves and most stems – epidermis is covered with a waxy cuticle to prevent water loss & protect against pathogen damage (fungi & bacteria)

• in woody plants – the periderm replaces the epidermis in older regions of the stem and roots

Trichromes

– epidermis:• outer walls contact the environment – regulate the exchange of materials• BUT the presence of cutin can inhibit the entry of CO2 needed for

photosynthesis• epidermis contains pairs of cells (guard cells) that surround a hole in the

epidermis (stomatal pore)• guard cells + stomatal pore = stoma (plural = stomata)• guard cells control the size of the pore – control CO2 entry

– absorb water and swell – increased turgid pressure curves the cell and opens the pore

• plant vascular system is NOT a circulatory system• vascular tissue: xylem & phloem– xylem: for the conduction of water & minerals

• conducting cells: tracheids & vessel elements• water and minerals enter the xylem in the roots and are

conducted upward to the leaves and stems

– phloem: for the conduction of sugars• conducting cells: sieve cells and sieve tube members• phloem picks up sugar from where it is abundant in the plant

and transports it to where it is needed

2. Vascular tissues

XylemPARENCHYMA CELLS

WATER-CONDUCTING CELLS OF THE XYLEM

Vessel elements withperforated end walls

Vesselelement

Tracheids

Pits

Tracheids and vessels(colorized SEM)

TracheidsVessel 100 µm

– two types of conducting cells: tracheids and vessel elements• either type of cell can be called a “tracheary

element”– both are types of conducting

sclerenchyma cells– tracheids and vessel elements develop

first as immature parenchyma cells with thin primary walls

– cell elongates and deposits and secondary wall

– cell then dies and the protoplasm degenerates – leaving a hollow dead cell comprised of two cell walls

– now a type of sclerenchyma cell

aligned perforations

pit-pair

• xylem:– tracheids: long cells with tapered

ends• tracheids obtain water from those under

them and pass the water on to those above them – connection is known as a pit-pair

• pit-pair has a pit membrane – provides some friction to water flow

– vessel elements: shorter and wider cells with flat ends • posses a large hole at either end of the

cell = perforation• the vessel elements align their

perforations to form a vessel• very little friction to water movement

(vs. tracheid)• while the vessel elements are short in

length – the vessels they create can run from root tip to shoot tip

hardwoods of angiosperms are mainly vessel elementssoftwoods of gymnosperms are mostly tracheids

PARENCHYMA CELLS

• xylem:– many tracheids or vessel elements

can be identified by their secondary wall

– secondary wall is organized as rings = annular thickenings • the majority of the primary wall is

uncovered - for water entry and exit• large surface area for water uptake and

movement– the strongest type of xylem cell is

called a pitted tracheary element• the majority of the primary wall is covered

with secondary wall except for small regions called circular bordered pits

• water uptake and movement through these is slow

tracheidsvessel elements

alignedperforations

Phloem– two cell types: sieve cells

and sieve tube members– either are referred to as

sieve elements– are parenchyma cells

with only a primary cell wall

– as sieve elements develop – the plasmodesmata enlarge and become sieve pores• sieve areas – clusters of

sieve pores• sieve plates are two

stacked sieve areas

SUGAR-CONDUCTING CELLS OF THE PHLOEM

Sieve-tube members:longitudinal view

30 µm

15 µm

Companioncell

Companioncell

Sieve-tubemember

Plasmodesma

Sieveplate

Sieve plate with pores (LM)

NucleusCytoplasm

Sieve-tube members:longitudinal view(LM)

Phloem– sieve cell is long and spindle-

shaped– sieve tube members are shorter

and wider with flat ends– these cells must remain alive

for function – yet their nuclei degenerate

– sieve tube members are associated with companion cells • still retain a nucleus and provide the

sieve cells with the necessary nuclear control over their metabolism

– sieve cells are associated with albuminous cells

Vascular Bundles• xylem and phloem occur

together in roots and stems as vascular bundles

• bundles are located just interior to the cortex

• the arrangement of the vascular bundles helps define a monocot stem from a eudicot stem

• also arranged differently according to whether they are in a root or a stem– e.g. in angiosperms there is a

solid central vascular cylinder called a stele

• tissues that are neither dermal or vascular– includes cells specialized for storage, photosynthesis,

and support– made up of:• 1. parenchyma – made up of parenchyma cells

– many parenchyma cells contain chloroplasts

• 2. sclerenchyma – made up of sclerenchyma cells– provides support to the plant

• 3. collenchyma- made up of collenchyma cells

– found in the cortex and pith of stem, the cortex of the root, the mesophyll of leaves and the endosperm of seeds

3. Ground Tissue

3 Plant organs

Shootsystem

Rootsystem

Reproductive shoot (flower)

Terminal bud

NodeInternode

Blade

Vegetableshoot

Terminalbud

Petiole

Axillarybud

Stem

Leaf

Taproot

Lateral roots

• plants respond to changes in their environment by altering their growth

• plants have organs each comprised of specific tissues– organ = made up of multiple tissues

• 3 organs – form a root system and a shoot system– 1. stems – for transport & support of leaves– 2. leaves – for photosynthesis– 3. roots – for absorption

Stems

• 1. organ that raises or separates leaves, exposing them to sunlight

• 2. also raises reproductive structures – facilitating the dispersal of pollen and fruit

• stem – is the main axis• shoot – stem plus any leaves, flowers or buds off of the

stem• stems or shoots consist of:

– a. an alternating system of nodes – where leaves are attached

– b. internodes – between the nodes

This year’s growth(one year old)

Terminal bud

Leaf scarGrowth of twoyears ago (three years old)

Last year’s growth(two years old)

One-year-old sshoot formedfrom axillary budnear shoot apex

Leaf scar

Bud scale

Axillary buds

Internode

StemNode

Leaf scar

Scars left by terminalbud scales of previouswinters

Stems• the arrangement of leaves on the stem = phyllotaxy

– so that two leaves don’t lie over each other and shade one another

– 1. opposite - two leaves on opposite sides of the stem at the node

– 2. whorled – three or more leaves per node– 3. alternate – leaves alternate up the stem; one leaf per

node– 4. spiral – the nodes themselves “spiral” up the plant; the

leaves can be opposite, alternate or whorled at the nodes

Evolutionary Adaptations of Stems• Rhizomes – a horizontal shoot that grows just

below the surface– can give rise to vertical shoots

• Bulbs – are vertical underground shoots– consist mainly of enlarged, fleshy leaves that store

food• Stolons – horizontal shoots that grow along

the surface– often called “runners” – asexual reproduction – plantlets can form along

the runner where it encounters a suitable habitat• Tubers – enlarged ends of rhizomes or stolons

– specialized for storing food– “eyes” – are clusters of axillary buds that mark the

nodes

rhizome

Stems• the angle formed by a stem and leaf – called an axil– the location of the axillary bud – structure that can form a

lateral shoot (e.g. branch stem or a leaf petiole) or a flower– vegetative bud if it forms a shoot – floral bud if it forms a flower– can be covered with thick modified leaves = bud scales– new growth from these buds results in “scars”

Axillary budsApical bud

Stems• most stem growth is concentrated near the stem tip –

consists of an apical or terminal bud– if the apical bud is near an axillary bud can prevent the growth

of a new shoot– apical dominance = inhibition of axillary bud growth by the

nearby apical bud– removal of the apical bud can stimulate growth of axillary buds

and new shoots • axillary and apical buds contain an undifferentiated tissue

called meristem

Stem growth:Apical meristems

• stems grow longer by creating new cells at their tips

• growth is at regions known as shoot apical meristems (SAMs)

• growth is via mitosis• as the small daughter cells grow to

the size of the parent – they push the meristem upward – lower and older cells mature and become part of the growing stem

• region below the apical meristem = subapical meristem– site of differentiation

• apical meristem is flanked by small, developing leaf primordia which protect the AM

apical meristem

axillary bud

developingvascular tissue

developing leaf primordia

subapicalmeristem

the apical meristem and leaf primordia = bud

• primary growth in the AM leads to the formation of the subapical meristem

• Subapical Meristem is composed of three types of subapical cells– 1. protoderm – gives rise to the epidermis– 2. provascular tissue – gives rise to primary xylem

and primary phloem – 3. ground meristem – gives rise to pith and cortex

Stem growth:Primary and Secondary growth

• primary growth is followed by secondary growth – continued differentiation– 1. in some plants - epidermis becomes the cork

cambium – 2. provascular tissue gives rise to the vascular

cambium - becomes the secondary xylem and phloem (woody tissues)

– 3. pith becomes the interfasicular cambium and the cortex forms the cork cambium (which forms cork)

Stem growth:Primary and Secondary growth

• subapical meristem – region where division and

differentiation takes place– some subapical meristem cells stop

dividing and form the first tracheids and vessel elements protoxylem or primary xylem (first xylem)• the remaining meristem cells

keep dividing but eventually stop and differentiate into larger tracheids and elements – called metaxylem

protoxylem

V

Tmetaxylem

Stem growth:Subapical meristem

– on the outer edge of the developing vascular bundle the exterior cells are called protophloem or primary phloem• those cells closest to the

metaxylem form metaphloem cells• metaphloem is bigger than

primary phloem and lasts longer– known often as just phloem

• metaphloem cells are much smaller than metaxylem

protoxylem

V

Tmetaxylem

Stem growth: Subapical meristem

Stems: Internal organization• epidermis – layer of parenchyma cells covered with a cuticle

• covered with a waxy cuticle to prevent water loss & protect against pathogen damage (fungi & bacteria)

• cortex – interior to the epidermis– simple and homogenous in most stems– composed of photosynthetic parenchyma and sometimes collenchyma

• vascular bundles – xylem and phloem– unique arrangement depending on whether the plant is a eudicot or a

monocot

• pith – most interior portion of the stem– region of parenchyma– similar to the parenchyma of the cortex

Stems: Internal organization

• vascular bundles– each bundle has xylem and phloem strands running parallel to each other– vascular bundles first contain both primary xylem and primary phloem

• then develop metaxylem and metaphloem– monocots – distributed as a complex network throughout the inner part of

the stem • frequently described as “scattered” in arrangement

– eudicots and gynmnosperms – vascular bundles are arranged in the periphery• surrounding an inner ground tissue of parenchyma called the pith

Stems: Internal organization

Key

Dermal

Ground

Vascular

Epidermis Cortex

A eudicot (sunflower) stem. Vascular bundles form a ring. Ground tissue toward the inside is called pith, and ground tissue toward the outside is called cortex. (LM of transverse section)

XylemPhloem

Pith

Vascularbundles

Epidermis

Vascularbundles

1 mm

Sclerenchyma(fiber cells)

Ground tissueconnectingpith to cortex

Ground tissue

A monocot (maize) stem. Vascular bundles are scattered throughout the ground tissue. In such an arrangement, ground tissue is not partitioned into pith and cortex. (LM of transverse section)

1 mm

• vascular bundles of monocots– between the bundles is parenchyma– frequently described as “scattered” in arrangement– more complex than a random arrangement

• vascular bundles of eudicots and gymnosperms– vascular bundles are arranged in the periphery

surrounding an inner tissue of parenchyma called the pith (ground tissue)

Dicot

MedullaryRays

Monocot Stem Vascular Bundles

• cap of sclerenchyma on top of primary phloem and metaphloem– called phloem in the figure

• below this is a region of large tracheids & vessel elements = metaxylem – called xylem in the figure

• smaller tracheids and vessel elements below this is primary xylem

• most interior layer is another layer of sclerenchyma

• cap of sclerenchyma• region of phloem (mostly

metaphloem)• below this is a region of

fascicular/vascular cambium– play a role in secondary growth

of a dicot stem– produces secondary xylem and

phloem in the “woody” stem– no VC is mature monocot stems

(no secondary growth)

Eudicot Stem Vascular Bundles

• below this is a region of primary xylem & metaxylem

• below these bundles is the parenchyma of the pith

• parenchyma between vascular bundles are medullary rays

Eudicot Stem Vascular Bundles

Leaves

• the main photosynthetic organ• consist of:– 1. a flattened blade– 2. stalk called the petiole - joins

the leaf to the stem at the node• contains vascular tissue in the

form of veins– monocots – parallel veins– eudicots – branched network

Axillary bud

Petiole

Blade

Vein

Leaves

Simple leaf

Axillary bud

Petiole

Compound leaf

Axillary bud

Petiole

Leaflet

Doubly compound leaf

Axillary budPetiole

Leaflet

• blade morphology:– simple – single undivided blade– compound – blade consists of

multiple leaflets– double compound – each leaflet

divides into smaller leaflets• like compound leaves – this

morphology may resist tearing by strong winds

Evolutionary Adaptations of Leaves

Tendrils.

Spines.

Storage leaves.

Bracts.

• tendrils –modified leaves or lateral branches capable of wrapping around small objects– e.g. pea plants, ivy

• spines – non-photosynthetic– e.g. cacti– photosynthesis carried out by the stem

• needles – capable of photosynthesis– usually seen in gymnosperms

• storage leaves – adapted to storing water– e.g. succulents

• reproductive – leaves that produce adventitious plantlets– e.g. succulents

• bracts – often mistaken for petals; modified leaves that surround a group of flowers– e.g. pointsettia

Keyto labels

DermalGroundVascular

Guardcells

Epidermalcells

Stomatal pore

Surface view of a spiderwort(Tradescantia) leaf (LM)

50 µm

Upperepidermis

Stoma

Lowerepidermis

Palisademesophyll

Spongymesophyll

Air spacesVein Guard cells

Transverse section of a lilac(Syringa) leaf (LM)

100 µm

Bundle-sheathcell

XylemPhloem Guard

cells

Vein

Cuticle

CuticleSclerenchymafibers

Guardcells

Cutaway drawing of leaf tissues

Leaf Tissues-leaves are comprised of three tissue types1. epidermis2. mesophyll3. vascular tissue - veins

Leaf Tissues– 1. epidermis: comprised of a cuticle, guard cells, stomata and trichomes

• water loss through the epidermis = transpiration• similar to the stem epidermis – large, flat epidermal cells with guard cells and

trichomes interspersed• epidermal cells: contain a coating of cutin and wax• guard cells & stomata: upper and lower epidermis have different characteristics –

upper surface has fewer stomata if any at all• trichomes – provide shade to the upper surface (deflects sunlight) and slow water

loss through the stomata on the lower surface• also make it difficult to eat the leaf by insects

Leaf Tissues– 2. mesophyll: ground tissues of the leaf located just below the

epidermis• upper surface is a layer of cells called the palisade parenchyma

– main photosynthetic tissue of the plant- usually one layer thick– BUT can be several layers thick in regions of intense sunlight

• layer below is made of spongy mesophyll– loosely packed parenchyma cells with many intercellular air spaces to permit

diffusion of CO2 toward the palisade cells – for photosynthesis

vascular bundles

• 3. vascular tissues or veins– located between the palisade and mesophyll layers– in dicots – branched venation– large midrib (or midvein) and several smaller lateral veins with narrower

branches called minor veins– primary xylem on the upper side and primary phloem on the lower side– vascular tissue of larger veins is surrounded by a bundle sheath (made of

bundle sheath cells)

collenchyma

DICOT LEAF

• 3. vascular tissues or veins– monocot – parallel venation

• distinct pattern of phloem and xylem• xylem with a few very large vessel

elements (3 of them)

bundle sheath

Roots

• organ that bears no leaves or node• have multiple functions:

– 1. anchors the vascular plant in the soil – done by the lateral roots• lateral root – plant organ that functions in increasing anchorage

– 2. absorbs minerals and water – mostly done at the tip of the root by root hairs• root hair = thin tubular extension of a root epidermal cell

– 3. stores carbohydrates– 4. can undergo vegetative reproduction– 5. production of hormones – e.g. cytokinin and gibberellin

(stem growth)• numerous types of roots• but two types of root systems: fibrous and taproot

Roots

• numerous types of roots– adventitious – form from unusual

locations – e.g. leaves or stems– aerial – growth above ground

• e.g. orchids

– aerating – grow up above the ground or water

– coarse – undergo secondary thickening and can be woody

– haustorial - seen in parasitic plants; substrate is the body of another plant• roots known as haustoria

Storage roots.

“Strangling” aerial roots.Aerating

Prop roots.

Roots

• numerous types of roots– prop – exposed adventitious roots

produced near the base of the stem– stilt – adventitious roots that grow

down from lateral branches off of a stem

– storage – for storage of food and water – includes taproots

– structural – large roots with secondary thickening, gives support to large woody plants and trees

Storage roots.

“Strangling” aerial roots.Aerating

Prop roots.

Taproot systems

• found in eudicots and gymnosperms

• develops from the embryonic root (known as the radicle)

• taproot gives rise to multiple lateral roots– lateral roots can also produce

smaller, lateral roots– lateral roots can become swollen

like the main taproot• e.g. sweet potatoes and cassava

• generally penetrate deeply • are well adapted to deep soils

where groundwater is not close to the surface

carrot turnip cassava

Fibrous Root Systems

• most monocots – e.g. grasses• mat of generally thin roots that spread out

below the soil surface• most of the roots are similarly sized• the embryonic root (radicle) dies early on

and doesn’t form a taproot• many small roots emerge from primordial

tissues found in the stem• each small root forms multiple lateral roots• does not penetrate the soil deeply• excellent at holding topsoil in place

Root Structure• fairly simple – no leaves, leaf

axils, axillary buds etc…• root tip:– tip of the root is where growth in

length occurs– root apical meristem (RAM)

present in the root tip– Root AM – more orderly than the

Shoot AM• there is a quiescent region in the RAM

called the quiescent center

Root Apicalmeristem

Root cap

Root Structure

• root cap:– the RAM is protected by a root

cap as the root pushes through soil

– golgi of the root cap cells secrete a mucilage or slime to help the root push through the soil

– cells are small and meristematic

Root Apicalmeristem

Root cap

Root growth• Growth occurs just behind the

root tip, in three zones of cells:– Zone of cell division

• region of mitotic division• can also be called the Meristematic zone

– Zone of elongation – located just behind the root apical meristem• cells begin differentiation in this region• 1. outermost cells called the protoderm -

form the epidermis• 2. center cells called provascular tissue -

form the primary xylem and phloem• as these cells move up and away from the

root tip – primary X and P becomes metaxylem and metaphloem

• permeable to water

Epidermis

Root hair

Cortex Vascular cylinder

Zone ofmaturation

Zone ofelongation

Zone of celldivision

Apicalmeristem

Root cap

Root growth• Growth occurs just behind the

root tip, in three zones of cells:– Zone of maturation – also called

the root hair zone• vascular bundles contain metaxylem and

metaphloem• many of the epidermal cells extend out to

form root hairs • innermost cells of the cortex

differentiates and form a cylinder called the endodermis – waterproof region that forms around

the developing vascular bundle

Epidermis

Root hair

Cortex Vascular cylinder

Zone ofmaturation

Zone ofelongation

Zone of celldivision

Apicalmeristem

Root cap

Roots: Internal organization

• primary growth of roots produces the epidermis, ground tissue, and vascular tissue• from outermost to innermost:

– 1. epidermis – from the protoderm of the developing root– 2. cortex – ground tissue– 3. endodermis – encircles the vascular cylinder – 4. vascular cylinder – contains xylem and phloem

• also contains a tissue called pericycle – layer of parenchyma or sclerenchyma cells• just inside the endodermis • in dicots – can form lateral roots

endodermis

Key

DermalGroundVascular

Epidermis

Cortex

Vascular cylinder

Endodermis

Core ofparenchymacells

Pericycle

Xylem

Phloem100 µm

Transverse section of a monocot root with parenchyma in the center. The stele of many monocot roots is a vascular cylinder with a core of parenchyma surrounded by a ring of alternating xylem and phloem.

Monocot Roots: Vascular Bundle

-Monocot roots:-vascular bundle consists of

a core of parenchyma (similar to the pith of a stem) -core is surrounded by a ring of alternating xylem and phloem tissue

Phloem

Xylem

Pericycle

Core ofparenchymacells

Endodermis

Key

DermalGroundVascular

Epidermis

Cortex

Vascular cylinder

Transverse section of a typical root. In the roots of typical gymnosperms and eudicots, as well as some monocots, the stele is a vascular cylinder consisting of a lobed core of xylem with phloem between the lobes.

100 µm

Endodermis

Core ofparenchymacells

Pericycle

Xylem

Phloem

Endodermis

Pericycle

Xylem

Phloem

50 µm

-Eudicots and gymnosperms:- vascular bundle is a single

vascular cylinder called a stele- surrounded by a pericycle and

endodermis

Dicot Roots: Vascular Bundle

LE 35-14

Emerginglateralroot

100 µm

Cortex

Vascularcylinder

Epidermis

Lateral root

Lateral roots arise from within the pericycle, the outermost cell layer in the vascular cylinder

Plants

• two designations: herbaceous (herb – non-woody) and woody plants

• wood: considered to be made of secondary xylem

Vascular Cambium• meristematic tissue that forms during primary growth

– also called a lateral meristem– source of secondary xylem and phloem– VC located between the xylem and phloem of the vascular bundle– in non-woody plants – the cambium ultimately stops dividing and

differentiates into more xylem and phloem

Vascular Cambium

– BUT in woody plants – the VC never undergoes cell arrest and continue to divide and differentiate• ultimately forms secondary xylem and

phloem– made up of two types of cells –

fusiform initials and ray initials• fusiform – long and tapered cells that

can differentiate into secondary xylem or phloem• ray – shorter and more cuboidal cells

that forms more parenchyma cells

Secondary vascular tissues• found in woody plants• vascular cambium differentiates into secondary xylem and

secondary phloem• secondary phloem – no arrangement of rings or early and late

wood• secondary xylem contains all the type of cells that are found in

primary xylem – tracheids, vessel elements, sclereid cells and parenchyma

Cork Cambium• another lateral meristem– forms from the outermost layer of secondary phloem

(cork)

bark

Cork Cambium– cork cambium divides only for a few

weeks then differentiates into cork cells (i.e. cork)

– cork cambium + cork = periderm tissue• also known as bark

– every few years a new CC must form –so several layers of cork and bark build up over the years

– the cork and bark are impermeable to water – problem!

– specific cork cells round up as they mature and form lenticels

secondaryphloem

LE 35-10

Shoot apicalmeristems(in buds)

Vascularcambium

Corkcambium

Lateralmeristems

Primaryphloem

PeridermCorkcambium

Secondaryxylem

Primaryxylem

Pith

Pith

Cortex

Secondary growth in stems

Secondaryphloem

Vascular cambium

Primary phloem

Primary xylem

Cortex

Primary growth in stems

Epidermis

Root apicalmeristems

SO what the heck is bark???

non woody plant?small woody plant?PERIDERM

big, honkin’ tree??BARK

– non-technical term– all tissues outside of the

vascular cambium– includes the:

• secondary phloem• cork cambium• cork (phellum)

– may also be called periderm• outer covering of non-woody stems

and small woody stems

Secondary growth: Wood• most commercial dicot woods are strong and

tough (hardwoods)• wood from conifers have a softer consistency

– softwoods• annual rings – vascular cambium becomes

quiescent during times of stress (extreme hot and cold) and stops forming new cells

• when the VC resumes its growth and starts to form new secondary X and P – forms new rings of secondary xylem or wood– first wood that forms – early wood or spring

wood – later in the season – late wood or summer

wood• late wood + early wood = annual ring

• early years of a tree are characterized by vigorous growth – innermost annual rings are wider – more secondary xylem

Secondary growth: Wood

• oldest regions of the wood found at the center = heartwood (darker)– as the tree grows older the components of

the xylem in the inner rings stop conducting water

– gradually converted to inactive wood and then heartwood (dead?)

• youngest region of the tree is found at the periphery= sapwood– most actively conducting wood

Some websites to check out

• http://www.slideshare.net/apurvanagvenker/anatomy-of-dicot-monocot-leaf

• http://waynesword.palomar.edu/index.htm• https

://www.youtube.com/watch?v=33H93Rlzk2w