1

Syllabus Theme ASyllabus Theme ASyllabus Theme ASyllabus Theme APlant Structure and FunctionPlant Structure and FunctionPlant Structure and FunctionPlant Structure and Function

A2: Structure and function of the basic plant organs

Campbell & Reece Chap. 35 Selected page numbers

ROOTS

� FunctionsAnchors the vascular plant� Absorbs minerals and water

� The absorption of water and minerals occurs near the root tips

� Vast numbers of tiny root hairs – function?

� Often stores organic nutrients

Root systems

� Taproot system� One main root formed from the radicle� Multiple lateral roots extend from the main

root

� Fibrous root system� Multiple adventitious roots of the same size

(developing from the end of the stem)� Multiple lateral roots branching off from the

adventitious roots

Taproot system

Fibrous root system

� Many plants have modified roots� What is a modified root?

Prop roots Storage roots “Strangling” aerialroots

Buttress roots Pneumatophores

Primary Growth of Roots

� The root tip is covered by a root cap� Function?

DermalGroundVascular

Key

Cortex Vascular cylinderEpidermis

Root hairZone ofmaturation

Zone ofelongation

Zone of celldivision

Apicalmeristem

Root cap

100 µm

2

� The primary growth of roots� Produces the epidermis, ground tissue, and

vascular tissue

CortexPith (monocots)Xylem

Phloem

Tissue Organization of RootsCortexVascularcylinder

Endodermis

Pericycle

Pith

Xylem

50 µm

Endodermis

Pericycle

Xylem

Phloem

Key

100 µm

VascularGroundDermal

Phloem100 µm

Epidermis

EUDICOT MONOCOT

� Lateral roots� Arise from within the pericycle, the

outermost cell layer in the vascular cylinder

Cortex

Vascularcylinder

Epidermis

Lateral root

100 µm

1 2

3 4

Emerginglateralroot

STEMS

� A stem is an organ consisting of � An alternating system of nodes, the points at which

leaves are attached� Internodes, the stem segments between nodes� An axillary bud

� Is a structure that has the potential to form a lateral shoot, or branch

� A terminal bud� Is located near the shoot tip and causes elongation of a young

shoot

� Lenticels � are sites of loosely-arranged cells allowing oxygen to diffuse into

interior of woody stem

This year’s growth(one year old)

Last year’s growth(two years old)

Growth of twoyears ago (threeyears old)

Scars left by terminalbud scales of previouswinters

Leaf scar

Leaf scar

Stem

Leaf scar

Bud scale

B

D

C

A� Many plants have modified stems

RHIZOMES

TUBERSBULBS

STOLONS

Storage leaves

Stem

Root Node

Rhizome

Root

3

Primary Growth of Shoots

� A shoot apical meristem� Is a dome-shaped mass of dividing cells at the

tip of the terminal bud� Gives rise to a repetition of internodes and leaf-

bearing nodes Apical meristem Leaf primordia

Developingvascularstrand

Axillary budmeristems

0.25 mm

Tissue Organization of Stems

� In gymnosperms and most eudicots� The vascular tissue consists of vascular

bundles arranged in a ring and a distinct cortex and pith XylemPhloem

Sclerenchyma(fiber cells)

Ground tissueconnecting pith to cortex

Pith

EpidermisVascularbundle

CortexKeyDermal GroundVascular1 mm

Groundtissue

Epidermis

Vascularbundles

1 mm

� In most monocot stems� The vascular bundles are scattered

throughout the ground tissue

Tissue Organization of Stems

� Occurs in stems and roots of woody plants but rarely in leaves

� Consists of the tissues produced by the vascular cambium and cork cambium� Name them!

Secondary Growth

4

Vascular Cambium and Secondary Vascular Tissue

� Vascular cambium� Cylinder of meristematic cells one cell thick� Develops from parenchyma cells� Produces

� Secondary xylem (wood) – to the inside� Secondary phloem (inner bark) – to the outside

Cork Cambium and the Production of Periderm

� The cork cambium� Gives rise to the secondary plant body’s protective

covering, the periderm

� Periderm� Consists of the cork cambium plus the layers of cork cells

it produces

� Bark� Consists of all the tissues external to the vascular

cambium, including secondary phloem and periderm

Secondary phloemVascular cambiumLate wood

Early woodSecondaryxylem

Corkcambium

CorkPeriderm

Xylem ray

Bark

0.5 mm0.5 mm

Vascular cambium

Vascular cambium

4 First cork cambium

Secondary xylem (twoyears ofproduction)

PithPrimary xylem

Vascular cambium

Primary phloem

2

1

6

Growth

Primary xylemSecondary xylem

Secondary phloem

Primary phloem Cork

Phloem ray3Xylem ray

Growth

Bark

8 Layers of periderm

7 Cork5 Most recentcork cambium

CortexEpidermis

9

Secondary phloem

� Vascular cambium� appears as a ring,

with regions of dividing cells

� fusiform initials –produce xylem and phloem tissue cells

� ray initials – produce vascular rays (move water and nutrients laterally)� Also store starch

Vascularcambium

C X CP

C

XC

XC

PP

PCX

X PC

XX

C C

� As a tree or woody shrub ages� The older layers of secondary xylem, the

heartwood, no longer transport water and minerals

� Is darker – has resin that protects against fungi/insects

� Closer to the centre of the stem

� The outer layers, known as sapwood� Still transport materials through the xylem� Is lighter� Closer to the vascular cambium

5

Growth ring

Vascularray

Heartwood

Sapwood

Vascular cambium

Secondary phloem

Layers of periderm

Secondaryxylem

Bark

LEAVES

� The main photosynthetic organ of most vascular plants

� Leaves generally consist of

� In classifying angiosperms� Taxonomists may use leaf morphology as

a criterion

Petiole

Simple leaf

Compound leaf

Double (bipinnately) compound leaf

Axillary bud

Leaflet

PetioleAxillary bud

Axillary bud

LeafletPetiole

� Monocots and dicots� Differ in the arrangement of veins, the vascular

tissue of leaves

� Most monocots� Have parallel veins

� Most dicots� Have branching veins (net veined)

� Pinnately� palmately

Leaf venation

Leaf arrangement

� Leaf arrangement on a stem may be� Alternate (one leaf at each node)� Opposite (two leaves at each node)� Whorled (three or more leaves at each node)

� Many plants have modified leaves

Tendrils

Spines

Storage leaves

Bracts

Reproductive leaves

6

Tissue Organization of Leaves

� The epidermal barrier� Has stomata – gaseous exchange and

transpiration� Coated by a waxy cuticle – why?

� The ground tissue - mesophyll� Is sandwiched between the adaxial and abaxial

epidermis

� The vascular tissue� Is continuous with the vascular tissue of the

stem

Keyto labels

DermalGround

Vascular

Cuticle

Sclerenchymafibers

Stoma

Adaxialepidermis

Palisademesophyll

Spongymesophyll

Abaxialepidermis

Cuticle

Vein

Guard cells

Xylem

Phloem

Guard cells

Bundle-sheathcell

Cutaway drawing of leaf tissues(a)

MONOCOTS

� Cannot distinguish between palisade parenchyma and spongy mesophyll

� Presence of bulliform cells (mainly grasses)

� Broad, flattened leaf blade� Stomata� Transparent epidermis� Air spaces in mesophyll tissue

Photosynthesis and leaf structure