1

BI 103: Leaves

An examination of leaves

Chapter 43 cont.

Learning Objectives

• What is the function of the plant leaf?

• How are specific cells and tissues

adapted in the leaf in order to help it

function?

• What do plants need to

photosynthesize?

• How are tissues in the leaf different

from the roots & stem?

• What are examples of specialized

leaves?

2

Plant Anatomy: Vegetative Organs

Leaves:

Roots:

Stem:

Form = Function

Photosynthesis

Gas exchange

Light absorption

Anchorage

Storage

Transport

Absorption

Support

Transport

Storage

Introduction

• Other functions of leaves:

– Wastes from metabolic processes accumulate in leaves and are disposed of when leaves are shed.

– Play major role in movement of water absorbed by roots

• Transpiration occurs when water evaporates from leaf surface.

• Guttation - Root pressure forces water out hydathodes at tips of leaf veins in some plants.

3

Adapted for Photosynthesis

• Leaves are usually thin

– High surface area-to-volume

ratio

– Promotes diffusion of gases

• Leaves are arranged to

capture sunlight

– Are held perpendicular to rays

of sun

– Arranged so they don’t shade

one another

Common Leaf Forms

petiole

blade

axillary

bud

node

blade

sheath

node

DICOT MONOCOT

4

leaf blade

leaf vein

stem

Leaf Vein

(one vascular

bundle) cuticle

Upper

Epidermis

Lower

Epidermis

Palisade

Mesophyll

Spongy

Mesophyll

50m

xylem

phloem

cuticle-coated cell

of lower epidermis

one stoma (opening

across epidermia)

Oxygen and

water vapor

diffuse out

of leaf at

stomata.

Carbon

dioxide in

outside air

enters leaf at

stomata.

Water, dissolved mineral ions from roots and stems move into leaf vein (blue arrow)

Photosynthetic

products (pink

arrow) enter

vein, will be

transported

throughout

plant body

Fig. 29-14, p.501

Epidermis: Cuticle

• Waxy cuticle secreted by epidermis cells

• Protective layer against disease

• Reduced water loss from cells

5

Dermal tissue

• Epidermis - Single layer of cells

covering the entire surface of the leaf

– Devoid of chloroplasts

– Coated with cuticle

– Functions to protect tissues inside

leaves

– Waste materials may accumulate in

epidermal cells.

– Different types of glands may also be

present in the epidermis.

Guard Cells

6

Vascular plants: have stomata

bordered by 2 guard cells

Open when water is abundant.

Close when water is scarce.

Bryophytes: pores

• Bryophytes like moss and

liverworts lack stomata

• Have pores that are always open

Cross section of liverwort thallus

(Conocephalum)

7

Trichomes on an Arabidopsis leaf. Trichomes are

extensions from the plant's epidermis and they occur in

numerous shapes and sizes in various plants.

Trichomes may provide defense against insects

Epidermis: Trichomes

Trichomes

Glandular trichome of Stinging Nettle Urtica

dioica

8

Prickles: epidermal

cells- larger tichomes

Examples: Rosa spp.

Epidermal tissue: Prickles

Mesophyll Most photosynthesis takes

place in the mesophyll between the

two epidermal layers.

– Compactly stacked,

barrel-shaped

parenchyma cells,

commonly in two rows

– Contains most of leaf’s

chloroplasts

II. Spongy Mesophyll

– Loosely arranged

parenchyma cells with

abundant air spaces

I. Palisade Mesophyll

9

Photosynthesis: The Components

? ?

?

? ?

What are the inputs?

What are the outputs?

10

Photosynthesis: The Components

6CO2 + 6H2O + Light energy C6H12O6 + 6O2

What are the inputs?

- light

- water

- carbon dioxide

What are the outputs?

- glucose

- oxygen

Light is a wave of energy:

colors are different

wavelengths

11

Photosynthesis: Input - Light

Intensity

The color you see

Energy (wavelength)

absorbed

Photosynthesis: Light Absorption

Plants are green because…

• Reflect green light.

• Absorb red, blue

and purple light.

12

Photosynthesis: Light Absorption

Plant pigments absorb light….

Absorb red, blue

and purple light Reflect green light

Leaf Veins: Vascular Bundles

• Xylem and phloem; often

strengthened with fibers

• In dicots, veins are netlike

• In monocots, they are parallel

13

Specialized Leaves

Carnivorous leaves

• Grow in swampy areas

and bogs • Nitrogen and other

elements are deficient in

soil.

• Specialized leaves trap

and digest insects.

– Pitcher Plants

• Insects trapped and

digested inside cone-

shaped leaves.

Pitcher plant

Specialized Leaves: Carnivorous

Sundews

• Have round to oval leaves

covered with glandular

hairs that have a sticky

fluid of digestive enzymes

at tip

Venus’s Flytraps

• Only in North Carolina and

South Carolina

• Blade halves trap insects.

Venus’s

Flytraps

Sundew

14

Specialized Leaves

• Floral Leaves (bracts)

– At bases of flowers or flower stalks

– Poinsettia - Flowers do not have petals, instead

brightly colored bracts surround flowers.

– Clary’s sage - Colorful bracts are at top of

flowering stalks above flowers.

Poinsettia Clary’s sage

Example: Flowering dogwood, Cornus nutallii

Specialized Leaves: Bracts

15

Specialized Leaves

• Leaves of Arid Regions

– Arid regions have limited availability of water,

– wide temperature ranges, and high light intensities.

– Leaves reduce loss of water by: • Thick, leathery leaves

• Fewer stomata or sunken stomata

• Succulent, water-retaining leaves, or no leaves

• Dense, hairy coverings

Specialized Leaves • Leaves of Arid Regions

– Conifers

– Sunken stomata

Tsuga canadensis

Picea

16

Specialized Leaves

• Leaves of Aquatic Areas – Less xylem and phloem

– Mesophyll not differentiated into palisade and spongy layers.

– Large air spaces

Water lily leaf Elodea leaf

Specialized Leaves

Tendrils

Spines

Tendrils

Spine

Thorn

Thorns - Modified stems

arising in the axils of

leaves of woody plants

17

Autumnal Changes in Leaf Color

• Chloroplasts of mature leaves contain

several groups of pigments:

– Chlorophylls - Green

– Carotenoids - Yellows

– In fall, chlorophylls break down and other

colors are revealed.

• Water soluble anthocyanins (red or blue)

and betacyanins (red) may also be present

in the vacuole.