CHAPTER 33

PLANT FORM AND FUNCTION

ORGANIZATION OF A VASCULAR PLANT

• A vascular plant is organized along a vertical axis.• The root penetrates the soil

and absorbs water and ions and it anchors the plant.

• The shoot consists of the stem and leaves.

• The stem serves as a framework for positioning the leaves.

• The leaves are where most photosynthesis takes place.

Apical meristem

Primary growth zone

Lateral meristems

Internode

Vascular system

Pith

Lateral root

Root

Primary root

Primary growth zoneApical meristem

Node

Axilary bud

petiole

BladeLeaf

Shoot

Terminal bud

Vein

ORGANIZATION OF A VASCULAR PLANT

• Plants contain growth zones of unspecialized cells called meristems.• Meristems are not only areas of actively

dividing cells that result in plant growth, but also continuously replenish themselves.

• In this way, meristem cells function much like stem cells in animals.

ORGANIZATION OF A VASCULAR PLANT

• Primary growth is initiated at the tips (of roots and shoots) by the apical meristems.• The growth of these meristems results primarily

in the extension of the plant body.

• Secondary growth involves the activity of the lateral meristems.• The continued divisions of their cells results

primarily in the thickening of the plant body.

ORGANIZATION OF A VASCULAR PLANT

• There are two kinds of lateral meristems:• Vascular cambium - gives rise to thick

accumulations of secondary xylem and phloem.• Cork cambium - forms the outer layers of bark

on both roots and shoots.

PLANT TISSUE TYPES

• Most plants have three tissue types: • Ground tissue - in which the vascular tissue is

embedded.

PLANT TISSUE TYPES

• Dermal tissue - the outer protective covering of the plant. Flattened epidermal cells are the most abundant cells in the plant’s outer layer, or epidermis.• The epidermis is often covered by a waxy layer

called the cuticle.• The epidermis and cuticle protect the plant and

provide an effective barrier against water loss.

Trichomes 186 µm

Trichome Epidermal cells Stomatal opening Guard cells

Stomata 137 µm

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

(top): © Andrew Syred/Science Photo Library/Photo Reseachers;(middle): © Dr. Jeremy Burgess/Science Photo Library/Photo Researchers; (bottom): © Dennis Drenner/Visuals Unlimited, Inc.

Roothairs

Root hairs

(a)

(b)(c)

PLANT TISSUE TYPES

• Vascular tissue - conducts water and dissolved minerals up the plant and conducts the products of photosynthesis throughout.

• There are two types of vascular tissue:• Xylem is the plant’s principal

water-conducting tissue.• It forms a continuous system

that runs throughout the plant body.

• Water (and dissolved minerals) pass from the roots to the shoots.

• When water reaches the leaves, most exits through the stomata.

• Phloem is the principal food-conducting tissue.

Xylem

(a) (b) (c)

TracheidVesselelement

Vesselelement

Vessel

TracheidsPoresPits

Courtesy of Wilfred Cote, SUNY College of Environmental Forestry

ROOTS

• Roots have a central column of xylem with radiating arms.• Alternating

within the radiating arms of xylem are strands of primary phloem.

H2OH2O

PericyclePrimaryxylem

PrimaryphloemEndodermis

Casparianstrip

Sectionedendodermalcells

Epidermis

Cortex

ROOTS

• The root elongates rapidly just behind its tip in the area known as the zone of elongation.

• Abundant root hairs, extensions of single epidermal cells, form above the elongation zone.• This area is called the zone of

differentiation.Epidermis

GroundmeristemProcambium

Protoderm

Apicalmeristem

Root cap

(b) Monocot(a) DicotRoot cap

Apical meristem

Zone ofelongation

Epidermis

Cortex

Protoderm

Groundmeristem

Procambium

Phloem

Xylem

Roothair

PericycleEndodermisPrimary phloemPrimary xylem

Zone ofdifferentiation

ROOTS

• Roots branching is initiated as a result of cell divisions in the pericycle.

• The developing lateral roots grow out of the cortex toward the surface of the root.

Endodermis

Developinglateral root

Pericycle

Cortex

Epidermis

STEMS

• Stems often experience both primary and secondary growth.• Stems are the source of an economically

important product—wood.

• In the primary growth of a shoot, leaves first appear as leaf primordia.• These are rudimentary leaves that cluster

around the apical meristem.• They unfold and grow as the stem elongates.

STEMS

• The places on the stem where leaves form are called nodes.• The portions of the stem

between these leaf attachment points are called internodes.

• As the leaves expand to maturity, a bud develops in the angle between the leaf and the stem from which it arises.• This area is called the

axil.

Terminalbud

Axillarybudarising fromtheaxil

Node

Internode

Terminalbudscalescars

Blade

Petiole

STEMS

• Within soft, young stems, the vascular tissue strands are arranged differently in dicots versus monocots.• In dicots, vascular bundles

(containing primary xylem and primary phloem) are arranged around the outside of the stem.

• In monocots, vascular bundles are scattered throughout the stem.

Epidermis(outerlayer)

Collenchyma(layers belowepidermis)

Pith

Vascularbundle

Xylem

Phloem

Cortex

Xylem

Phloem

Ground tissue

Vascular bundles

(a)

(b)

STEMS

• In stems, secondary growth is initiated by the differentiation of the vascular cambium.• This is a thin layer of actively dividing cells

located between the bark and the main stem in woody plants, running between the xylem and the phloem.• Cells that divide from the vascular cambium

outwardly become secondary phloem.• Cells that divide from the vascular cambium

inwardly become secondary xylem.

Corkcambium

Cork

Annualgrowthlayers

Periderm

Secondaryphloem

Primaryphloem

Secondaryxylem

Primaryxylem

Secondaryphloem

Vascularcambium

VASCULAR CAMBIUM AND SECONDARY GROWTH

STEMS

• While the vascular cambium is being established, a second kind of lateral cambium develops in the stem’s outer layer.• The cork cambium consists of plates of

dividing cells that move deeper and deeper into the stem as they divide.• Outwardly, this cambium divides to form

densely packed cork cells.• Inwardly, this cambium divides to produce a

layer of parenchyma (ground) cells.

STEMS

• The term bark refers to all of the tissues of a mature stem or root outside of the vascular cambium.

• Wood is accumulated secondary xylem.

STEMS

• Because of the way it is accumulated, wood often displays rings.• The vascular cambium

divides more actively in the spring and the summer than in the fall and winter.

• The growth rate differences are reflected in alternating rings of growth of different thickness.

LEAVES

• Leaves are usually the most prominent shoot organ and are structurally diverse.• Growth occurs by means

of marginal meristems.• The marginal meristems

grow outward and ultimately form the blade (the flattened portion) of the leaf.

LEAVES

• Leaf blades come in a variety of forms:• Simple leaves have a single, undivided blade.• Compound leaves have a blade divided into

leaflets.• Pinnately compound describes leaflets that

are arranged in pairs along a central axis.• Palmately compound describes leaflets that

radiate out from a common point at the blade end of the petiole.

LEAVES

• Veins, comprised of xylem and phloem, run through the leaf.• In most dicots, the veins have a net or

reticulate venation.• In most monocots, the veins are parallel.

LEAVES

• Leaves can be arranged in different patterns:• Alternate leaves spiral around a shoot.• Opposite leaves occur on opposite sides of a

shoot.• Whorled leaves circle the stem as a group.

Alternate (spiral):Ivy

Opposite:Periwinkle

Whorled:Sweet woodruff

LEAVES

• A typical leaf contains masses of parenchyma, called mesophyll, through which the vascular bundles, or veins, run.

Vein

Air spaces Stoma Guard cellAir spacesVeinStomaGuard cell

Upperepidermis

Palisademesophyll

Spongymesophyll

Cuticle

Lowerepidermis

WATER MOVEMENT

• Vascular plants have conducting systems for transporting fluids and nutrients throughout the plant.• Water and minerals enter a plant through the roots

and are transported in the xylem.• Carbohydrates synthesized by photosynthesis are

transported throughout the plant in the phloem.

H2O

H2O

H2O

H2O and minerals

Phloem

Xylem

Water and carbohydratestravel to all parts of the plant.

H2Oand

minerals

H2Oand

minerals

H2Oand

minerals

Carbohydrates

Water exits theplant through stomatain leaves.

StomaH2O

vapor

Carbohydrates

Water enters theplant through the roots.

Spongymesophylllayer

Xylem

Water and mineralspass up throughxylem.

WATER MOVEMENT

• Several factors are at work to move water up the height of a plant.• Osmosis: Water moves into the cells of the root

because the fluid in the xylem contains more solutes than the surroundings.• This osmotic force is called root pressure

but, by itself, is not sufficient to “push” water up a plant’s stem.

WATER MOVEMENT

• In addition to root pressure, capillary action adds “pull” to the movement of water up a plant stem.• Capillary action results from

the tiny electrical attractions of polar water molecules to surfaces that carry electrical charge.• This attraction is called

adhesion.• But capillary action, by itself, is

not strong enough to “pull” water up the plant stem.

WATER MOVEMENT

• A final “pull” to the process of moving water up a plant shoot is provided by transpiration.• Water evaporating from the top (leaf) of the

tube pulls the column of water from the bottom (root).

• The column of water does not collapse because water molecules are attracted to each other.• This process is called cohesion.

• The narrower the diameter of the tube, the more tensile strength, or resistance to separation, of the water column.

WATER MOVEMENT

• The combination of gravity, tensile strength, and cohesion affects water movement.• The whole process is explained by the

cohesion-adhesion-tension theory.

WATER MOVEMENT

• Transpiration is the process by which water leaves a plant.• More than 90% of the water taken in by a plant is lost to

the atmosphere, mostly through the leaves.• Water first passes into the pockets of air in the spongy

mesophyll and then evaporates through the stomata.• High humidity and low temperatures increase

transpiration rates.

H2O

H2O

H2O

H2O

H2O H2O

H2O

1 2 3

Dry air passes across the leaves andcauses water vapor to evaporate outof the stomata.

The loss of water from the leavescreates a type of “suction” that drawswater up the stem through the xylem.

New water enters the plant through the roots to replace the water moving up the stem.

Dry air

WATER MOVEMENT

• The only way that plants can control water loss on a short-term basis is to close their stomata.• But plants need to balance closing their

stomata with keeping them open for providing access to carbon dioxide.

• The stomata open and close because of changes in the water pressure of their guard cells.

WATER MOVEMENT

• When the guard cells are plump and swollen with water, they are said to be turgid and the stoma is open.

• When the guard cells lose water, the stoma closes.

H2O H2O H2O H2O

H2O

H2O H2O H2O H2O

H2OH2O

Thickenedinner wall

Stoma closedStoma open

Guard cell Chloroplasts Epidermal cell Nucleus

H2O

(b)(a)

WATER MOVEMENT

• Root hairs greatly increase the surface area of roots.• Root hairs are turgid

because they contain a higher concentration of dissolved solutes than the soil.

• Root hairs also contain a variety of ion transport channels that transport specific ions.• This may involve active transport.• The minerals are transported by the xylem while

dissolved in water.

CARBOHYDRATE TRANSPORT

• Translocation is the process by which most of the carbohydrates manufactured in plants are moved through the phloem.• Carbohydrates are transported by mass flow, a

passive process.• Mass flow occurs because of water pressure -

when carbohydrates are loaded into sieve tubes, water also enters due to osmosis; the water pressure forces the carbohydrates down the plant.

CARBOHYDRATE TRANSPORT

• An area where sucrose is made is called a source and an area where sucrose is delivered from the sieve tubes is called a sink.

• Sucrose moves from a source to a sink by a process described by the pressure-flow hypothesis.

H2O

1

2

3

4

Sugar created in the leaves by photosynthesis (“source”)enters the phloem by active transport.

When the sugar concentration in the phloem increases,water is drawn into phloem cells from the xylem by osmosis.

The addition of water from the xylem causes pressure tobuild up inside the phloem and pushes the sugar down.

Sugar from the phloem enters the root cells (“sink”) byactive transport.

Leaf cells Sugar

XylemPhloem

Root cells

Sugar

Sugar