Ch. 31 – Plant Structure, Growth and Differentiation

Plant Body

• Root system– Underground– Anchor and absorb

• Shoot system– Vertical stem, leaves (flowers, fruits w/seeds)– photosynthesis

Fig. 35-2Reproductive shoot (flower)

Apical bud

Node

Internode

Apicalbud

Shootsystem

Vegetativeshoot

LeafBlade

Petiole

Axillarybud

Stem

Taproot

Lateralbranchroots

Rootsystem

Plant Cells and Tissues

• Ground tissue system – majority– Photosynthesis, storage, support

• Vascular tissue system– Conduction, strength, support

• Dermal tissue system– Covering, protection

All 3 are Interconnected throughout the plant

Fig. 35-8

Dermaltissue

Groundtissue Vascular

tissue

Ground Tissue System

• Parenchyma, collenchyma, sclerenchyma tissue

• Primary cell wall – secreted by growing cell; stretches and expands as cell grows

• Secondary cell wall – secreted when cell stops growing; thick and strong (inside primary)

Parenchyma• Living, metabolizing• Most common• Soft parts• Function

– Photosynthesis – green chloroplasts– Storage – starch, oil, water, salt– Secretion – resins, tannins, hormones, enzymes, nectar

• Can differentiate if plant injured (i.e. xylem cells)

Fig. 35-10a

Parenchyma cells in Elodea leaf,with chloroplasts (LM) 60 µm

Parenchyma

Collenchyma

• Flexible, structural support (nonwoody parts)• Elongated cells• Alive at maturity• Primary CW – unevenly thick, thicker in

corners• Near stem surface, leaf veins

Fig. 35-10b

Collenchyma cells (in Helianthus stem) (LM)

5 µm

Collenchyma

Sclerenchyma

• Structural support• Primary and secondary CW (strong and hard,

extreme thickening, so can’t stretch, elongate)• Cells dead at maturity• 2 types:

– Sclereids – variable shape, nut shells, pits of stone fruits, pears gritty (clusters of sclereids)

– Fibers – long, tapered – patches, clumps; wood, inner bark, leaf veins

Sclerenchyma

Fig. 35-10c

5 µm

25 µm

Sclereid cells in pear (LM)

Fiber cells (cross section from ash tree) (LM)

Cell wall

Vascular tissue

• Embedded in ground tissue• Transport• Xylem and phloem

Xylem

• Conducts water, dissolved nutrient minerals roots stems, leaves

• Support• Angiosperms –

– tracheids, vessel elements - conduct– parenchyma cells - storage– fibers - support

Tracheids and vessel elements

• Dead at maturity hollow, CW remain• Tracheids – long, tapering, patches/clumps;

water passes from 1 tracheid to another by pits (thin areas where sec. wall did not form)

• Vessel elements – larger in diameter than tracheid; end walls have perforations; stacked water goes between; stack = vessel; pits in side walls for lateral water transport

Fig. 35-10d

Perforationplate

Vesselelement

Vessel elements, withperforated end walls Tracheids

Pits

Tracheids and vessels(colorized SEM)

Vessel Tracheids 100 µm

Phloem

• Conducts food• Support• Angiosperms

– Sieve tube members, companion cells – conduct– Fibers – support– Parenchyma cells

Sieve tube members

• Conduct food in solution• Joined end-to-end long tubes• CW ends = sieve plates; cytoplasm extends

between cells• Living at maturity – many organelles

shrink/disintegrate• Can function w/o nuclei

Companion cells

• Adjacent to each sieve tube member (stm)• Assists stm• Living w/ nucleus – directs activities of both

cells• Plasmodesmata between stm and companion• Helps move sugar into stm

Fig. 35-10e

Sieve-tube element (left)and companion cell:cross section (TEM)

3 µmSieve-tube elements:longitudinal view (LM)

Sieve plate

Companioncells

Sieve-tubeelements

Plasmodesma

Sieveplate

Nucleus ofcompanioncells

Sieve-tube elements:longitudinal view Sieve plate with pores (SEM)

10 µm

30 µm

Food conducting

Dermal tissue system

• Epidermis and periderm• Protective covering• Herbaceous – single layer = epidermis• Woody – epidermis splits w/ growth

– Periderm – layers thick, under epidermis; replaces epidermis in stems, roots, composing outer bark

Epidermis

• Unspecialized dermal cells• Special guard cells + trichomes• Single layer, flat cells• Usually no chloroplasts transparent

– Allow light through

Fig. 35-18a

Keyto labels

Dermal

Ground

VascularCuticle Sclerenchyma

fibersStoma

Bundle-sheathcell

Xylem

Phloem

(a) Cutaway drawing of leaf tissuesGuardcells

Vein

Cuticle

Lowerepidermis

Spongymesophyll

Palisademesophyll

Upperepidermis

Fig. 35-18b

Guardcells

Stomatapore

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

Epidermalcell

(b)

50 µ

m

Fig. 35-18c

Upperepidermis

Palisademesophyll

Keyto labels

Dermal

Ground

Vascular

Spongymesophyll

Lowerepidermis

Vein Air spaces Guard cells

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

(c)

100

µm

Cuticle

• Aerial parts• Secreted by epidermal cells• Waxy – water loss• Slows diffusion of CO2 – stomata help• Stomata

– Open – day – photosynthesis, evaporative cooling– Closed – night– Closed in day if drought

Trichomes

• Outgrowths or hairs• Many shape, sizes, functions• Ex:

– Roots hairs – increase SA– Salty env. – remove excess salt– Aerial parts – increase light reflection, cooler– Protections – stinging nettles

Growth at Meristems

• Cell division– Increase # cells

• Cell elongation– Vacuole fills, increase pressure on CW, expands

• Cell differentiation– Specialize into cell types

• Meristems = where plant cells divide, mitosis– No differentiation

2 kinds of Growth

• Primary growth– Increase stem, root length– All plants, soft tissues

• Secondary growth– Increase width– Gymnosperms, woody dicots– Wood + bark

Fig. 35-11

Shoot tip (shootapical meristemand young leaves)

Lateral meristems:

Axillary budmeristem

Vascular cambiumCork cambium

Root apicalmeristems

Primary growth in stems

Epidermis

Cortex

Primary phloem

Primary xylem

Pith

Secondary growth in stems

Periderm

Corkcambium

Cortex

Primaryphloem

Secondaryphloem

Pith

Primaryxylem

Secondaryxylem

Vascular cambium

Primary growth• Increase in length• Apical meristem – tips of roots + shoots (buds)• Buds = dormant embryonic shoot (develop into

branches next spring• Root tip

– Root cap – protective layer of cells, covers root tip– Root apical meristem – directly behind root cap– Cell elongation – behind meristem, push tip ahead,

some differentiation

Fig. 35-13

Ground

Dermal

Keyto labels

Vascular

Root hair

Epidermis

Cortex Vascular cylinder

Zone ofdifferentiation

Zone ofelongation

Zone of celldivision

Apicalmeristem

Root cap

100 µm

Fig. 35-14a1

Root with xylem and phloem in the center(typical of eudicots)

(a)

100 µm

Epidermis

Cortex

Endodermis

Vascularcylinder

Pericycle

Xylem

Phloem

Dermal

Ground

Vascular

Keyto labels

Fig. 35-14a2

Vascular

Ground

Dermal

Keyto labels

Root with xylem and phloem in the center(typical of eudicots)

(a)

Endodermis

Pericycle

Xylem

Phloem

50 µm

Fig. 35-14b

Epidermis

Cortex

Endodermis

Vascularcylinder

Pericycle

Core ofparenchymacells

Keyto labels

Dermal

Ground

Vascular

Xylem

Phloem

Root with parenchyma in the center (typical ofmonocots)

(b)

100 µm

• Shoot apex = terminal bud– Shoot meristem– Give rise to leaf primordia and bud primordia

Fig. 35-16

Shoot apical meristem Leaf primordia

Youngleaf

Developingvascularstrand

Axillary budmeristems

0.25 mm

Fig. 35-17a

Sclerenchyma(fiber cells)

Phloem Xylem

Ground tissueconnectingpith to cortex

Pith

CortexEpidermisVascularbundle

1 mm

Cross section of stem with vascular bundles forminga ring (typical of eudicots)

(a)

Dermal

Ground

Vascular

Keyto labels

Fig. 35-17b

Groundtissue

Epidermis

Keyto labels

Cross section of stem with scattered vascular bundles(typical of monocots)

Dermal

Ground

Vascular

(b)

Vascularbundles

1 mm

Secondary Growth

• Increase in width• Make secondary tissues: sec. xylem, sec.

phloem, periderm• Lateral meristem – cells divide, not elongate• 2 types:

– Vascular cambium • Between wood and bark• Make sec. xylem (wood) + sec. phloem (inner bark)

Fig. 35-20

Vascular cambium Growth

Secondaryxylem

After one yearof growth

After two yearsof growth

Secondaryphloem

VascularcambiumX X

X X

X

X

P P

P

P

C

C

C

C

C

C

C C C

C C

CC

Fig. 35-22

Growthring

Vascularray

Secondaryxylem

Heartwood

Sapwood

Bark

Vascular cambium

Secondary phloem

Layers of periderm

– Cork cambium• In outer bark• Form cork to outside +parenchyma (storage)• Periderm = cork, parenchyma, cork cambium

• Bark – outermost covering of woody stems– Everything outside of vascular cambium– 2 regions:

• Living inner bark of secondary phloem• Mostly dead outer bark of periderm

Fig. 35-19a3

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Primary and secondary growthin a two-year-old stem

(a)

Periderm (mainlycork cambiaand cork)

Secondary phloem

Secondaryxylem

Epidermis

Cortex

Primary phloem

Vascular cambiumPrimary xylem

Pith

Vascular ray

Secondary xylem

Secondary phloem

First cork cambium

Cork

Growth

Cork

Bark

Most recent corkcambium

Layers ofperiderm

Fig. 35-19b

Secondary phloemVascular cambium

Secondary xylem

Bark

Early woodLate wood Cork

cambium

Cork

Periderm

0.5

mm

Vascular ray Growth ring

Cross section of a three-year-old Tilia (linden) stem (LM)

(b)

0.5 mm

You should now be able to:

1. Compare the following structures or cells:– Dermal, vascular, and ground tissues – Parenchyma, collenchyma, sclerenchyma, water-

conducting cells of the xylem, and sugar-conducting cells of the phloem

– Sieve-tube element and companion cell2. Describe in detail the primary and secondary growth

of the tissues of roots and shoots3. Describe the composition of wood and bark