PRIMARY AND SECONDARY GROWTH

IN PLANTS

Meristems generate cells for new organs

Shoot apicalmeristems(in buds)

VascularcambiumCorkcambium

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

Woody plants

•In woody plants, primary and secondary growth occur simultaneously but in different locations

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 sidebranch formedfrom axillary budnear shoot apex

Leaf scar

Bud scale

Axillary buds

Internode

StemNode

Leaf scar

Scars left by terminalbud scales of previouswinters

Primary Growth of Roots

Key

DermalGroundVascular

Epidermis

Root hair

Cortex Vascular cylinder

Zone ofmaturation

Zone ofelongation

Zone of celldivision

Apicalmeristem

Root cap

100 µm

Primary Growth of Shoots

•A shoot apical meristem gives rise to a repetition of internodes and leaf-bearing nodes

Developingvascularstrand

Axillary budmeristems

0.25 mm

Leaf primordiaApical meristem

How a plant grows from a seed – primary growth

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Secondary growth adds girth to stems and roots in woody plants

•Occurs in stems and roots of woody plants but rarely in leaves•Produced by the vascular cambium and cork cambium

Primary and secondary growth in a two-year-old stem

EpidermisCortex

GrowthXylemray

Vascularcambium

Primaryphloem

Pith

Primaryxylem Phloem ray

EpidermisCortex

Vascular cambiumPrimary phloem

PithPrimary xylem

Vascular cambium

Primary phloem

Primaryxylem

Secondary phloem

Secondary xylem

First cork cambium Cork

Growth

Vascular cambium

Primary phloemSecondary phloem

Secondary xylem

Periderm(mainly corkcambiaand cork)

Primary xylem

Pith

Vascular cambiumSecondary phloem

Secondaryxylem (twoyears ofproduction)

Cork

Bark

Layers ofperiderm

Most recentcork cambium

Secondary growth adds girth to stems and roots in woody plants

0.5 mm

Vascular cambiumSecondary phloem

Secondaryxylem

Transverse sectionof a three-year-old Tilia (linden)stem (LM)

Late woodEarly wood

0.5 mm

Cork cambium

Cork

Periderm

Xylem rayBark

Secondary xylem

•As a tree ages, the older layers of secondary xylem no longer transport water and minerals

Growth ring

Vascularray

Secondaryxylem

Heartwood

Sapwood

Vascular cambium

Secondary phloem

Layers of peridermBark