Primary growth occurs at the shoot and root apical meristems, adding new cells and increasing length. Secondary growth occurs through the vascular cambium in woody stems and roots, adding girth over time. The vascular cambium produces secondary xylem towards the inside and secondary phloem towards the outside, resulting in increased thickness. Cork cambium also forms layers of protective cork over years of secondary growth.

1. PRIMARY AND
SECONDARY GROWTH
IN PLANTS

2. Meristems generate cells for new organs
Primary growth in stems
Shoot apical
meristems Epidermis
(in buds)
Cortex
Primary phloem
Primary xylem
Vascular
cambium Lateral Pith
Cork meristems
cambium
Secondary growth in stems
Periderm
Cork
Pith cambium
Cortex
Primary Primary
xylem phloem
Root apical Secondary
meristems xylem Secondary
phloem
Vascular cambium

3. Woody plants
Terminal bud
Bud scale
•In woody Axillary buds
plants, primary Leaf scar
This year’s growth Node
and secondary (one year old) Stem
growth occur Internode
simultaneously One-year-old side
branch formed
but in different from axillary bud
near shoot apex
locations Last year’s growth
Leaf scar
(two years old)
Scars left by terminal
bud scales of previous
winters
Growth of two Leaf scar
years ago (three
years old)

4. Primary Growth of Roots
Cortex Vascular cylinder
Epidermis
Key Zone of
Root hair maturation
Dermal
Ground
Vascular
Zone of
elongation
Apical Zone of cell
meristem division
Root cap
100 µm

5. Primary Growth of Shoots
Apical meristem Leaf primordia
•A shoot
apical
meristem
gives rise to a
repetition of Developing
vascular
internodes strand
and leaf-
bearing nodes
Axillary bud
meristems
0.25 mm

6. How a plant grows from a seed –
primary growth
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7. Secondary growth adds girth to stems
and roots in woody plants
•Occurs in stems Primary and secondary growth
in a two-year-old stem
and roots of
Epidermis
woody plants Cortex
Primary
phloem
Pith
Primary xylem
Vascular cambium
but rarely in Vascular
cambium
Primary
Primary phloem
Cortex
Epidermis
xylem Phloem ray
leaves Pith Xylem
ray
•Produced by Primary
xylem
the vascular Secondary xylem
Vascular cambium
Secondary phloem
Primary phloem
cambium and Periderm
First cork cambium Cork
cork cambium (mainly cork
cambia
and cork)
Primary
phloem
Secondary
phloem Secondary
Vascular xylem (two
cambium years of
Secondary production)
xylem Vascular cambium
Secondary phloem Bark
Primary
xylem Most recent
Pith cork cambium Layers of
Cork periderm

8. Secondary growth adds girth to stems
and roots in woody plants
Secondary phloem
Vascular cambium
Late wood Cork
Secondary cambium Periderm
xylem Early wood
Cork
Transverse section
of a three-year-
old Tilia (linden)
Xylem ray stem (LM)
Bark
0.5 mm 0.5 mm

9. Secondary xylem
•As a tree Growth ring
ages, the
older layers Vascular
of ray
secondary
xylem no Heartwood
Secondary
longer xylem Sapwood
transport
water and Vascular cambium
minerals Secondary phloem
Bark
Layers of periderm