Plant Structure, Growth, and Development

Page
5

Lateral meristems add thickness to woody plants, a process called secondary growth.

There are two lateral meristems: the vascular cambium and the cork cambium.

The vascular cambium adds layers of vascular tissue called secondary xylem = wood and secondary phloem.

The cork cambium replaces the epidermis with periderm, which is thicker and tougher.

Slide 37

An overview of primary and secondary growth

Shoot tip (shoot

apical meristem

and young leaves)

Lateral meristems:

Axillary bud

meristem

Vascular cambium

Cork cambium

Root apical

meristems

Primary growth in stems

Epidermis

Cortex

Primary phloem

Primary xylem

Pith

Secondary growth in stems

Periderm

Cork

cambium

Cortex

Primary

phloem

Secondary

phloem

Pith

Primary

xylem

Secondary

xylem

Vascular cambium

Slide 38

Primary Growth - Lengthens Roots and Shoots

The root tip is covered by a root cap, which protects the apical meristem as the root pushes through soil.

Growth occurs just behind the root tip, in three zones of cells:

Zone of cell division

Zone of elongation

Zone of maturation - differentiation.

Slide 39

Primary growth of a root

Ground

Dermal

Key

to labels

Vascular

Root hair

Epidermis

Cortex

Vascular cylinder

Zone of

differentiation

Zone of

elongation

Zone of cell

division

Apical

meristem

Root cap

100 µm

Slide 40

The primary growth of roots produces the epidermis, ground tissue, and vascular tissue.

In most roots, the stele is a vascular cylinder.

The ground tissue fills the cortex, the region between the vascular cylinder and epidermis.

The innermost layer of the cortex is called the endodermis.

Slide 41

Epidermis

Cortex

Endodermis

Vascular

cylinder

Pericycle

Core of

parenchyma

cells

Xylem

Phloem

100 µm

Root with xylem and phloem in the center

(typical of eudicots)

(a)

Root with parenchyma in the center (typical of

monocots)

(b)

100 µm

Endodermis

Pericycle

Xylem

Phloem

50 µm

Key

to labels

Dermal

Ground

Vascular

Organization of primary tissues

in young roots

Slide 42

Lateral roots arise from within the pericycle, the outermost cell layer in the vascular cylinder