CH 26- Secondary Growth

Secondary growth

• Occurs in woody plants

  • all gymnosperms

  • 20% of dicots

  • 5% of monocots

• Occurs in stems and roots after they are no longer growing in length

Lateral meristems

• Produce secondary growth

• Cylinders rather than clusters of undifferentiated stem cells

• Secondary growth is radial

• New cells are added bidirectionally (toward center and surface)

Cambia

• Cambial cells are derived from cells that dedifferentiate

• Cambium means “to exchange”

Cambia forms 2 lateral meristems

• Vascular cambium

• Cork cambium

Vascular cambium

• Derived from the cortex and procambium

• Produces secondary xylem and phloem

Cork cambium

• Forms from parenchyma cells and phloem

• Produces new dermal tissue

Stem Vascular cambium

• Produces secondary xylem and phloem

• Vascular cambium cells arise from residual procambium cells (between 1 xylem and phloem)

• Can also arise from parenchyma cells between vascular bundles

Root Vascular Cambium

• First forms a discontinuous series of sections that eventually form a closed ring

• Pericycle cells become part of the vascular cambium

The secondary xylem and phloem

• Expand the plant’s conducting capacity

• Replace older cells that no longer conduct

• Vascular cambia produce much more xylem than phloem

Secondary Xylem

• Wood!

• Consists of largely dead cells

• Only the more recently formed layers of secondary xylem conduct water and minerals

• Primary and older secondary xylem become inactive (vessels have broken columns of water; trac

Secondary Phloem

• One more recent layers of living secondary phloem conduct food

• Primary and older secondary phloem become stretched and broken as vascular cambium push them outward

Secondary growth

• Enhances conduction and support

• Function of cells in secondary xylem and phloem is the same as that of primary tissues

• Secondary xylem tends to have thicker walls (lignin constitutes up to 25% of dry weight of wood)

• Lignin is the second most common organic compound on Earth

After several years of growth

Cork cambium

2 types of dermal tissue in vascular plants:

1. Epidermis

2. Periderm

During secondary growth, epidermis and cortex are replaced by periderm

Periderm

• Is produced by the cork cambium

• It consists of:

  • Cork (phellum)

  • Phelloderm

  • Cork cambium cells (phellogen)

Cork phellum

• Forms to the outside of the cork cambium

• Consists of dead cells when mature

Phelloderm

• Forms to the inside of the cork cambium

• Consists of a thin layer of living parenchyma cells

  Unlike vascular cambium, cork cambium does not grow in diameter (forms inside old one each year or so)

Stem cork cambium

• First cork cambium arises from parenchyma cells in cortex

• Each new cork cambium arises from cortex tissue to the inside

– until cortex is used up

• Expansion causes cortex to break apart and fall off stem

• Subsequent cork cambia arise from secondary phloem to the inside

Root cork cambium

Endodermis and pericycle

• Endodermis-filtering no longer needed

• Pericycle-no more branch roots

• Outer layer of enlarged pericycle gives rise to the first cork cambium forms periderm

Bark

Consists of all tissues external to the vascular cambium

Two distinct regions:

1. Inner bark

2. Outer bark

• Typically thinner than the woody portion of the stem

• Pattern s are variable but characteristic for different species

• Essential for tree viability

Inner bark

Living secondary phloem, dead phloem between the vascular cambium and inner-most cork cambium, and any remaining cortex

• Inner bark carries sugar and other organic molecules

Outer bark

dead tissue (includes dead secondary phloem, all layers of periderm outside of most recent cork cambium)

• Dead outer bark provides protection

Primary and secondary growth

Heartwood

Older non-conducting rings of xylem found in the center of the trunk or root

Sapwood

Consist of the outer xylem rings that still conduct water and minerals

• Antibacterial and antimicrobial compounds protect heartwood from rotting

Growth rings in wood

• Yearly growth of xylem forms rings

• The sizes of the cells that make up these rings varies with season

• Cells tend to be small in late summer and large in the spring

Dendrochronology

• Science of tree ring dating and climate interpretation

• Tree rings provide information about tree and climate variations

Reaction Wood

Develops to counterbalance wind and gravity

• Tension wood (dicots)

• Compression wood (gymnosperms):

Tension wood (dicots)

Forms on the upper side of leaning trunks or branches-pulls

Compression wood (gymnosperms)

Forms on the underside of trunk or branches-pushes

Protrecting the Outside at a cost

• Cork cambium forms protective layers of tissues preventing water loss, infection by disease and predation

• Cork cambium cannot grow in diameter and must reform inside the old one (toward a source of water and nutrients)

• Cork cells enlarge and become coated and impregnated with suberin

Lenticels

Small openings in outer bark of stems and roots that allow gas exchange which is blocked by suberin

Antimicrobials

• Latex

• Resins

Flowering plants - produce forms of latex

• Milky substance

• Blocks entry of disease-causing organisms

• Contains compounds with growth inhibiting

properties

Pines and SVP’s - produce resins

• Flow through canals in secondary xylem and phloem, periderm and leaves (turpentine and rosin mix)