Download Lecture 14, Secondary growth in stems and roots

Secondary Growth in Stems
and Roots
•  Review of introduction to secondary growth
•  Secondary Growth in Stems
–  Initiation Vascular Cambium
–  Annual Growth in Stems
–  Initiation of initial and subsequent Cork Cambia
•  How the Vascular Cambium produces cells
–  Fusiform and Ray Initials
–  Additive and Multiplicative Divisions
•  Secondary Growth in Roots
–  Initiation of Vascular Cambium
–  Initial and subsequent cork cambia
Secondary growth
(can be identified by radial alignment of cells)
•  Formed at a secondary meristem
•  (re-differentiated from “mature” non-dividing cells)
•  Responsible for increase in girth (diameter)
•  Meristems called cambia and are located laterally on stems and roots
(less often in leaves)
•  Two lateral meristems are common in seed plants
–  Vascular Cambium (Xylem & Phloem)
–  Phellogen = cork cambium; produces
cork or the bark)
Changes in Stem Structure
with primary and
secondary growth
Growth Rings
Represent cones
of secondary tissue
1. 
Vascular cambium (V.C.) is initiated in layer of Residual Procambium
between the primary xylem and primary phloem in the bundle (Fascicular
V.C.)
2. 
Vascular cambium is initiated in ground tissue between primary bundles of the
stele (Interfascicular V.C.)
3. 
Is Bifacial – producing Secondary Xylem (wood) to the inside and secondary
Phloem to the outside.
4. 
Consists of Fusiform and Ray Initials
Vascular Cambial Activity
at the Cellular Level of Organization
One, Two, & Three year old stems
1.  Growth rings
2.  Increase in Girth (thickness of wood)
3.  Increase in Size of Cambial Cylinder
Diagramatic Representations of Stem Growth
Page 618 textbook
Vascular cambium of Pinus
(Tangential Section)
Fusiform and Ray Initials
Additive Divisions
a.  Divisions are Periclinal
b.  Add new cells to radial files
c.  Increase thickness of wood & phloem
Diagramatic Representations of Stem Growth
Additive Divisions
Multiplicative Divisions
a.  Divisions are anticlinal
b.  Add new cells to vascular cambium
c.  Increase circumference of vascular cambium
Cell divisions in the vascular cambium
Multiplicative = anticlinal division-adds to the
width of the vascular cambium to keep up
with the growth in diameter
Additive = periclinal division-produces the
new xylem and phloem cells
Periderm
1. 
2. 
3. 
4. 
• 
• 
Protective tissue
Secondary origin
Found in bark-or the bark system
Consists of 3 parts
Phellogen = cork cambium = lateral meristem
Phellem = cork-produced to outside-cells
suberized and sometimes also lignified
•  Phelloderm = parenchyma produced toward
the inside-usually thin-walled cells
Stages in periderm formation
ep
pl
ph
pd
ep
pl
ph
ph=phellogen
pd=phelloderm
pl=phellem
ep=epidermis
Fate of the epidermis and cuticle is to be
sloughed off the outside
of the stem as phellem (cork) is
produced
Periderm in x.s.
Phellem cells
can contain
tannins
Can deter insects
and other animals
from feeding on the
bark
Phellogen can start to form in
one of several places
1. Just under the epidermis
(in hypodermis)
2. In the epidermis
3. In the outer cortex
4. In the secondary phloem
In older stems there are
several periderms
“Bark”
•  Informal –used for trees or shrubs for everything outside of the
vascular cambium
•  Includes secondary phloem
•  Any primary tissues that still might be there in the inner cortex
•  The periderm
•  Tissues still outside of periderm –epidermis and outer cortex
•  Term often used inconsistently or to mean several things to different
people
•  Some use “bark” just to mean phellem
lenticels
Lenticel=
A specialized region of the
periderm containing intercellular spaces allowing for
gas exchange (O2 + CO2)
Lenticel in cross section
Interruption in epidermis- a part of the periderm in which
the phellogen is more active than elsewhere
The phellogen bends inward here
Usually forms beneath a stoma
Lenticel allows for
gas exchange
Note: cuticle still
present outside
Found in both stems and
roots
Vary in size and shape
Secondary Growth in Roots
1. 
2. 
Basically Similar to that in Stems, but
Vascular cambium and Periderm are initiated differently
Salix root x.s.
Vascular Cambium initiated
between protoxylem ribs
Periderm initiated in pericycle
Periderm
Cortex and Epidermis
Meristematic Growth in Stems
Meristems and their Derivitives
Secondary
meristems