Download Secondary growth in stems

Secondary growth
in stems
Secondary growth
Some of the meristematic
cells in plants with
secondary growth keep their
meristematic state and
become cells of the
cambium.
The addition of secondary
vascular tissue and
periderm = increase the
girth or diameter of the plant
Annuals, biennials and
perennials.
• Description of the time for
growth
• Woody perennials produce
flowers only when they become
adults.
• Woody plants in temperate
regions are deciduous.
Secondary growth in roots and stems
Secondary xylem and phloem
provide physical continuity
between primary xylem and
phloem of roots and stems
Leaves have minimal, if any,
secondary growth, generally is
restricted to strengthening vein
tissue.
Origin of the secondary growth
Secondary growth originates from two tissues:
Vascular cambium and cork cambium.
Both of them are lateral meristems
The vascular cambium
Meristematic cells of vascular cambium are highly vacuolated.
Two types of cells:
• Fusiform Initials (elongated) è axial elements = vertical
conduction
• Ray Initials (isodiametric) è ray parenchyma = radial conduction
and storage
• Are radial (lateral)
transport system cells
• ray initials in xylem and
phloem = parenchyma
• axial (vertical) transport system
cells
• In xylem and phloem =
• sieve-tube
member
• companion
cells
• tracheids
• vessels
• fibers
• Can be storied or nonstoried
Cambium exhibits seasonal dormancy.
This contributes to the growth rings common in wood.
In many tropical species cambium is always active, and
wood lacks distinctive rings.
Cambium exhibits seasonal dormancy.
The inner lighter toned ring is that laid
down during early spring growth, and is
known as spring wood. This is relatively
soft and the cells ( tracheids ) are thin
walled and carry sap.
The outer ring, sometimes being quite dark
is usually a harder band and is laid in
summer; it is called summer wood. These
Tracheids are thick walled and provide
rigidity and stability to the bole.
Lignin in the wall
Development of secondary vascular tissues
Fascicular cambium
Interfascicular cambium
fascicle = vascular bundle
Periderm
cork = phellem
cork cambium = phellogen
cork cortex = phelloderm
periderm
Cork Cu'cle epidermis phelloderm cortex Lenticels
• Lenticels are portions of periderm with intercellular spaces
• Present in stems, roots and fruits
• Commonly appear below of a stoma or a group of stomata
Bark
Transversal Xylem ray Outer bark Inner bark With phloem Tangen*al surface Xylem ray Radial surface Bark refers to all the tissues outside the vascular cambium
Dark area= heart wood
Lighter part of the wood= sapwood
Aging of bark
Increment in the girth è stress
on the older tissues of the bark.
The older phloem gets crushed with
the lateral expansion of the stem.
New periderm layers separate the
newer phloem from the older phloem
The older phloem and periderm
layers are eventually sloughed off
External features of woody
Aging stems
of bark
Branches and twig are small extensions of the
stem of certain angiosperms
Scales are transformed and reduced leaves.
They cover and protect the embryonic parts.
Twig Leaf scar 1
1. 
2. 
3. 
4. 
5. 
6. 
7. 
Terminal bud
Internode
Terminal bud-scale scars
Lenticel
Lateral bud
Node area
Stem
Lateral bud Leaf scar Bundle scars Len'cel Terminal bud-­‐scale scares 5
2
6
3
4
7
Plants with secondary protective tissues
bark
The manner in which new periderm is formed the kind of tissues isolated
determine the surface of the bark
Cork
Quercus suber
After the first cork is removed from the epidermis, new cork
cambium is formed in the cortex
Conifer wood
• Lack of vessels
• Reduced wood parenchyma
• Long tapering tracheids
• Resin ducts in the
parenchyma cells
Conifer wood
Transverse radial tangen'al Early wood Late wood Ray Early wood Late wood Resin duct ray Pit-pairs of Pinus tracheids
Torus border Torus border Reaction wood
In conifers = compression
wood
• dense wood
• lignin
• lower side of branches
In angiosperms = tension
wood
• soft wood
• cellulose
• upper side of branches