Secondary Growth of
Stems - due to division of lateral meristems
1. Vascular Cambium -
• derived from parenchyma cells
• located between the primary phloem (towards outside) and primary xylem (towards inside)
• produces secondary xylem and secondary phloem
2. Products of Secondary Growth
• wood = accumulated layers of secondary xylem
• bark = secondary phloem, and all tissues external to it http://www.puc.edu/Faculty/Gilbert_Muth/art0056.jpg

http://mason.gmu.edu/~jlawrey/biol304/biol304/notes/Image18.gif

• Periderm = secondary tissue
– replaces epidermis in roots and stems
– consists of phellem, phellogen, and phelloderm
• Phellem
– corky tissues
– non-living suberized cells
– produced by the cork cambium (phellogen) to outside of stem
• Phellogen
– cork cambium
– produces cork to the outside
– produces phelloderm to the inside
• Phelloderm
– parenchyma-like cells
– produced toward inside of stem by the cork cambium (phellogen)

Forming the Periderm (A.K.A. cork and bark)
1.
increase in diameter of the stem occurs due to activity of vascular cambium
2.
causes the protective epidermis to crack and split open
3.
need meristematic layer at the outer edge of the phloem to produce protective layer internal tissues
4.
layer of cork cambium forms outside of the phloem.
5.
cylinder of cork cambium increases in diameter as stem increases in diameter
This outer non-living part of the bark is called the rhytidome .

Phase 1: As the layers of cells outside the vascular cambium die, they are sloughed off as bark
Phase 2: In the young stem the bark contains: epidermis, cork, cork cambium, phelloderm, cortex, and phloem
Phase 3: In the old stem the bark contains: cork, cork cambium, phelloderm, and phloem

In the young stem (1 year old or less) a. Cortical cells just under the epidermis become meristematic b. Produces a layer 1-2 cells thick of cork cambium called phellogen c. Phellogen produces a layer of cork cells 4-6 cells thick external (toward the epidermis) to the phellogen d. Phellogen produces a single layer of cells, phelloderm, internal (toward the xylem and phloem) to the phellogen
Pelargonium (geranium) stem with cork cambium and several layers of cork .
http://www.uri.edu/artsci/bio/plant_anatomy/142.html

Pelargonium (geranium) stem with cork and cork cambium.
*Note that the epidermis is tearing and peeling away. http://www.uri.edu/artsci/bio/plant_anatomy/143.html

1. Cells are flattened and cell walls contain suberin , a waxy substance
2. In old stems (more than 1 year old, generally 3-
4 years) a. new cambium forms because the former phellogen dies as it is crushed by expanding xylem b. forms in the outer region of the still-living phloem c.
reforms every ~ 1 to 4 years depending upon the species of tree

1. Cork - generally impervious to fluids and gases
2. Special structures for gas exchange required to provide oxygen to the living cells of the secondary growth region.
3. Lenticels = weak
"eruptions" of parenchyma cells through which gases can diffuse.
4. Also contribute to the appearance of bark

lenticels birch
( Betula sp.
)

– can be shed from tree by sloughing off
– sloughing results in unique bark patterns
Rhytidome of white ash

1.
Result of seasonal dormancy
2.
Growth - active in spring and tapers in summer, ceasing in fall
3.
Spring wood larger vessels, more porous fewer, smaller rays
4.
Summer wood denser, smaller cells, thicker walls.

1. Distribution and size of vessels a. ring porous = abrupt transition between spring and summer wood
– spring vessels - large and fewer in number
– summer vessels numerous and small b. diffuse porous vessels more uniformly distributed in both spring and summer wood

2. Pattern of rays a. rays in clumps - detectable without magnification b. single cell wide rays - visible only microscopically
3. Aging in wood a. heartwood - non-functioning older xylem
– can rot away, leaving a hollow core filled with a variety of substances (oils, gums, resins, tannins)
– typically darkened in appearance b. sapwood functioning xylem toward the exterior

1.
Much of the wood used commercially is from conifers.
2.
Conifers are often called softwood
• only tracheids, no vessels
• minimal parenchyma
(appears more uniform)
3.
Resin ducts (or canals)
• lined by a ring of parenchyma cells.
• defense mechanism
4.
Conifer tracheids have prominent bordered pits along their walls

1. Comprised of vessels, fiber and parenchyma rays
2. Frequently referred to as hardwood (but has no real meaning in terms of strength)
3. Larger diameter vessels, and more numerous fibers

1.
Most monocots are small and herbaceous
2.
Generally lack secondary growth
3.
Exceptions with secondary-like growth:
– PALMS - produce a wide procambium region --> large diameter base with many strong vascular bundles and much vascular parenchyma

1.
Most monocots are small and herbaceous
2.
Generally lack secondary growth
3.
Exceptions with secondary-like growth:
– AGAVE - cambium that produces additional vascular bundles, but not
"wood”; increases volume but not exactly a tree

1.
Most monocots are small and herbaceous
2.
Generally lack secondary growth
3.
Exceptions with secondary-like growth:
– BANANA - Sheath stem with giant leaves that have extraordinary vascular tissue (vein) connections (includes
Veins, the vascular many sclerenchyma fibers). Can achieve big dimensions, but shortlived