1. Secondary Growth of Stems - due to division of lateral meristems
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

2. Derivation from the Apical Meristem

3. A Few Definitions Periderm = secondary tissue Phellem Phellogen
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)

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

8. Bark (Cork) Formation
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

9. Anatomy of cork formation
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.

10. Pelargonium (geranium) stem with cork and cork cambium.
*Note that the epidermis is tearing and peeling away.

11. Structure and longevity of cork cells
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)
new cambium forms because the former phellogen dies as it is crushed by expanding xylem
forms in the outer region of the still-living phloem
reforms every ~ 1 to 4 years depending upon the species of tree

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

13. lenticels birch
(Betula sp.)

14. Aging of Bark
Due to constant expansion of growing stem, bark must increase in girth
Old bark is continuously being pushed outward
can be shed from tree by sloughing off
sloughing results in unique bark patterns
Rhytidome of white ash

15. Characteristics of Wood: Growth Rings
Result of seasonal dormancy
Growth - active in spring and tapers in summer, ceasing in fall
Spring wood - larger vessels, more porous fewer, smaller rays
Summer wood - denser, smaller cells, thicker walls.

16. Characteristics of Wood, continued
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

17. Characteristics of Wood, continued
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

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

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

20. Secondary Growth in Monocots
Most monocots are small and herbaceous
Generally lack secondary growth
Exceptions with secondary-like growth:
PALMS - produce a wide procambium region --> large diameter base with many strong vascular bundles and much vascular parenchyma

21. Secondary Growth in Monocots
Most monocots are small and herbaceous
Generally lack secondary growth
Exceptions with secondary-like growth:
AGAVE - cambium that produces additional vascular bundles, but not "wood”; increases volume but not exactly a tree

22. Secondary Growth in Monocots
Most monocots are small and herbaceous
Generally lack secondary growth
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 short-lived