1. Secondary Growth

2. Concepts:
Woody plants produce secondary tissue.
Wood = secondary xylem
Bark = secondary phloem and cork

3. Advantages Ability to live through harsh conditions
Once it is rooted, it has the ability to grow back and does not have to reseed
They give the plant a greater number of vascular tissues to move water.
Greater ability to move water and minerals up; carbohydrates down
Each ring has more capacity than the previous ring
Photosynthetic capacity increases
Allows the plant to occupy a favorable site for years
Provides strength and size advantages

4. Disadvantages Must fight disease for much longer
Larger plants are bigger targets for pathogens
Wood is “expensive” to produce—takes a lot of energy
Reproductive maturity can be delayed for several years (or decades)
Greater chance of death before reproduction

5. * Secondary growth has risen only 3 times (2 lines died out)—the woody plants of today began evolving about 370 mya. (Fig. 24.4)
* Secondary growth occurs in gymnosperms and dicots—not in ferns or monocots

6. VASCULAR CAMBIUM Meristem cells that produce secondary growth
Divides longitudinally (anticlinal walls)
Gives rise to secondary growth
Secondary xylem—pushed toward the inside of the plant
Secondary phloem—pushed to the outside of the plant
Fusiform initials produce vessel elements and phloem
Ray initials produce short cells used for storage

8. Vascular Bundles Primary growth- only 1 set of phloem and xylem
Phloem- Carbohydrates and other photosynthetic materials are carried throughout plant—From Leaves
Xylem- water and nutrients are carried throughout plant--From Roots
Grow together in Vascular bundles

9. Monocots- Vascular bundles are scattered
Dicots- Vascular bundles are in a ring

10. Secondary
Secondary Phloem- Created by the vascular cambium. Pushed toward the outside of the stem
Secondary Xylem- Created by the Vascular cambium. Pushed toward the insided of the stem
Multiplicative division allows for continuous band around the stem.
Additive division makes the stem thicker.

12. Secondary Xylem
Answer questions 1 and 5 on page 227.

13. Secondary Xylem Hardwoods-
contain large amounts of fiber, making them tough.
Originate from dicots or deciduous trees
Includes balsa (no fiber)
Softwoods- Have less amounts of fiber making them softer. Originate from conifers or needle trees
Rays are used to store material during dormancy and to transport material laterally for short distance

14. Annual Rings New xylem and phloem produced seasonally- forms the rings
Tells the age of the tree
Indicates good or bad growing season
Older xylem toward the middle (inside)
Early wood (Spring wood)
First wood formed
Must have high capacity to move water because leaves cuticle is not developed
Late wood (Summer wood)
Lower proportion of vessels for moving water because the cuticle is formed.
Larger number of fibers to provide strength for the increased size of the plant

15. Heartwood Sapwood Darker wood at center of tree
Vessels of heartwood do not function any more due to breaking of the vessels from insects, vibration, etc.
Plugs fill the vessels to prevent infection—TYLOSIS
On average, one ring/year added to heartwood
Sapwood
Lighter colored rings
Vessels are full of “xylem sap” (hence the name)
Is this where maple syrup is found?
One ring of sapwood added each year

18. Reaction Wood Also known as tension wood in hardwoods
Gelatinous fibers in wood exert tension on the branch
Prevents branches from drooping
Rings are much wider at the top of the branch
Also known as compression wood in conifers
Enriched with lignin
Rings much wider at the bottom of the branch

19. Secondary Phloem Also conducts up and down the stem
Also derived from vascular cambium
Outermost phloem is the oldest
Secondary phloem conducts for less than one year (no rings)
Rays of secondary phloem equal to those of xylem because they originate from same ray initial.

20. Outer Bark
Secondary phloem develops into another cambium outside the vascular cambium = cork cambium (phellogen)
Cork cambium produces the outer bark
Outer bark made of cork cells (phellem cells)
Phellem cells are dead, waterproof and fire resistant
Phellem cells are constantly lost sloughed off like our skin
Phellem = Cork = Outer Bark

21. Inner Bark “a dog’s conscience” 
All the phloem between the vascular cambium and the cork cambium.
Lenticels are loosely packed cork cells that allow the diffusion of oxygen into the trunk.
Included Phloem is phloem in secondary root tissue that is included in the xylem.

23. Homework