1. Bellwork: What is the difference between primary and secondary growth of stems? How are they related? Why does this result in certain plants being able to grow taller?

2. Stems
Section 23.3

3. What are the 3 main functions of stems?
Produce leaves, branches and flowers
Hold leaves up to the sun
Stems transport substances through the plant

4. Stem anatomy
Stems contain all three tissue types – dermal, vascular and ground tissue
Layer of epidermal cells that have thick cell walls and a waxy protective coating
Node – where leaf is attached to the stem
Bud – contains apical meristem that can produce new stems and leaves
Stem can develop woody tissues that help support leaves and flowers

5. Vascular bundle patterns
Monocots have clusters of xylem and phloem tissues called vascular tissues
In dicots vascular bundles are arranged in a cylinder or a ring

6. Plant stem growth Primary vs Secondary growth
Primary growth - of new cells produced by the apical meristems of roots and stems
Increase in length in a plant is due to primary growth from year to year
Primary growth of stems is the result of elongation of cells produced in the apical meristem
Occurs in all seed plants
Secondary growth – Stems must increase in thickness as well as length if the plant gets big and have more mass to support
Common in dicots and non flowering seed plants such as pines
Rare in monocots
Explains why most monocots have limited girth

7. Secondary growth continued…
Most dicots have meristems within their stems and roots
Enable true secondary growth
Increased width is able to support increased height of tree
In conifers and Dicots, secondary growth occurs in meristems called the vascular cambium and the cork cambium
Vascular cambium – produces vascular tissue and increases stem thickness over time
Cork cambium – produces the outer covering of stems
Similar types of cambium are also found in roots]

8. Growth from the Vascular Cambium
In a young dicot stem, bundles of xylem and phloem are arranged in a ring
Secondary growth begins as a thin, cylindrical layer of cells between clusters of vascular tissue
New meristem forms between the xylem and the phloem of each vascular bundle
Division of vascular cambium gives rise to new layers of the xylem and phloem
Each year the cambium continues to produce new layers of vascular tissue, causing the stem to become thicker and thicker.

9. How does wood form?
Most of what we call wood is layers of secondary xylem produced by the vascular cambium
These cells build up year after year, layer upon layer
Older xylem – heartwood – found in the center no longer conducts water
Darkens with age as it collects colored deposits
Sapwood surround heartwood – is lighter in color, and is active in fluid transport

10. What is a tree ring? How does it form?
Tree growth is seasonal in temperate zones
When growth begins in the spring, the vascular cambium begins to grow rapidly, producing large, light colored xylem cells with thin cell walls
Results in light colored earthy wood
As growing season continues, cells grow less and have thicker cell walls, forming a layer of dark late wood
Alternation of dark and light wood produces tree rings
Usually one ring is a year of growth
Tree ring cross sections can be used to estimate a trees age
Size of rings can provide additional information about weather conditions

11. How does Bark form?
In a mature stem all of the tissue outside of the vascular cambium makes up bark
Includes Phloem, Cork Cambium and cork
As a tree expands in width, the phloem layer must grow as well
Expansion may cause the oldest tissues to split and fragment as the expanding stem stretches them
The cork cambium surrounds the cortex and produces a thick , protective layer of waterproof cork that prevents water loss from the stem
As stem increases in size, outer layers of dead bark often crack and flake off the tree

12. Key points questions What are three important functions of a stem?
How does the arrangement of vascular bundles differ between monocots and dicots?
How do the functions of a stem relate to the functions of roots ad leaves?
Define primary and secondary growth