Secondary Growth
The cambial Layer is responsible for two conducting systems

The cambium must produce cells that add to diameter (radius) and accommodate increasing diameter (perimeter)

Separate Bundles
Rings of Vascular Tissue

Development of cork cambium provides covering tissue

Lenticels are openings in the surface which allow oxygen to enter stems
The cork cambium isolates layers of exterior tissue.
•	As time goes by, old epidermis and cortex are sloughed off
•	Layers of cork form the surface, and they too are lost
Ultimately, phloem tissue becomes meristematic and produces a cork cambium.

Xylem, trapped in the interior accumulates

Parts of Woody Stem
•	Wood (old xylem)
–	Heartwood, plugged, no flow
–	Sapwood, open, moving water
•	Bark (old cortex, old phloem, phloem)
–	Living, functional phloem “inner bark”
–	Corky periderm “outer bark”

Conifer Wood
•	Dominated by tracheids
•	Prominent resin ducts

Tracheids Have Bordered Pits

Bordered Pits Prevent Embolisms

Growth Rings in Wood
Knots
Knots 
Always Prune a Tree Properly