Tree growth begins with photosynthesis in the spring growing season. Growth occurs both upward and outward from the crown downward, directed by the plant hormone auxin. New growth occurs through cell division, elongation and maturation in the meristem tissue. Annual growth rings are visible when examining the transverse plane of tree trunks and branches, showing incremental growth over time. Key parts of the tree include the phloem, cambium, xylem, and bark.

1. Wood Anatomy of Tree Rings

2. Tree growth begins with photosynthesis to produce new
wood when the growing season begins.

3. Like
“melting
wax” !!
Trees grow upward and outward (including root tips), but…
… tree growth actually
begins in the crown and
moves downward due to the
growth regulator, auxin.

4. Like
“melting
wax” !!

5. Meristems:
apical and
lateral
growth
D: cell
division
E: cell
elongation
M: cell
maturation

6. Meristems:
annual
growth can
also be seen
in the
branching
patterns of
many tree
species (esp.
conifers)

7. When we
dissect the
trunk, we can
see this
annual
incremental
growth, both
upward and
outward.
STEM
ANALYSIS

8. Locally Absent Rings
Rings may be
locally absent along
the length of the
tree.

9. False Ring
Cells leading into the
false ring will
gradually decrease in
size and then
gradually increase
back to earlywood
cells.

10. Viewing wood: tangential, radial, and transverse planes. We are
only interested in which of these?

11. Definitions:
Cambium: the growing (generative) layer between the xylem and phloem.
Xylem: principle strengthening and water conducting tissue of the stem,
roots, and branches.
Phloem: inner bark, principal function to distribute manufactured
foodstuffs.
Bark: dead, outer tissue that protects the cambium from the external
environment and exposure to pathogens and physical injury.
Vessel: the composite, tube-like structure found in hardwoods from the
fusion of cells in a longitudinal column.
Fiber: an elongated cell with pointed ends and a thick or infrequently thin
wall.
Rays: ribbon-shaped tissue extending in a radial direction across the grain
of the wood.

12. Note the five major portions of the tree trunk.

13. Phloem (inner bark)
Cambium
Xylem (wood)

14. Role of
heartwood
is…?
Role of
sapwood
is…?

15. The Wood Cell:
1. Holocellulose
a. alpha-cellulose (40-50%) = non-soluble =
long-chain polymers (glucose)
b. hemicellulose (20-35%) = readily soluble =
short-chain polysaccharides
2. Lignin (15-35%) – non-carbohydrate materials in
cell wall, very complex chemical structure.
3. Numerous Extractives – can be removed!
a. tannins
b. oils and resins
c. other complex organic compounds

16. Cell
lumen
Cell wall