I. Plants increase in size through concerted processes of
A. Cell division = mitosis + cytokinesis
B. Cell expansion = turgor pressure leads to elastic
expansion of
cell wall
II. Plant Cell Walls
A.Composed primarily of cellulose, with various amounts of
other molecules
B. How plants stand up to gravity
1. Primary cell walls + turgor pressure
2. Secondary
cell walls of dead cells
III. Growth of a plant stem
A. Primary Growth via Shoot Apical Meristem
1. Increase in length and girth
Primary tissue diffferentiation
2. Epidermis
3. Cortex
4. Procambium ->
a. Primary Phloem
b. Fascicular Cambium -> Vascular Cambium
c. Primary Xylem
5. Pith
B. Secondary Growth via Vascular Cambium
1. Increase in girth only
2. Structural support
3. Maintains continuity in Transport Pathways
a. transpiration in xylem
b. translocation in phloem
2. Toward the end of primary growth
a. Formation of Fascicular Cambium from Procambium
+ formation of Interfascicular Cambium from
Primary Ray Parenchyma
b. Vascular Cambium forms via merger of Fascicular
and Interfasicular Cambia
c. Cell Division + Cell Expansion of cells
comprising Vascular Cambium give rise to
2. Vascular Cambium laterally
3. Secondary Phloem to outside
3. Massive stem trunks of trees and shrubs are produced
by cyclic periods of cell division + cell expansion
in vascular cambium (& cork cambium)
a. Period of dormancy
b. New secondary xylem formed first to outside of
existing xylem
c. New secondary phloem then formed to inside of
existing phloem
d. As development and accretion of new seconday
xylem increases girth of stem, new vascular
cambium cells develop to maintain continuity
of the vascular cambium cylinder
C. Secondary Growth via Cork Cambium
1. Increase in girth only
2. Provides protection against
a. water loss
b. desease vectors
c. fire
3. Lenticels provide regions of gaseous exchange
4. Initial cork cambium may be derived from
a. Epidermis
b. Cortex
5. Subsequent cork cambia derived
a. Interior to initial cork cambium
b. Cortex
c. Parenchyma within primary or secondary phloem
6. Cork cambia can be continuous or discontinuous
7. Most of the cells derived from cork cambium are
a. to the outside
b. heavily suberized and dead at maturity
8. Bark texture is anatomically complex
a. Continuity of cork cambium
b. Types of cells that are incorporated from
secondary phloem into outer bark
