The illustration above shows an herbaceous
stem of a dicotyledonous plant. The white stripe between xylem & phloem
is the cambium layer.Note:
The diagrams that students will draw should show zones or general regions. Individual
cells need not be illustrated.
Internal structures
to know of herbaceous stems:
Pith: Large central area for storage
& support.
Cambium: Found as a circle around
inner stem & outer surface. Forms woody secondary tissue for support.
Cortex: Storage area between cambium
and epidermis.
Epidermis: Thin layer of skin cells.
Protection.
Xylem: Water conduction up.
Phloem: Sap (organic molecules)
conduction, usually down to roots.
Note:The diagrams that students will draw should show zones or
general regions. Individual cells need not be illustrated.
Internal Anatomy of Woody Stems:
Pith: Original stem at very center
of stem.
Xylem: Water conduction tubes connect
leaf to roots. Inner most xylem dies & forms wood.
Phloem: Outer tubes just inside
bark to carry food from leaves to roots.
Heartwood: Dead wood (xylem) in
center of stem. It is either dry or filled with tars. Frequently darker than
live wood.
Sapwood: Live outer wood conducting
water and sap.
Cambium: Special cells that make
new wood (xylem) & new phloem & bark to make tree trunk thicker. Found
between xylem & phloem.
Bark: Outer protective (from insects,
fire, and injury) and waterproof layer of stem. Made by cambium. Outer barks
is dead, inner is alive.
Springwood: Light colored rings
of xylem in wood made when growing season is good (spring & early summer).
Summerwood: Darker colored tree
rings made when growing season is poor (late summer, fall, & winter).
The upper diagram shows a young woody dicot stem before it
has started to grow in width. The lower diagram shows the stem after several
layers of wood have been layed down.
