Stem Lecture
Functions
Support of Leaves, Flowers, Fruits
Conduction of Water, Minerals, Sugars, etc.
Photosynthesis
Storage
Defense
Stems support a display of
leaves.
Stems orient the leaves
toward the light with minimal
overlap among the leaves.
Asclepias - milkweed
The stem supports a display of flowers
Cercis canadensis - redbud
The stem supports a display of fruits.
The stem of a vine “twines” around
objects in the environment circumnutation!
Ipomoea nil - morning glory
The stem does photosynthesis…and stores water.
Opuntia-prickly pear
This stem does
photosynthesis, stores
water, but also produces a
defense chemical:
mescaline…a hallucinogen.
Lophophora williamsii - peyote
Stem Lecture
Structure to Provide Functions
Support of Leaves, Flowers, Fruits
Conduction of Water, Minerals, Sugars, etc.
Photosynthesis
Storage
Defense
Typical Stem Cross Section
Helianthus annuussun flower annual
Epidermis
Cortex
A ring of vascular bundles
Pith
Epidermis
- window, reduce water loss
Cortex Collenchyma
- extensible support
Cortex Parenchyma
- photosynthesis, etc.
Phloem Fibers
- rigid support
Functional Phloem
- conduct sugars etc. away
from leaf to rest of plant
Vascular Cambium
- adds 2° xylem and 2° phloem
Xylem
-conduct water and minerals
up from soil
Pith
-water storage, defense?
VIP Stem: Provide both name and function labels:
Epidermis: reduce evaporation, gas exchange
Cortex: photosynthesis, collenchyma support
Vascular Bundles: conduction
Pith: water storage? defense? disintegrate?
outside
Vascular Bundle:
conduct CH2O away from leaf
to center
outside
Phloem Fibers: support
Functional Phloem:
Vascular Cambium:
add 2° Xylem and 2° Phloem
Xylem:
to center
conduct minerals up from soil
Vitis vinifera - grape
Notice how the vascular cambia
of adjacent vascular bundles
line up side by side.
Notice that cambium tissue
differentiates between the
bundles, connecting the cambia
together.
Vitis vinifera - grape
The vascular cambium makes 2° tissues:
Vitis vinifera - grape
Each year the cambium
produces a layer of secondary
xylem and a layer of secondary
phloem.
This photo shows secondary
xylem from parts of three years
in Pinus strobus (white pine).
spring of the next year
winter of that year
fall of that year
mid-summer of one year
Three years of Secondary Growth
Tilia - basswood
Secondary
Phloem
Secondary
Xylem
The study of the growth rings in wood: Dendrochronology
This tree is Pinus aristata
(bristlecone pine).
One individual of this species
shows more than 5000 growth
rings!
Inner wood, harvested by
boring, was used to validate
carbon-14 dating.
Imagine the stories that this
California tree could
tell…perhaps something of
migration of Asian peoples
down the western coast of
North America! They were
contemporaries of Pharaohs!
The epidermis will be stretched and torn if not replaced
Sambucus canadensis - elderberry
A cork cambium differentiates and produces a periderm.
Epidermis
cutin
suberin
Cork Cells
Cork Cambium
Phelloderm
Over time, the epidermis dies.
The cork cells build up to for
a thick layer for the bark of a
tree. We use this to make
stoppers for wine bottles and
so on.
When suberin is fully
developed, the cortex cells
will eventually be in the
dark. So these chloroplasts
will lose their function!
The thick periderm can be quite thick and assist in survival of
forest fires!
Sequoia sempervirens - giant sequoia
Randy is about six-feet tall!
The bark covers and stiffens the spines on many woody trees
and shrubs.
Bark =
epidermis +
periderm +
cortex +
phloem +
vascular
cambium
Wood =
secondary
xylem only!
Pith =
a small
percentage of
tree diameter
at maturity