Download CB098-008.46_The_Stem_A

The Stem:
The Shoot System I
Functions & Organization of the Shoot System
-The shoot system is composed
of the stem and its lateral
appendages: leaves, buds and
flowers.
-Stems are made up of the 3 tissue
systems (Ground Tissue System mainly
in the form of cortex, pith and
parenchyma; Dermal Tissue System
mainly in the form of epidermis and
periderm: Vascular Tissue System in the
form of vascular bundles containing
mainly xylem and phloem conducting
tissues.
-Vascular tissue runs the length of the
stem in vascular bundles. Each vascular
bundle contains xylem tissue on the
inside, phloem tissue on the outside and
meristem tissue in between the two.
-Two groups of flowering plants, dicots
and monocots, have major differences in
the distribution of their stem tissues.
Anatomy of A Dicot Stem
Vascular bundles are arranged in a ring
around the edge of the stem.
Anatomy of A Monocot Stem
Vascular bundles are scattered throughout the stem. The terms pith
and cortex usually are not used when the bundles are scattered;
instead the term ground tissue is used for all parenchyma tissue
surrounding the vascular bundles.
Primary Growth and Stem Anatomy
Primary tissues of
dicot stems
develop from the
primary
meristems. The
SAM is composed
of dividing cells.
It is responsible
for the initiation
of new leaves and
buds and for
making the
3 primary
meristems.
The Pattern of Primary Vascular Development in a Dicot Stem
The primary meristem,
Procambium, gives rise to
primary xylem and primary
phloem. The primary
meristem, Protoderm, gives
rise to epidermis. The
primary meristem, Ground
Meristem, gives rise to pith
and cortex. All resulting are
primary tissues in the stem
of the primary plant body.
Parenchyma cells in the outside region of the cortex
sometimes contain chloroplasts for photosynthesis. The
pith region of the stem in some plants may become hollow
by the breakdown of the centrally located parenchyma cells.
In some plants exhibiting secondary growth, some
procambium between the primary xylem and phloem
remains undifferentiated; such cells are called residual
procambium.
The Distribution of the Primary
Vascular Bundles Depends on
the Position of Leaves.
Primary Vascular System
The Vascular
Bundles that
network into the
attached leaves are
called leaf traces.
The organization of
primary vascular
bundles in stems
depends on the
number and
distribution of
leaves and on the
number of traces
that branch into the
leaves.
Leaf Arrangement Patterns
Primary Growth Differs in Monocot and Dicot Stems
Monocot stem diameter is about
the same near its apex as its
base. Monocots have primary
thickening meristems (PTM),
which are absent in dicots. The
PTM does elongation and lateral
growth, a characteristic resulting
in an umbrella-like shape.
Iris (Monocot) Shoot Tip
Monocot Shoot Apex in More Detail
The SAM and the primary
meristems also are present in
these shoot tips. Some
monocots, like wheat are hollow
in the stem internodes rather
than having ground tissue there.
BIO 141 Botany with Laboratory
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