Shoot and Stem Structure
Exercise A: Shoot Morphology and Primary Growth in the Stem
Part 1: Herbaceous Dicot Shoot Morphology
* Examine a live herbaceous dicot such as coleus (see p. 548) and note that leaves are
paired and opposite one another at the node. Also note the relative orientation of
adjacent pairs of leaves.
* Identify the nodes and internodes and the lateral (axillary) buds (or shoots if they have
already expanded) located in the leaf axils. Note that some of the axillary shoots are
flowers (specialized reproductive shoots). Locate the region of the shoot apical
meristem of the plant. The lateral shoots near the stem tip are less well developed than
the shoots nearer the base, in part because they are younger and in part because of
apical dominance (we will discuss apical dominance later in the semester).
Part 2: Herbaceous Dicot Stem
In herbaceous (non-woody) plants, or in the young stems of woody plants, the primary
vascular tissues (i.e., vascular tissues produced by primary growth) are arranged in
bundles. These bundles may form a ring around the pith (dicots), or they may be
arranged in several rings or scattered throughout the ground tissue (monocots).
* Examine a prepared slide of a Helianthus stem cross section with separate bundles.
Note that the vascular bundles are arranged in one ring around the pith. Identify the
epidermis, cortex, vascular bundles, pith, and pith rays. Within the cortex, can you
distinguish the parenchyma and collenchyma cells? How? All of the vascular tissue in
this section is produced by primary growth. You should be able to distinguish primary
xylem on the inner side of the bundle and primary phloem on the outer side. Note the
cap of fibers associated with the phloem (see Fig. 25-10; p. 555 for assistance).
Part 3: Monocot Stem
* Examine a prepared slide of a cross section of a Zea mays (corn) stem (text p. 556).
Corn has vascular bundles scattered throughout the ground tissue. There is no
procambium remaining between the xylem and phloem of these bundles, thus no
vascular cambium can form within the bundle as occurs in dicots with secondary growth
(see below). Bundles that lack residual procambium are called closed. Bundles in which
a vascular cambium eventually forms are called open (e.g., Helianthus and Medicago).
Note that the vascular bundles in corn are surrounded by a bundle sheath of
sclerenchyma cells. Primary phloem is found toward the outside of the stem within the
bundle and consists of sieve-tube elements and companion cells. Primary xylem is
found toward the inside of the stem within the bundle and consists of vessel elements
and parenchyma cells. During cell elongation, the xylem vessels may break, forming an
air space.
* Identify the bundle sheath, primary phloem, companion cells, sieve-tube elements,
xylem, vessel elements, xylem parenchyma, ground tissue, and an air space (Fig 25-13;
pp. 557).
Part 4. Coleus shoot tip
* Examine a prepared slide of a longitudinal section of a Coleus stem tip (text p. 548
and 568)
You should see a stem with several pairs of opposing leaves. At the top of the stem is
the dome-shaped apical meristem, which is surrounded by leaves. It gives rise to the
primary tissues of the stem: protoderm (develops into the epidermis), procambium
(develops into the primary vascular tissues), and ground meristem (develops into the
cortex and pith). Along the sides of the apical meristem are the leaf primordia with
axillary bud primordia, which will give rise to new leaves and buds, respectively.
Although the vascular bundles are arranged in a ring around the stem, the vascular
tissue appears in median longitudinal section to consist of a vascular strand on each
side of the pith. Each vascular strand contains xylem and phloem. At each node, the
vascular stand continues into the leaf as a leaf trace. You should be able to identify
these tissues and regions.
Exercise B: Secondary Growth
Woody plants can live for many years, and every year they produce new tissues.
Primary growth results in an increase in the length of stems and roots and the
production of new organs. Secondary growth results in an increase in the girth of stems
and roots. Primary growth in the shoot is the result of the activity of the apical meristem
in the stem. Secondary growth is the result of the activity of two lateral meristems: the
vascular cambium, which produces secondary vascular tissues (xylem and phloem),
and the cork cambium (phellogen), which produces the periderm. The periderm
develops as the woody plant ages; it replaces the epidermis. Periderm consists of
several layers including the cork and cork cambium. The cork cambium produces cork
to the outside. The term bark refers to all tissues outside the vascular cambium (i.e., the
phloem and the periderm).
Part 1: The Onset of Secondary Growth
*Examine a prepared slide of a cross section of an old stem of Helianthus (compare to
Medicago p. 555). This section has been taken from a level in the stem where the
primary tissues are mature, and secondary vascular tissues have begun to be produced
by a vascular cambium.
In dicots, the vascular cambium consists of an alternation of fascicular cambium, which
develops from residual procambium within each vascular bundle, and interfascicular
cambium, which differentiates in the pith rays between the bundles. The fascicular and
interfasicular cambia connect to form a continuous cylinder of vascular cambium (which
appears as a ring in cross section). The cells of the vascular cambium divide to produce
phloem to the outside of the cambium and xylem to the inside.
Identify the epidermis, cortex (including parenchyma and collenchyma), vascular
bundles (with xylem, phloem, and phloem fibers), fascicular cambium, interfascicular
cambium, and pith.
Part 2: Woody Stems of Different Ages
* Examine the photomicrograph of a Tilia (basswood) young stem cross section (Fig 258; p. 552). Note that the vascular tissues are arranged in a continuous ring around the
pith.
* Now compare this photomicrograph with prepared slides of Tilia one-, two-, and threeyear old stem cross sections (p. 584 and 585). Note the thick-walled primary phloem
fibers.
As the stem grows, the vascular cambium produces secondary xylem to the inside and
secondary phloem to the outside. Therefore, the primary xylem is next to the pith and
the primary phloem (if still present) is near the cortex or periderm.
Each year, new secondary xylem (wood) is laid down in an annual ring. Often, the
xylem laid down in the spring has many large vessel elements in it, while the xylem laid
down later in the year has smaller vessel elements (look at a piece of oak wood). The
abrupt contrast between the small-celled late wood and the large-celled early wood of
the following year is what helps to make a growth ring visible. From the number of rings
of xylem, one can determine the age of a stem.
A smaller increment of new phloem is laid down every year, but previously formed
phloem is pushed toward the outside of the stem in the process and is crushed and
finally sloughed off. Therefore there is much less phloem than xylem in a cross section
of a woody stem.
*Identify (proceeding outward from the center of the stem) pith, primary and secondary
xylem, early xylem (early wood), late xylem (late wood), vascular cambium, secondary
phloem, primary phloem, fibers, cortex, periderm, and lenticels (areas of the bark that
contain many airs spaces and play a part in gas exchange).
Part 3: Gross Anatomy of Old Woody Stems
* Examine the cross, radial, and tangential sections of a woody stem. Radial and
tangential sections are types of longitudinal sections. Radial sections pass through the
exact center of the stem (i.e., they go through the pith), whereas tangential sections do
not pass through the stem center. Note how the cells look in the different sections. Note
the annual growth rings. Do you understand how they form?
Part 4: External Features of Woody Stems
* Examine the external features of the various twigs (see p. 589). At the tip of the twig is
the terminal bud. There are no leaves present, but leaf scars have been left on the twig
where the leaves were once located. Inside the leaf scars are tiny vascular bundle
scars. The small dots along the twig are the lenticels, areas of the bark that contain
many airs spaces and play a part in gas exchange. There are also terminal bud-scale
scars that wrap around the twig and indicate the location of previous years’ terminal
buds. Locate these features on the twigs.
Excluded 2003
Part 4: Woody Roots
Dicots with secondary growth usually have woody roots as well.
*Examine the cross sections of a mature herbaceous dicot root with primary growth only
(Ranunculus) and a woody dicot with primary and secondary growth (Salix). How do
these two differ? How does a woody root compare to a woody stem?