Stem Structure

How is the stem organized in cross-section?

Moving from the outside toward the center of the stem, you will observe these structures: the epidermis, the cortex, the vascular cylinder , and then pith (not in monocots)

Epidermis

Outer layer of cells that protects the internal stem tissues

Cortex

The area beneath the epidermis, up to the vascular cylinder
This is mostly ground tissues that store materials and provide peripheral strength

Vascular Cylinder or Stele

The vascular cylinder, or stele, are the names given to the area that includes the xylem and phloem
In young stems, the stele is composed of primary xylem and primary phloem, which formed from the shoot apical meristem
In monocots, the vascular tissues are arranged in scattered bunches, called vascular bundles

Pith

The area interior to the vascular cylinder, composed mostly of parenchyma cells, and use for storage by the plant
The pith provides a low-cost way to increase twig circumference and thereby space for attachment of large leaves, which increases their ability to biomechanically support large leaves (Mendieta et al. 17Nov2021)

Above: Eudicot stem, showing epidermis, cortex, vascular cylinder, and pith

Below: Monocot stem, showing epidermis and dispersed vascular bundles throughout the cortex

How is the stem organized lengthwise?

Moving from shoot tip toward the plant body (proximal), you will observe these structures: shoot apical meristem, primary meristems (i.e. procambium, protoderm, ground meristem), and then primary tissues (xylem, phloem, parenchyma, etc.)

Shoot Apical Meristem (SAM)

Area of actively dividing cells (active mitosis)
Increases the length of the stem (tunica-corpus system)
The SAM creates primary growth in the plant

Primary growth in stems

- Mitotic growth from the shoot apical meristem (SAM)
- Increases length and/or depth of shoots of the plant
- Each apical meristem creates three (3) intermediate primary meristems which are the sites of cell differentiation and maturation: protoderm, ground meristem, and procambium

Protoderm

- Protoderm is the thin of cells on the periphery of the developing shoots and roots that will eventually become the mature epidermis

Ground meristem

- The ground meristem are the bulk mass of cells in the developing shoots or roots that eventually become mature ground tissues, such parenchyma and sclerenchyma

Procambium

- The procambium are the cells within the developing shoots or roots that will mature into the vascular tissues, such as xylem and phloem

Above: the shoot tip of Coleus, showing the SAM, leaf primordia, protoderm, procambium, ground meristem, and newly formed axillary buds

Morphology of Stems

Externally, stems have many visible features that can be used for identification: internodes, nodes with buds and leaves

Nodes

- Part of shoot where a leaf (or leaves) attaches to a stem; an axillary bud is also located at nodes
- Plants with one leaf per node are labeled "alternate"; the leaves appear to alternate on the stem.
- Plants with two leaves per node are labeled "opposite"; leaves appear opposite of each other on the stem.
- Plants with three or more leaves per node are labeled "whorled"; leaves appear in a whorled ring around the stem

Internodes

- Area of stem where no leaves (or buds) are attached (in between nodes)
- In some plants such as horsetails and grasses,there is an intercalary meristem just above the node (see "Special Primary Growth" in Primary Meristems)

Buds

Buds are inactive or dormant shoot apical meristems
Buds create new shoot systems; they create branching!

Axillary bud

- Inactive apical meristem found in the "axils" of leaves
- In flowering plants, an axillary bud appears above each leaf.
- Axillary buds are created by the distal-most SAM during primary growth
- These buds function as a branching mechanism, although this branching may be immediate or delayed.
- Lateral stems (branches) form from axillary buds on the outside of the stem. This is called exogenous growth
  - n.b. Contrast this with how lateral roots form

Terminal bud

- Term used for terminal SAM during a hibernation period (e.g winter)
- Created when the distal-most or dominant SAM goes dormant due to seasonal change
- Terminal bud scales are created to protect the SAM from harsh environmental conditions.

Delayed branching (proleptic growth)

- Discontinuous development of new lateral shoots, with its apical meristem experiencing a resting period (e.g. winter)
- The new branched shoot usually shows successive short internodes at its base;
- The first leaves of the branched shoot are also reduced when compared to the following leaves, or the bearer leaves.
- On the shorter internodes, spines or cataphylls may be found instead of reduced leaves.
- Common in temperate plants, although there are exceptions
- e.g. Acer, Quercus, Platanus, Tree tomato (Cyphomandra)

Above: On the new shoot, the first internodes are short and bear reduced leaves. Above left: Tree tomato (Cyphomandra hartwegii) inwhich the diameter of the new shoots is significantly reduced when compared to the bearer. Above right: Sycamore (Platanus sp.) inwhich the scars on this past branch shows a short hypopodium

Immediate branching (sylleptic growth)

- Branches result from the development of a newly initiated lateral axis without having had an intervening rest period
- The new branched shoot shows a long first internode (i.e. a long hypopodium), and the first leaves of the branched shoot also show a similar type and size then the next leaves, or the bearer leaves.
- Common in tropical plants, although there are exceptions
- e.g. Persea, Acacia, Liquidambar, Liriodendron

Above left: Persea americana, Above right: Acacia dealbata. Both first leaves and internodes of the branched shoot are similar to the other ones, and the first internode is long (Photos and drawing D. Barthélémy, CIRAD).

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