Stem Anatomy

Stem Anatomy

Stems usually grow above the soil surface and towards the light from the sun. Depending on the hardness of the stem, we can distinguish between herbaceous stems, which are leafy non-woody structures, and woody stems. Woody stems are harder than herbaceous stems.

Stems have four main functions.

• Support for the plant as it holds leaves, flowers and fruits upright above the ground. Stems keep the leaves in the light and provide an attachment for flowers and fruits.
• Transport of water, mineral salts and sugars between roots and shoots in the xylem and phloem.
• Storage of nutrients.
• Production of new living tissue: stems contain meristematic tissue which generates new tissue.

The main stem develops from the plumule of the embryo and the lateral branches develop from the buds. Nodes and internodes are regions found on the stem. Nodes are the regions from which leaves and lateral branches develop, and the regions between nodes are known as internodes (shown in the figure below). Stomata, or pores, can be found in stems of younger plants. We will subsequently discuss the tissues present in the dicot stem. The trunk of a tree (shown below) is the stem.

Stem showing internode and nodes plus leaf petioles.

Photo of a redwood. The trunk of the tree is its stem.

The internal structure of the dicotyledonous stem

The figure below shows a schematic arrangement of tissues in a dicotyledonous stem. Details of each tissue type are described in this section.

Epidermis: A single layer of cells that covers the stem, and is in turn covered by a waxy cuticle. The waterproof cuticle helps prevent water loss and thus prevents the inner tissues drying out. Since the function of the epidermis is to protect underlying tissues, epidermal cells are tightly packed and have thickened walls. The epidermis may contain hair-like outgrowths known as trichomes, and stomata with guard cells. Stomata present in the epidermis allow for transpiration and gaseous exchange for respiration and photosynthesis.

Cortex: A region which comprises of collenchyma, parenchyma and the endodermis

• Collenchyma: A few layers of living cells that lie under the epidermis. These cells are not lignified but do have unevenly thickened cell walls. Collenchyma cells are thickened in the corners, but thin-walled elsewhere. The thickened corners strengthen the stem and provide support, while the thin-walled sections allow for flexibility in the wind. Collenchyma cells contain chloroplasts which produce food for the plant during photosynthesis.
• Parenchyma: Found beneath the collenchyma cells and makes up the bulk of the cortex. The cells are thin-walled, and there are intercellular spaces which are important in gaseous exchange. Parenchyma stores synthesised organic food (mostly starch) produced elsewhere in the plant.

Cross-section of a dicotyledonous stem showing tissue distribution.

Vascular cylinder or stele: Comprised of the pericycle, vascular bundles and pith

• Pericycle: Commonly found in roots and, in lower vascular plants, also in stems. In higher vascular plants, however, a distinct layer of cells may not be present. The pericycle, if present, may be composed of either thin walled parenchyma cells or sclerenchyma cells with relatively thin or heavily thickened walls. In plants undergoing secondary growth, the pericycle contributes to the vascular cambium often diverging into a cork cambium.
• Vascular bundles: Characteristically organised in a ring inside the pericycle of the dicot plant. Mature vascular bundles are made up of water-conducting xylem, cambium, and food-conducting phloem. The phloem is located on the outside of the bundle and the xylem towards the centre (see the figure above). The phloem and xylem is separated by meristematic tissue known as cambium, which is responsible for secondary thickening. Xylem has lignified cell walls which helps it fulfil its two important roles, namely; strengthening and supporting the stem, and transporting water and minerals from the root system to the leaves. The function of phloem is to transport synthesised food from the leaves to other parts of the plant.
• Pith (or medulla): Occupies the large, central part of the stem. The pith is made up of thin-walled parenchyma cells containing intercellular spaces. Where the parenchyma extends between vascular bundles as thin bands it is known as medullary rays, and can be continuous with the pith and cortex of the parenchyma. The cells of the pith store water and starch, while the intercellular spaces allow for gaseous exchange. The medullary rays facilitate transport of substances from xylem and phloem to the inner and outer parts of the stem.