Seed Plant Anatomy: Stems

Helianthus stem

Helianthus is also known as a sunflower. It's an annual flowering plant (an angiosperm). Since it's an annual, there is no secondary growth visible in the stem; no growth rings or lateral meristems such as you might see in a woody stem.

Helianthus is a dicot, and the vascular bundles form a ring around the perimeter of the stem. Each bundle contains xylem and phloem.

The xylem is toward the inside. The xylem cells are large and thick-walled, and they are stained red in this slide.

The phloem lies just outside the xylem; the phloem cells are smaller, with thin walls and blue staining. Just outside the active phloem there are phloem fiber cells. These tough, reinforced cells are stained red and look similar to xylem cells. However, they don't seem to be involved in transport within the plant. Instead, they apparently function mainly to strengthen the stem.

The middle of the stem is filled with a ground tissue called pith.

The outer layer of the stem is the epidermis, which protects the stem and reduces water loss.

Woody dicots: Tilia stem

Tilia (also called basswood or linden) is a tree; it has woody stems. These stems look different from the sunflower stems above, because they are structured for secondary growth. Secondary growth thickens the stem and produces distinct rings of tissue.

Tilia 1-year stem

This image shows vascular tissue forming a continuous ring around the stem -- unlike the discrete bundles of the annual Helianthus stem. The is a 1-year-old stem, with no secondary growth visible; there is only one layer of xylem. This stem does have two lateral meristems: the vascular cambium and the cork cambium. These meristems would produce secondary growth in subsequent years. The vascular cambium will add new layers of xylem to the outside of the existing xylem, and new phloem to the inside of the existing phloem.

Tilia 2-year stem

The 2-year-old stem shows two distinct rings of xylem. The inner ring is the primary xylem from the first year of growth; the outer ring was added by the vascular cambium during the second year.

Tilia 3-year stem

This image shows a three-year-old stem, with two layers of secondary xylem visible. Each layer represents one year of growth. As a woody stem or tree trunk continues to grow, most of the cross-sectional area will be filled with secondary xylem.

New vascular tissue is added at the vascular cambium, which is one of the two lateral meristems. The vascular cambium continually adds new secondary xylem to the outside of the existing xylem (just inside the vascular cambium), so the xylem area grows thicker and thicker. However, this stretches out the band of phloem, which must continue to add new cells to fill in the phloem band. The new secondary phloem cells are added to the inner part of the ring of phloem. The annual rings of phloem are not as clearly defined as those of xylem.

Phloem contains several types of cells; it includes both the intensely red cells and the pale blue cells visible here.

There are two kinds of lateral meristems in this slide. The vascular cambium produces vascular tissue inside the stem (xylem and phloem); the cork cambium produces new cells on the outside, forming a protective layer for the stem. Since stems grow from the inside, the outer layers tend to get stretched thinner and thinner; the cork cambium fills in additional cells to ensure that the protective cork and epidermis form a continuous layer of bark around the stem.

Pinus stem

The stem of a pine tree, a gymnosperm, is quite similar to that of a woody angiosperm.

The image above shows one ring of primary xylem; there is no secondary growth visible.

Pine stems and leaves have resin channels. When the tree is alive, these would be filled with resin, a hydrocarbon-containing substance that seems to help protect the tree. Resin is the sticky substance that sometimes drips from damaged pine trees.