A stem is an organ that raises or separates leaves, exposing them to sunlight. Stems also raise reproductive structures, facilitating dispersal of pollen and fruit. Each stem consists of an alternating system of nodes, the points at which leaves are attached, and internodes, the stem segments between nodes.
In the upper angle (axil) formed by each leaf and the stem is an axillary bud, a structure that can form a lateral shoot, commonly called a branch. Young axillary buds typically grow very slowly. Most of the growth of a young shoot is concentrated near the shoot tip, which consists of an apical bud, or terminal bud, that is composed of developing leaves and a compact series of nodes and internodes.
The stems can show variation because many stems are specialized. With specialized stems, plants can exploit a diversity of niches in virtually all terrestrial ecosystems.
Like roots, the stems of vascular plants are made of dermal, vascular, and ground tissues.
- A single-celled layer of epidermis protects and waterproofs the stem and controls gas exchange. In trees, some of the epidermal tissue is replaced by bark. Bark is a combination of tissues that provides a tough, woody external covering on the stems of trees. The inner part of bark is alive and growing; the outer part is dead and provides strength, support, and protection.
- Ground tissue forms the interior of the stem. The large central vacuoles of ground tissue cells fill with water to support the plant. The cells may also store food.
- Bundles of vascular tissue run through the ground tissue of a stem and transport fluids. Plants may vary in how these bundles are arranged.
The vital function of stem is transporting water and minerals from roots to leaves and carrying food from leaves to the rest of the plant. Without this connection between roots and leaves, plants could not survive high above ground in the air. In many plants, stems also store food or water during cold or dry seasons.