A stem is one of two main structural axes of a vascular plant, the other being the root. It supports leaves, flowers and fruits, transports water and dissolved substances between the roots and the shoots in the xylem and phloem, photosynthesis takes place here, stores nutrients, and produces new living tissue. The stem can also be called halm or haulm or culms.
The stem is normally divided into nodes and internodes: The nodes are the points of attachment for leaves and can hold one or more leaves. There are sometimes axillary buds between the stem and leaf which can grow into branches (with leaves, conifer cones, or flowers). Adventitious roots may also be produced from the nodes. Vines may produce tendrils from nodes. The internodes distance one node from another.
The term "shoots" is often confused with "stems"; "shoots" generally refers to new fresh plant growth, including both stems and other structures like leaves or flowers.
In most plants, stems are located above the soil surface, but some plants have underground stems.
Stems have several main functions:
Support for and the elevation of leaves, flowers, and fruits. The stems keep the leaves in the light and provide a place for the plant to keep its flowers and fruits. Transport of fluids between the roots and the shoots in the xylem and phloem. Storage of nutrients. Production of new living tissue. The normal lifespan of plant cells is one to three years. Stems have cells called meristems that annually generate new living tissue. Photosynthesis.
Stems have two pipe-like tissues called xylem and phloem. The xylem tissue arises from the cell facing inside and transports water by the action of transpiration pull, capillary action, and root pressure. The phloem tissue arises from the cell facing outside and consists of sieve tubes and their companion cells. The function of phloem tissue is to distribute food from photosynthetic tissue to other tissues. The two tissues are separated by cambium, a tissue that divides to form xylem or phloem cells.
Specialized terms
Stems are often specialized for storage, asexual reproduction, protection, or photosynthesis, including the following:
Acaulescent: Used to describe stems in plants that appear to be stemless. Actually these stems are just extremely short, the leaves appearing to rise directly out of the ground, e.g. some Viola species. Arborescent: Tree with woody stems normally with a single trunk. Axillary bud: A bud which grows at the point of attachment of an older leaf with the stem. It potentially gives rise to a shoot. Branched: Aerial stems are described as being branched or unbranched. Bud: An embryonic shoot with immature stem tip. Bulb: A short vertical underground stem with fleshy storage leaves attached, e.g. onion, daffodil, and tulip. Bulbs often function in reproduction by splitting to form new bulbs or producing small new bulbs termed bulblets. Bulbs are a combination of stem and leaves so may better be considered as leaves because the leaves make up the greater part. Caespitose: When stems grow in a tangled mass or clump or in low growing mats. Cladode (including phylloclade): A flattened stem that appears leaf-like and is specialized for photosynthesis, e.g. cactus pads. Climbing: Atems that cling or wrap around other plants or structures. Corm: A short enlarged underground storage stem, e.g. taro, crocus, gladiolus.